DMTD Mixer Terminations
I am in the process of designing a DMTD system. As an experiment to do
basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in
series to ground with a 47 ohm metal film resistor. Where the capacitor
and resistor meets, another resistor is attached (390 ohms) that goes to
ground. The idea is to provide a 50 ohm termination at 20 Mhz and a
lighter termination at audio frequencies. I seen this is a NBS note and
I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A
schematic is attached of what I am experimenting with at the moment. A
HP5370B is the recording instrument. The noise floor from 1 days
observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at
10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000
secs. It will be interesting when the project is completed to see how
much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD
system. It occurs to me that maybe a capacitor designed for 50 ohms at
20 mhz may be a better termination (for the IF port) for this mixer. A
16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it
would be 100 meg-ohms, which would give maximum amplitude at 10 hertz.
As I understand, a capacitor terminated mixer will give a triangle wave
output, which is very beneficial to the design - as the end result is to
get maximum slope out of the mixer. I would say, unqualified as I am,
the capacitor termination matches the 20 mhz signal, and helps
attenuates the harmonics of the mixer, and has no , or very little
effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on
the 10 hertz beat signal - so maybe a capacitive termination on the 10
hertz signal only and something resistive on the 20 mhz
signal........another idea use the 16 pF direct off the mixer, then a
series resistor for isolation and then a large capacitor on the 10 hertz
beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on
the THAT1512 so I can make better measurements on the mixer. Bruce has
provided a lot of good suggestions and helpful comments on my project
and Ulrich has provided me quite a bit of user support on his program,
Plotter. Thanks to all.
Comments ? Brian KD4FM
Hi
A couple of things to try:
Just drop a 0.01 uf to ground directly on the output of the mixer.
Take the 390 to ground up to 3.9K ohms.
Depending on your mixer either / both may help or hurt.
You also may be able to run slightly more power into the mixer. More power may also smoke the mixer.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to
do basic measurements on the chosen mixer, I used a capacitor (0.01
uF) in series to ground with a 47 ohm metal film resistor. Where the
capacitor and resistor meets, another resistor is attached (390 ohms)
that goes to ground. The idea is to provide a 50 ohm termination at
20 Mhz and a lighter termination at audio frequencies. I seen this is
a NBS note and I can say, its a starting point for my experiments.
You will need a bit more filtering of the mixer IF output, or signal
rectification effects in the bipolar opamp may be an issue.
The amplitude of the sum frequency component seen by the opamp input
needs to be reduced to a value such that the effect of signal
rectification by the opamp input stage is insignificant.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A
schematic is attached of what I am experimenting with at the moment.
A HP5370B is the recording instrument. The noise floor from 1 days
observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13
at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at
10,000 secs. It will be interesting when the project is completed to
see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the
DMTD system. It occurs to me that maybe a capacitor designed for 50
ohms at 20 mhz may be a better termination (for the IF port) for this
mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at
10 hertz, it would be 100 meg-ohms, which would give maximum amplitude
at 10 hertz. As I understand, a capacitor terminated mixer will give
a triangle wave output, which is very beneficial to the design - as
the end result is to get maximum slope out of the mixer. I would say,
unqualified as I am, the capacitor termination matches the 20 mhz
signal, and helps attenuates the harmonics of the mixer, and has no ,
or very little effect on the audio frequencies that we are interested in.
Whilst in narrowband systems (a DMTD is a narrow band system) reactive
termination of the mixer/phase detector RF port will reduce the noise,
the idea is to reflect all of the sum frequency component back into the
mixer. This can be done using a capacitive termination where the
impedance of the capacitor is low at the (20MHz) sum frequency.
The capacitor impedance should be high at the (10Hz) difference
frequency to avoid attenuating the difference frequency component.
Using a capacitor with a 50 ohm reactance at the sum frequency will not
reflect all of the sum frequency back into the mixer.
Note with saturated mixer input ports capacitive termination as outlined
above of the IF port will not produce a triangular beat frequency waveform.
The waveform should be quasi trapezoidal with rounded peaks.
The slew rate at the zero crossing will be increased and the noise to
slope ration improved over that achieved with a more conventional
termination.
And saying/rambling on... that if maximum slope is needed, its needed
on the 10 hertz beat signal - so maybe a capacitive termination on the
10 hertz signal only and something resistive on the 20 mhz
signal........another idea use the 16 pF direct off the mixer, then a
series resistor for isolation and then a large capacitor on the 10
hertz beat for maximum slope.
There are a series of NIST papers that show the effect of the IF port
termination on the noise and beat frequency waveform.
Since its very easy to measure the beat frequency waveform slope and
noise at the zero crossing its probably better to calibrate your
speculations with actual measurements.
At the present, I am awaiting parts to build a low noise preamp base
on the THAT1512 so I can make better measurements on the mixer. Bruce
has provided a lot of good suggestions and helpful comments on my
project and Ulrich has provided me quite a bit of user support on his
program, Plotter. Thanks to all.
Comments ? Brian KD4FM
Bruce
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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The LT1037 is shown with a gain of ~1690x, if this amplifier is used to
amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network
to limit the large signal gain to 5x (so that the LT1037 remains stable
as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037
then avoids saturation and the opamp input stage remains in the linear
region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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Hi
The gotcha here is that saturation / slew limiting is one of the few things that will give you better data than the oscillators are really doing. Most error sources have the "nice" property of making things worse.
Bob
On Feb 27, 2010, at 6:40 PM, Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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The values in the schematics are wrong for the op amp gain. The drawing
was from an earlier drawing where I made a preamp to start checks on the
mixers, and I sent it to you (Bruce G). Thats when you determined I did
not have enough gain to get near the noise floor. The THAT1512/1646 ICs
were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts
peak to peak, sine-wave. I know from previous readings I see about 500
mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I
bought several taped reels of Dale RN55D resistors when a local business
went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the
power rails are +/- 15 volts). Also not shown on the schematic is a
0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to
limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was
marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and
stay up all night....and my employer just thinks I party too
hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used
to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback
network to limit the large signal gain to 5x (so that the LT1037
remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the
LT1037 then avoids saturation and the opamp input stage remains in the
linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a
chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax
capacitance. The LT1037 is quite happy driving a 600 ohm load. You
could easily drop the impedance at that point below 300 ohms. That
should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You
don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to
do basic measurements on the chosen mixer, I used a capacitor (0.01
uF) in series to ground with a 47 ohm metal film resistor. Where
the capacitor and resistor meets, another resistor is attached (390
ohms) that goes to ground. The idea is to provide a 50 ohm
termination at 20 Mhz and a lighter termination at audio
frequencies. I seen this is a NBS note and I can say, its a
starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A
schematic is attached of what I am experimenting with at the
moment. A HP5370B is the recording instrument. The noise floor
from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1
sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and
7x10-16 at 10,000 secs. It will be interesting when the project is
completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the
DMTD system. It occurs to me that maybe a capacitor designed for
50 ohms at 20 mhz may be a better termination (for the IF port) for
this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for
comparison at 10 hertz, it would be 100 meg-ohms, which would give
maximum amplitude at 10 hertz. As I understand, a capacitor
terminated mixer will give a triangle wave output, which is very
beneficial to the design - as the end result is to get maximum slope
out of the mixer. I would say, unqualified as I am, the capacitor
termination matches the 20 mhz signal, and helps attenuates the
harmonics of the mixer, and has no , or very little effect on the
audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its
needed on the 10 hertz beat signal - so maybe a capacitive
termination on the 10 hertz signal only and something resistive on
the 20 mhz signal........another idea use the 16 pF direct off the
mixer, then a series resistor for isolation and then a large
capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base
on the THAT1512 so I can make better measurements on the mixer.
Bruce has provided a lot of good suggestions and helpful comments on
my project and Ulrich has provided me quite a bit of user support on
his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
<DMTD_Plans.pdf>_______________________________________________
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Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load > 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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My simulations indicate that terminating the Mixer IF port in an RF
short (with both RF and LO ports saturated) increases the beat frequency
zero crossing slope by more than a factor of 2 (exact value depends on
mixer component characteristics) but doesnt significantly increase the
beat frequency amplitude over that with a high value resistive
termination. To achieve this the IF port termination impedance needs to
be high at the beat frequency and its significant harmonics. The value
above which the impedance is considered high depends on mixer details
such as transformer turns ratio, RF source impedance, diode
characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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Hi
Here's some data:
The setup is very simple:
Two oscillators at 10 MHz driving common base / 50 ohm output buffers. The buffers ensure that the source impedance is really 50 ohms. One puts out 9.3 dbm the other 9.5 dbm. They can be tuned for a beat note in the 0 to 100 Hz range.
The basic mixer termination filter is a pair cascaded / identical L networks. Both have 10 uh in the series leg and 0.047 uf to ground in the shunt leg. The "audio end" of the filter hooks straight into a digitizing scope.
The termination options for the mixer are:
- inductive - just running into the 10 uh of the first L network.
- 50 ohms - drop a 57 ohm in series with 0.047 from the mixer output to ground
- Capacitive - 0.047 uf to ground at the mixer output.
The data is computed from the time to cross the center 50% of the output waveform. If the output is 1V p-p then the data would cover the range -.25 to +.25 volts. I've normalized it to "volts / cycle". Divide by 2* pi if you want to get volts / radian.
mixer 50 ohms inductive capacitive
ZAD-3 3.51 2.96 9.98
RPD-1 #1 17.77 10.50 18.85
RPD-1 #2 17.40 10.058 18.53
10514A #1 5.796 4.396 10.31
10514A #2 5.826 4.406 10.33
10534A 5.402 4.078 10.88
ZP3-MH 8.06 5.81 11.28
ZAD-1H 7.73 5.93 9.38
Since not everybody has memorized mixer catalogs:
ZAD-3 typical minicircuits 7 to 10 dbm mixer
RPD-1 500 ohm output phase detector (50 ohms is the "wrong" termination for it)
10514, 10534 HP products from a ways back
ZP3-MH a 13 dbm class mixer, 9 dbm should be under driving it
ZAD-1H a 17 dbm class mixer, should be 8 db under driven.
Bottom line - Capacitive termination helps some parts more than others. The RPD-1 does not get a real big boost, but the ZAD-1 certainly does. There's no real way to know what it's going to do without checking your mixer under your conditions.
A few other notes:
-
The measurement technique slightly under states the slope for the 50 ohm case. Since the beat note is approximately a sine wave in all cases, the true slope at zero is a bit higher than this technique indicates.
-
The Inductive termination gives the widest linear region. The output is very nearly an ideal triangle wave. It would make the best "wide range" phase detector.
-
The terminations are not precise, but they are identical in all cases. A more purely inductive load could be constructed. The parts are just what I had lying around.
-
No strange bumps or peaks were detected in the beat notes of any of the mixers. Never seen one, regardless of what NIST says they have seen.
-
Eventually I'll go back and check the RPD's with 500 ohms. I stuck with 50 simply to keep everything as "same same" as I could.
-
Sweeping the beat note from 100 Hz to 1 Hz showed no change in the output amplitude.
-
Contrary to my previous post the peak-peak output voltages are within 10% for all terminations. Slope and peak to peak are different things.....
-
All mixers are running into essentially an open circuit load at audio. The scope input is > 1 M ohm and the capacitive reactances are >100 K ohms.
-
No attempt was made to set up directional couplers and figure out what the "real" input to the mixers actually is. Ditto on playing with series resistors to improve the match.
So there it is. Anybody else got some data to compare to.
Bob
On Feb 28, 2010, at 3:53 PM, Bruce Griffiths wrote:
My simulations indicate that terminating the Mixer IF port in an RF short (with both RF and LO ports saturated) increases the beat frequency zero crossing slope by more than a factor of 2 (exact value depends on mixer component characteristics) but doesnt significantly increase the beat frequency amplitude over that with a high value resistive termination. To achieve this the IF port termination impedance needs to be high at the beat frequency and its significant harmonics. The value above which the impedance is considered high depends on mixer details such as transformer turns ratio, RF source impedance, diode characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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Hi
Left out:
All the data was taken with a beat note of roughly 10.2 Hz
Bob
On Feb 28, 2010, at 8:11 PM, Bob Camp wrote:
Hi
Here's some data:
The setup is very simple:
Two oscillators at 10 MHz driving common base / 50 ohm output buffers. The buffers ensure that the source impedance is really 50 ohms. One puts out 9.3 dbm the other 9.5 dbm. They can be tuned for a beat note in the 0 to 100 Hz range.
The basic mixer termination filter is a pair cascaded / identical L networks. Both have 10 uh in the series leg and 0.047 uf to ground in the shunt leg. The "audio end" of the filter hooks straight into a digitizing scope.
The termination options for the mixer are:
- inductive - just running into the 10 uh of the first L network.
- 50 ohms - drop a 57 ohm in series with 0.047 from the mixer output to ground
- Capacitive - 0.047 uf to ground at the mixer output.
The data is computed from the time to cross the center 50% of the output waveform. If the output is 1V p-p then the data would cover the range -.25 to +.25 volts. I've normalized it to "volts / cycle". Divide by 2* pi if you want to get volts / radian.
mixer 50 ohms inductive capacitive
ZAD-3 3.51 2.96 9.98
RPD-1 #1 17.77 10.50 18.85
RPD-1 #2 17.40 10.058 18.53
10514A #1 5.796 4.396 10.31
10514A #2 5.826 4.406 10.33
10534A 5.402 4.078 10.88
ZP3-MH 8.06 5.81 11.28
ZAD-1H 7.73 5.93 9.38
Since not everybody has memorized mixer catalogs:
ZAD-3 typical minicircuits 7 to 10 dbm mixer
RPD-1 500 ohm output phase detector (50 ohms is the "wrong" termination for it)
10514, 10534 HP products from a ways back
ZP3-MH a 13 dbm class mixer, 9 dbm should be under driving it
ZAD-1H a 17 dbm class mixer, should be 8 db under driven.
Bottom line - Capacitive termination helps some parts more than others. The RPD-1 does not get a real big boost, but the ZAD-1 certainly does. There's no real way to know what it's going to do without checking your mixer under your conditions.
A few other notes:
-
The measurement technique slightly under states the slope for the 50 ohm case. Since the beat note is approximately a sine wave in all cases, the true slope at zero is a bit higher than this technique indicates.
-
The Inductive termination gives the widest linear region. The output is very nearly an ideal triangle wave. It would make the best "wide range" phase detector.
-
The terminations are not precise, but they are identical in all cases. A more purely inductive load could be constructed. The parts are just what I had lying around.
-
No strange bumps or peaks were detected in the beat notes of any of the mixers. Never seen one, regardless of what NIST says they have seen.
-
Eventually I'll go back and check the RPD's with 500 ohms. I stuck with 50 simply to keep everything as "same same" as I could.
-
Sweeping the beat note from 100 Hz to 1 Hz showed no change in the output amplitude.
-
Contrary to my previous post the peak-peak output voltages are within 10% for all terminations. Slope and peak to peak are different things.....
-
All mixers are running into essentially an open circuit load at audio. The scope input is > 1 M ohm and the capacitive reactances are >100 K ohms.
-
No attempt was made to set up directional couplers and figure out what the "real" input to the mixers actually is. Ditto on playing with series resistors to improve the match.
So there it is. Anybody else got some data to compare to.
Bob
On Feb 28, 2010, at 3:53 PM, Bruce Griffiths wrote:
My simulations indicate that terminating the Mixer IF port in an RF short (with both RF and LO ports saturated) increases the beat frequency zero crossing slope by more than a factor of 2 (exact value depends on mixer component characteristics) but doesnt significantly increase the beat frequency amplitude over that with a high value resistive termination. To achieve this the IF port termination impedance needs to be high at the beat frequency and its significant harmonics. The value above which the impedance is considered high depends on mixer details such as transformer turns ratio, RF source impedance, diode characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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The results for the RPD-1's are about what I expected: there's little
difference in slope between either a 50 ohm +47nF termination or a 47nF
termination.
The slopes are about 6.5x greater than the values given by Minicircuits.
(8mv/degree = 2.88V/cycle). I assume that they use 500 ohms connected
directly to ground not via a capacitor.
So there's something in NISTs claims of improved slope at least for the
RPD-1.
I suspect that NISTs original 50 ohm terminations were actually 50 ohms
direct to ground not via a capacitor.
Using a series capacitor increases the termination impedance at the beat
frequency substantially over that when the resistor is connected
directly to ground.
Since its is also claimed by NIST and others that reactive termination
reduces the noise, one also needs to measure the output noise spectral
density for the various IF port terminations.
Bruce
Bob Camp wrote:
Hi
Here's some data:
The setup is very simple:
Two oscillators at 10 MHz driving common base / 50 ohm output buffers. The buffers ensure that the source impedance is really 50 ohms. One puts out 9.3 dbm the other 9.5 dbm. They can be tuned for a beat note in the 0 to 100 Hz range.
The basic mixer termination filter is a pair cascaded / identical L networks. Both have 10 uh in the series leg and 0.047 uf to ground in the shunt leg. The "audio end" of the filter hooks straight into a digitizing scope.
The termination options for the mixer are:
- inductive - just running into the 10 uh of the first L network.
- 50 ohms - drop a 57 ohm in series with 0.047 from the mixer output to ground
- Capacitive - 0.047 uf to ground at the mixer output.
The data is computed from the time to cross the center 50% of the output waveform. If the output is 1V p-p then the data would cover the range -.25 to +.25 volts. I've normalized it to "volts / cycle". Divide by 2* pi if you want to get volts / radian.
mixer 50 ohms inductive capacitive
ZAD-3 3.51 2.96 9.98
RPD-1 #1 17.77 10.50 18.85
RPD-1 #2 17.40 10.058 18.53
10514A #1 5.796 4.396 10.31
10514A #2 5.826 4.406 10.33
10534A 5.402 4.078 10.88
ZP3-MH 8.06 5.81 11.28
ZAD-1H 7.73 5.93 9.38
Since not everybody has memorized mixer catalogs:
ZAD-3 typical minicircuits 7 to 10 dbm mixer
RPD-1 500 ohm output phase detector (50 ohms is the "wrong" termination for it)
10514, 10534 HP products from a ways back
ZP3-MH a 13 dbm class mixer, 9 dbm should be under driving it
ZAD-1H a 17 dbm class mixer, should be 8 db under driven.
Bottom line - Capacitive termination helps some parts more than others. The RPD-1 does not get a real big boost, but the ZAD-1 certainly does. There's no real way to know what it's going to do without checking your mixer under your conditions.
A few other notes:
-
The measurement technique slightly under states the slope for the 50 ohm case. Since the beat note is approximately a sine wave in all cases, the true slope at zero is a bit higher than this technique indicates.
-
The Inductive termination gives the widest linear region. The output is very nearly an ideal triangle wave. It would make the best "wide range" phase detector.
-
The terminations are not precise, but they are identical in all cases. A more purely inductive load could be constructed. The parts are just what I had lying around.
-
No strange bumps or peaks were detected in the beat notes of any of the mixers. Never seen one, regardless of what NIST says they have seen.
-
Eventually I'll go back and check the RPD's with 500 ohms. I stuck with 50 simply to keep everything as "same same" as I could.
-
Sweeping the beat note from 100 Hz to 1 Hz showed no change in the output amplitude.
-
Contrary to my previous post the peak-peak output voltages are within 10% for all terminations. Slope and peak to peak are different things.....
-
All mixers are running into essentially an open circuit load at audio. The scope input is> 1 M ohm and the capacitive reactances are>100 K ohms.
-
No attempt was made to set up directional couplers and figure out what the "real" input to the mixers actually is. Ditto on playing with series resistors to improve the match.
So there it is. Anybody else got some data to compare to.
Bob
On Feb 28, 2010, at 3:53 PM, Bruce Griffiths wrote:
My simulations indicate that terminating the Mixer IF port in an RF short (with both RF and LO ports saturated) increases the beat frequency zero crossing slope by more than a factor of 2 (exact value depends on mixer component characteristics) but doesnt significantly increase the beat frequency amplitude over that with a high value resistive termination. To achieve this the IF port termination impedance needs to be high at the beat frequency and its significant harmonics. The value above which the impedance is considered high depends on mixer details such as transformer turns ratio, RF source impedance, diode characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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Hi
The Minicircuits guys claim 800 to 1000 mv / radian. In my units that would be 5 to 6.2 volts per cycle. I believe I'm getting ~ 3 X that mostly from the open circuit termination at audio. It's certainly something I could head back downstairs and check.
The < 10% increase in slope between resistive and capacitive termination has never really been enough with the RPD-1 to make it seem to be worth it. It's certainly worth it with a ZAD-3.
Bob
On Feb 28, 2010, at g8:39 PM, Bruce Griffiths wrote:
The results for the RPD-1's are about what I expected: there's little difference in slope between either a 50 ohm +47nF termination or a 47nF termination.
The slopes are about 6.5x greater than the values given by Minicircuits. (8mv/degree = 2.88V/cycle). I assume that they use 500 ohms connected directly to ground not via a capacitor.
So there's something in NISTs claims of improved slope at least for the RPD-1.
I suspect that NISTs original 50 ohm terminations were actually 50 ohms direct to ground not via a capacitor.
Using a series capacitor increases the termination impedance at the beat frequency substantially over that when the resistor is connected directly to ground.
Since its is also claimed by NIST and others that reactive termination reduces the noise, one also needs to measure the output noise spectral density for the various IF port terminations.
Bruce
Bob Camp wrote:
Hi
Here's some data:
The setup is very simple:
Two oscillators at 10 MHz driving common base / 50 ohm output buffers. The buffers ensure that the source impedance is really 50 ohms. One puts out 9.3 dbm the other 9.5 dbm. They can be tuned for a beat note in the 0 to 100 Hz range.
The basic mixer termination filter is a pair cascaded / identical L networks. Both have 10 uh in the series leg and 0.047 uf to ground in the shunt leg. The "audio end" of the filter hooks straight into a digitizing scope.
The termination options for the mixer are:
- inductive - just running into the 10 uh of the first L network.
- 50 ohms - drop a 57 ohm in series with 0.047 from the mixer output to ground
- Capacitive - 0.047 uf to ground at the mixer output.
The data is computed from the time to cross the center 50% of the output waveform. If the output is 1V p-p then the data would cover the range -.25 to +.25 volts. I've normalized it to "volts / cycle". Divide by 2* pi if you want to get volts / radian.
mixer 50 ohms inductive capacitive
ZAD-3 3.51 2.96 9.98
RPD-1 #1 17.77 10.50 18.85
RPD-1 #2 17.40 10.058 18.53
10514A #1 5.796 4.396 10.31
10514A #2 5.826 4.406 10.33
10534A 5.402 4.078 10.88
ZP3-MH 8.06 5.81 11.28
ZAD-1H 7.73 5.93 9.38
Since not everybody has memorized mixer catalogs:
ZAD-3 typical minicircuits 7 to 10 dbm mixer
RPD-1 500 ohm output phase detector (50 ohms is the "wrong" termination for it)
10514, 10534 HP products from a ways back
ZP3-MH a 13 dbm class mixer, 9 dbm should be under driving it
ZAD-1H a 17 dbm class mixer, should be 8 db under driven.
Bottom line - Capacitive termination helps some parts more than others. The RPD-1 does not get a real big boost, but the ZAD-1 certainly does. There's no real way to know what it's going to do without checking your mixer under your conditions.
A few other notes:
-
The measurement technique slightly under states the slope for the 50 ohm case. Since the beat note is approximately a sine wave in all cases, the true slope at zero is a bit higher than this technique indicates.
-
The Inductive termination gives the widest linear region. The output is very nearly an ideal triangle wave. It would make the best "wide range" phase detector.
-
The terminations are not precise, but they are identical in all cases. A more purely inductive load could be constructed. The parts are just what I had lying around.
-
No strange bumps or peaks were detected in the beat notes of any of the mixers. Never seen one, regardless of what NIST says they have seen.
-
Eventually I'll go back and check the RPD's with 500 ohms. I stuck with 50 simply to keep everything as "same same" as I could.
-
Sweeping the beat note from 100 Hz to 1 Hz showed no change in the output amplitude.
-
Contrary to my previous post the peak-peak output voltages are within 10% for all terminations. Slope and peak to peak are different things.....
-
All mixers are running into essentially an open circuit load at audio. The scope input is> 1 M ohm and the capacitive reactances are>100 K ohms.
-
No attempt was made to set up directional couplers and figure out what the "real" input to the mixers actually is. Ditto on playing with series resistors to improve the match.
So there it is. Anybody else got some data to compare to.
Bob
On Feb 28, 2010, at 3:53 PM, Bruce Griffiths wrote:
My simulations indicate that terminating the Mixer IF port in an RF short (with both RF and LO ports saturated) increases the beat frequency zero crossing slope by more than a factor of 2 (exact value depends on mixer component characteristics) but doesnt significantly increase the beat frequency amplitude over that with a high value resistive termination. To achieve this the IF port termination impedance needs to be high at the beat frequency and its significant harmonics. The value above which the impedance is considered high depends on mixer details such as transformer turns ratio, RF source impedance, diode characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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Hi
Ok, RPD-1 #1 puts out 9.97 volts into a 500 ohm resistor to ground termination (no blocking capacitor). That's still well above the catalog spec. I'm running 25% more voltage than their 7 dbm. That still does not fully explain what I'm seeing.
The scope does indeed indicate 15 volts when I hook it to a 15 volt supply. Given the number of broken pieces of test gear I seem to own that was worth checking. ...
Bob
On Feb 28, 2010, at 8:54 PM, Bob Camp wrote:
Hi
The Minicircuits guys claim 800 to 1000 mv / radian. In my units that would be 5 to 6.2 volts per cycle. I believe I'm getting ~ 3 X that mostly from the open circuit termination at audio. It's certainly something I could head back downstairs and check.
The < 10% increase in slope between resistive and capacitive termination has never really been enough with the RPD-1 to make it seem to be worth it. It's certainly worth it with a ZAD-3.
Bob
On Feb 28, 2010, at g8:39 PM, Bruce Griffiths wrote:
The results for the RPD-1's are about what I expected: there's little difference in slope between either a 50 ohm +47nF termination or a 47nF termination.
The slopes are about 6.5x greater than the values given by Minicircuits. (8mv/degree = 2.88V/cycle). I assume that they use 500 ohms connected directly to ground not via a capacitor.
So there's something in NISTs claims of improved slope at least for the RPD-1.
I suspect that NISTs original 50 ohm terminations were actually 50 ohms direct to ground not via a capacitor.
Using a series capacitor increases the termination impedance at the beat frequency substantially over that when the resistor is connected directly to ground.
Since its is also claimed by NIST and others that reactive termination reduces the noise, one also needs to measure the output noise spectral density for the various IF port terminations.
Bruce
Bob Camp wrote:
Hi
Here's some data:
The setup is very simple:
Two oscillators at 10 MHz driving common base / 50 ohm output buffers. The buffers ensure that the source impedance is really 50 ohms. One puts out 9.3 dbm the other 9.5 dbm. They can be tuned for a beat note in the 0 to 100 Hz range.
The basic mixer termination filter is a pair cascaded / identical L networks. Both have 10 uh in the series leg and 0.047 uf to ground in the shunt leg. The "audio end" of the filter hooks straight into a digitizing scope.
The termination options for the mixer are:
- inductive - just running into the 10 uh of the first L network.
- 50 ohms - drop a 57 ohm in series with 0.047 from the mixer output to ground
- Capacitive - 0.047 uf to ground at the mixer output.
The data is computed from the time to cross the center 50% of the output waveform. If the output is 1V p-p then the data would cover the range -.25 to +.25 volts. I've normalized it to "volts / cycle". Divide by 2* pi if you want to get volts / radian.
mixer 50 ohms inductive capacitive
ZAD-3 3.51 2.96 9.98
RPD-1 #1 17.77 10.50 18.85
RPD-1 #2 17.40 10.058 18.53
10514A #1 5.796 4.396 10.31
10514A #2 5.826 4.406 10.33
10534A 5.402 4.078 10.88
ZP3-MH 8.06 5.81 11.28
ZAD-1H 7.73 5.93 9.38
Since not everybody has memorized mixer catalogs:
ZAD-3 typical minicircuits 7 to 10 dbm mixer
RPD-1 500 ohm output phase detector (50 ohms is the "wrong" termination for it)
10514, 10534 HP products from a ways back
ZP3-MH a 13 dbm class mixer, 9 dbm should be under driving it
ZAD-1H a 17 dbm class mixer, should be 8 db under driven.
Bottom line - Capacitive termination helps some parts more than others. The RPD-1 does not get a real big boost, but the ZAD-1 certainly does. There's no real way to know what it's going to do without checking your mixer under your conditions.
A few other notes:
-
The measurement technique slightly under states the slope for the 50 ohm case. Since the beat note is approximately a sine wave in all cases, the true slope at zero is a bit higher than this technique indicates.
-
The Inductive termination gives the widest linear region. The output is very nearly an ideal triangle wave. It would make the best "wide range" phase detector.
-
The terminations are not precise, but they are identical in all cases. A more purely inductive load could be constructed. The parts are just what I had lying around.
-
No strange bumps or peaks were detected in the beat notes of any of the mixers. Never seen one, regardless of what NIST says they have seen.
-
Eventually I'll go back and check the RPD's with 500 ohms. I stuck with 50 simply to keep everything as "same same" as I could.
-
Sweeping the beat note from 100 Hz to 1 Hz showed no change in the output amplitude.
-
Contrary to my previous post the peak-peak output voltages are within 10% for all terminations. Slope and peak to peak are different things.....
-
All mixers are running into essentially an open circuit load at audio. The scope input is> 1 M ohm and the capacitive reactances are>100 K ohms.
-
No attempt was made to set up directional couplers and figure out what the "real" input to the mixers actually is. Ditto on playing with series resistors to improve the match.
So there it is. Anybody else got some data to compare to.
Bob
On Feb 28, 2010, at 3:53 PM, Bruce Griffiths wrote:
My simulations indicate that terminating the Mixer IF port in an RF short (with both RF and LO ports saturated) increases the beat frequency zero crossing slope by more than a factor of 2 (exact value depends on mixer component characteristics) but doesnt significantly increase the beat frequency amplitude over that with a high value resistive termination. To achieve this the IF port termination impedance needs to be high at the beat frequency and its significant harmonics. The value above which the impedance is considered high depends on mixer details such as transformer turns ratio, RF source impedance, diode characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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The potentially ~ 3dB lower noise with capacitive termination may be
worthwhile.
As usual the NIST papers are somewhat ambiguous as to what they mean by
a 50 ohm termination.
The meaning probably varies from one paper to the next.
A Minicircuits application note on their phase detectors shows 500 ohm
to ground at the IF output followed by an RC low pass filter (5k + 10nF?).
The RPD-1 datasheet definitely says 8mV/degree.
The 800mV to 1V is the maximum output.
Bruce
Bob Camp wrote:
Hi
The Minicircuits guys claim 800 to 1000 mv / radian. In my units that would be 5 to 6.2 volts per cycle. I believe I'm getting ~ 3 X that mostly from the open circuit termination at audio. It's certainly something I could head back downstairs and check.
The< 10% increase in slope between resistive and capacitive termination has never really been enough with the RPD-1 to make it seem to be worth it. It's certainly worth it with a ZAD-3.
Bob
On Feb 28, 2010, at g8:39 PM, Bruce Griffiths wrote:
The results for the RPD-1's are about what I expected: there's little difference in slope between either a 50 ohm +47nF termination or a 47nF termination.
The slopes are about 6.5x greater than the values given by Minicircuits. (8mv/degree = 2.88V/cycle). I assume that they use 500 ohms connected directly to ground not via a capacitor.
So there's something in NISTs claims of improved slope at least for the RPD-1.
I suspect that NISTs original 50 ohm terminations were actually 50 ohms direct to ground not via a capacitor.
Using a series capacitor increases the termination impedance at the beat frequency substantially over that when the resistor is connected directly to ground.
Since its is also claimed by NIST and others that reactive termination reduces the noise, one also needs to measure the output noise spectral density for the various IF port terminations.
Bruce
Bob Camp wrote:
Hi
Here's some data:
The setup is very simple:
Two oscillators at 10 MHz driving common base / 50 ohm output buffers. The buffers ensure that the source impedance is really 50 ohms. One puts out 9.3 dbm the other 9.5 dbm. They can be tuned for a beat note in the 0 to 100 Hz range.
The basic mixer termination filter is a pair cascaded / identical L networks. Both have 10 uh in the series leg and 0.047 uf to ground in the shunt leg. The "audio end" of the filter hooks straight into a digitizing scope.
The termination options for the mixer are:
- inductive - just running into the 10 uh of the first L network.
- 50 ohms - drop a 57 ohm in series with 0.047 from the mixer output to ground
- Capacitive - 0.047 uf to ground at the mixer output.
The data is computed from the time to cross the center 50% of the output waveform. If the output is 1V p-p then the data would cover the range -.25 to +.25 volts. I've normalized it to "volts / cycle". Divide by 2* pi if you want to get volts / radian.
mixer 50 ohms inductive capacitive
ZAD-3 3.51 2.96 9.98
RPD-1 #1 17.77 10.50 18.85
RPD-1 #2 17.40 10.058 18.53
10514A #1 5.796 4.396 10.31
10514A #2 5.826 4.406 10.33
10534A 5.402 4.078 10.88
ZP3-MH 8.06 5.81 11.28
ZAD-1H 7.73 5.93 9.38
Since not everybody has memorized mixer catalogs:
ZAD-3 typical minicircuits 7 to 10 dbm mixer
RPD-1 500 ohm output phase detector (50 ohms is the "wrong" termination for it)
10514, 10534 HP products from a ways back
ZP3-MH a 13 dbm class mixer, 9 dbm should be under driving it
ZAD-1H a 17 dbm class mixer, should be 8 db under driven.
Bottom line - Capacitive termination helps some parts more than others. The RPD-1 does not get a real big boost, but the ZAD-1 certainly does. There's no real way to know what it's going to do without checking your mixer under your conditions.
A few other notes:
-
The measurement technique slightly under states the slope for the 50 ohm case. Since the beat note is approximately a sine wave in all cases, the true slope at zero is a bit higher than this technique indicates.
-
The Inductive termination gives the widest linear region. The output is very nearly an ideal triangle wave. It would make the best "wide range" phase detector.
-
The terminations are not precise, but they are identical in all cases. A more purely inductive load could be constructed. The parts are just what I had lying around.
-
No strange bumps or peaks were detected in the beat notes of any of the mixers. Never seen one, regardless of what NIST says they have seen.
-
Eventually I'll go back and check the RPD's with 500 ohms. I stuck with 50 simply to keep everything as "same same" as I could.
-
Sweeping the beat note from 100 Hz to 1 Hz showed no change in the output amplitude.
-
Contrary to my previous post the peak-peak output voltages are within 10% for all terminations. Slope and peak to peak are different things.....
-
All mixers are running into essentially an open circuit load at audio. The scope input is> 1 M ohm and the capacitive reactances are>100 K ohms.
-
No attempt was made to set up directional couplers and figure out what the "real" input to the mixers actually is. Ditto on playing with series resistors to improve the match.
So there it is. Anybody else got some data to compare to.
Bob
On Feb 28, 2010, at 3:53 PM, Bruce Griffiths wrote:
My simulations indicate that terminating the Mixer IF port in an RF short (with both RF and LO ports saturated) increases the beat frequency zero crossing slope by more than a factor of 2 (exact value depends on mixer component characteristics) but doesnt significantly increase the beat frequency amplitude over that with a high value resistive termination. To achieve this the IF port termination impedance needs to be high at the beat frequency and its significant harmonics. The value above which the impedance is considered high depends on mixer details such as transformer turns ratio, RF source impedance, diode characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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Unless Minicircuits have made significant changes to the RPD-1 there has
to be some kind of calibration error or an as yet poorly understood effect.
Did you try the load and filter shown in the attached application note?
Replicating Minicircuits measurements within 10% or so is probably
necessary to correctly assess the effect of various termination networks.
Bruce
Bob Camp wrote:
Hi
Ok, RPD-1 #1 puts out 9.97 volts into a 500 ohm resistor to ground termination (no blocking capacitor). That's still well above the catalog spec. I'm running 25% more voltage than their 7 dbm. That still does not fully explain what I'm seeing.
The scope does indeed indicate 15 volts when I hook it to a 15 volt supply. Given the number of broken pieces of test gear I seem to own that was worth checking. ...
Bob
On Feb 28, 2010, at 8:54 PM, Bob Camp wrote:
Hi
The Minicircuits guys claim 800 to 1000 mv / radian. In my units that would be 5 to 6.2 volts per cycle. I believe I'm getting ~ 3 X that mostly from the open circuit termination at audio. It's certainly something I could head back downstairs and check.
The< 10% increase in slope between resistive and capacitive termination has never really been enough with the RPD-1 to make it seem to be worth it. It's certainly worth it with a ZAD-3.
Bob
On Feb 28, 2010, at g8:39 PM, Bruce Griffiths wrote:
The results for the RPD-1's are about what I expected: there's little difference in slope between either a 50 ohm +47nF termination or a 47nF termination.
The slopes are about 6.5x greater than the values given by Minicircuits. (8mv/degree = 2.88V/cycle). I assume that they use 500 ohms connected directly to ground not via a capacitor.
So there's something in NISTs claims of improved slope at least for the RPD-1.
I suspect that NISTs original 50 ohm terminations were actually 50 ohms direct to ground not via a capacitor.
Using a series capacitor increases the termination impedance at the beat frequency substantially over that when the resistor is connected directly to ground.
Since its is also claimed by NIST and others that reactive termination reduces the noise, one also needs to measure the output noise spectral density for the various IF port terminations.
Bruce
Bob Camp wrote:
Hi
Here's some data:
The setup is very simple:
Two oscillators at 10 MHz driving common base / 50 ohm output buffers. The buffers ensure that the source impedance is really 50 ohms. One puts out 9.3 dbm the other 9.5 dbm. They can be tuned for a beat note in the 0 to 100 Hz range.
The basic mixer termination filter is a pair cascaded / identical L networks. Both have 10 uh in the series leg and 0.047 uf to ground in the shunt leg. The "audio end" of the filter hooks straight into a digitizing scope.
The termination options for the mixer are:
- inductive - just running into the 10 uh of the first L network.
- 50 ohms - drop a 57 ohm in series with 0.047 from the mixer output to ground
- Capacitive - 0.047 uf to ground at the mixer output.
The data is computed from the time to cross the center 50% of the output waveform. If the output is 1V p-p then the data would cover the range -.25 to +.25 volts. I've normalized it to "volts / cycle". Divide by 2* pi if you want to get volts / radian.
mixer 50 ohms inductive capacitive
ZAD-3 3.51 2.96 9.98
RPD-1 #1 17.77 10.50 18.85
RPD-1 #2 17.40 10.058 18.53
10514A #1 5.796 4.396 10.31
10514A #2 5.826 4.406 10.33
10534A 5.402 4.078 10.88
ZP3-MH 8.06 5.81 11.28
ZAD-1H 7.73 5.93 9.38
Since not everybody has memorized mixer catalogs:
ZAD-3 typical minicircuits 7 to 10 dbm mixer
RPD-1 500 ohm output phase detector (50 ohms is the "wrong" termination for it)
10514, 10534 HP products from a ways back
ZP3-MH a 13 dbm class mixer, 9 dbm should be under driving it
ZAD-1H a 17 dbm class mixer, should be 8 db under driven.
Bottom line - Capacitive termination helps some parts more than others. The RPD-1 does not get a real big boost, but the ZAD-1 certainly does. There's no real way to know what it's going to do without checking your mixer under your conditions.
A few other notes:
-
The measurement technique slightly under states the slope for the 50 ohm case. Since the beat note is approximately a sine wave in all cases, the true slope at zero is a bit higher than this technique indicates.
-
The Inductive termination gives the widest linear region. The output is very nearly an ideal triangle wave. It would make the best "wide range" phase detector.
-
The terminations are not precise, but they are identical in all cases. A more purely inductive load could be constructed. The parts are just what I had lying around.
-
No strange bumps or peaks were detected in the beat notes of any of the mixers. Never seen one, regardless of what NIST says they have seen.
-
Eventually I'll go back and check the RPD's with 500 ohms. I stuck with 50 simply to keep everything as "same same" as I could.
-
Sweeping the beat note from 100 Hz to 1 Hz showed no change in the output amplitude.
-
Contrary to my previous post the peak-peak output voltages are within 10% for all terminations. Slope and peak to peak are different things.....
-
All mixers are running into essentially an open circuit load at audio. The scope input is> 1 M ohm and the capacitive reactances are>100 K ohms.
-
No attempt was made to set up directional couplers and figure out what the "real" input to the mixers actually is. Ditto on playing with series resistors to improve the match.
So there it is. Anybody else got some data to compare to.
Bob
On Feb 28, 2010, at 3:53 PM, Bruce Griffiths wrote:
My simulations indicate that terminating the Mixer IF port in an RF short (with both RF and LO ports saturated) increases the beat frequency zero crossing slope by more than a factor of 2 (exact value depends on mixer component characteristics) but doesnt significantly increase the beat frequency amplitude over that with a high value resistive termination. To achieve this the IF port termination impedance needs to be high at the beat frequency and its significant harmonics. The value above which the impedance is considered high depends on mixer details such as transformer turns ratio, RF source impedance, diode characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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Hi
All of the mixers I'm using are safe from any issues related to "recent changes" in the product. For the most part they are old enough to vote.
Here's some more data from the original run:
Peak to peak output voltage into a cap load (other loads are close):
ZAD-3 .648
RPD-1 #1 2.48
RPD-1 #2 2.4
10514 #1 .971
10514 #2 .989
10534 .914
ZP3-MH 1.378
ZAD-1H 1.296
All are "open circuit" at audio.
The data for the ZAD-3 is .644 V into a 50 ohm with blocking capacitor load. That's pretty close to what they show on page 4 of the app note. It would be lower with a 50 ohm full load, but they imply they are using 500 ohms. I believe the ZAD-3 is similar to the SAM-1 except for frequency range. At the "eyeball level" the waveform they show is the waveform I'm seeing with resistive termination. Their graph indicates an slightly asymetric response (check at 0 degrees) that I'm not seeing.
The setup and circuit are deliberately pretty simple. Not a lot of stuff to go crazy.
Any other data out there?
Bob
On Feb 28, 2010, at 10:58 PM, Bruce Griffiths wrote:
Unless Minicircuits have made significant changes to the RPD-1 there has to be some kind of calibration error or an as yet poorly understood effect.
Did you try the load and filter shown in the attached application note?
Replicating Minicircuits measurements within 10% or so is probably necessary to correctly assess the effect of various termination networks.
Bruce
Bob Camp wrote:
Hi
Ok, RPD-1 #1 puts out 9.97 volts into a 500 ohm resistor to ground termination (no blocking capacitor). That's still well above the catalog spec. I'm running 25% more voltage than their 7 dbm. That still does not fully explain what I'm seeing.
The scope does indeed indicate 15 volts when I hook it to a 15 volt supply. Given the number of broken pieces of test gear I seem to own that was worth checking. ...
Bob
On Feb 28, 2010, at 8:54 PM, Bob Camp wrote:
Hi
The Minicircuits guys claim 800 to 1000 mv / radian. In my units that would be 5 to 6.2 volts per cycle. I believe I'm getting ~ 3 X that mostly from the open circuit termination at audio. It's certainly something I could head back downstairs and check.
The< 10% increase in slope between resistive and capacitive termination has never really been enough with the RPD-1 to make it seem to be worth it. It's certainly worth it with a ZAD-3.
Bob
On Feb 28, 2010, at g8:39 PM, Bruce Griffiths wrote:
The results for the RPD-1's are about what I expected: there's little difference in slope between either a 50 ohm +47nF termination or a 47nF termination.
The slopes are about 6.5x greater than the values given by Minicircuits. (8mv/degree = 2.88V/cycle). I assume that they use 500 ohms connected directly to ground not via a capacitor.
So there's something in NISTs claims of improved slope at least for the RPD-1.
I suspect that NISTs original 50 ohm terminations were actually 50 ohms direct to ground not via a capacitor.
Using a series capacitor increases the termination impedance at the beat frequency substantially over that when the resistor is connected directly to ground.
Since its is also claimed by NIST and others that reactive termination reduces the noise, one also needs to measure the output noise spectral density for the various IF port terminations.
Bruce
Bob Camp wrote:
Hi
Here's some data:
The setup is very simple:
Two oscillators at 10 MHz driving common base / 50 ohm output buffers. The buffers ensure that the source impedance is really 50 ohms. One puts out 9.3 dbm the other 9.5 dbm. They can be tuned for a beat note in the 0 to 100 Hz range.
The basic mixer termination filter is a pair cascaded / identical L networks. Both have 10 uh in the series leg and 0.047 uf to ground in the shunt leg. The "audio end" of the filter hooks straight into a digitizing scope.
The termination options for the mixer are:
- inductive - just running into the 10 uh of the first L network.
- 50 ohms - drop a 57 ohm in series with 0.047 from the mixer output to ground
- Capacitive - 0.047 uf to ground at the mixer output.
The data is computed from the time to cross the center 50% of the output waveform. If the output is 1V p-p then the data would cover the range -.25 to +.25 volts. I've normalized it to "volts / cycle". Divide by 2* pi if you want to get volts / radian.
mixer 50 ohms inductive capacitive
ZAD-3 3.51 2.96 9.98
RPD-1 #1 17.77 10.50 18.85
RPD-1 #2 17.40 10.058 18.53
10514A #1 5.796 4.396 10.31
10514A #2 5.826 4.406 10.33
10534A 5.402 4.078 10.88
ZP3-MH 8.06 5.81 11.28
ZAD-1H 7.73 5.93 9.38
Since not everybody has memorized mixer catalogs:
ZAD-3 typical minicircuits 7 to 10 dbm mixer
RPD-1 500 ohm output phase detector (50 ohms is the "wrong" termination for it)
10514, 10534 HP products from a ways back
ZP3-MH a 13 dbm class mixer, 9 dbm should be under driving it
ZAD-1H a 17 dbm class mixer, should be 8 db under driven.
Bottom line - Capacitive termination helps some parts more than others. The RPD-1 does not get a real big boost, but the ZAD-1 certainly does. There's no real way to know what it's going to do without checking your mixer under your conditions.
A few other notes:
-
The measurement technique slightly under states the slope for the 50 ohm case. Since the beat note is approximately a sine wave in all cases, the true slope at zero is a bit higher than this technique indicates.
-
The Inductive termination gives the widest linear region. The output is very nearly an ideal triangle wave. It would make the best "wide range" phase detector.
-
The terminations are not precise, but they are identical in all cases. A more purely inductive load could be constructed. The parts are just what I had lying around.
-
No strange bumps or peaks were detected in the beat notes of any of the mixers. Never seen one, regardless of what NIST says they have seen.
-
Eventually I'll go back and check the RPD's with 500 ohms. I stuck with 50 simply to keep everything as "same same" as I could.
-
Sweeping the beat note from 100 Hz to 1 Hz showed no change in the output amplitude.
-
Contrary to my previous post the peak-peak output voltages are within 10% for all terminations. Slope and peak to peak are different things.....
-
All mixers are running into essentially an open circuit load at audio. The scope input is> 1 M ohm and the capacitive reactances are>100 K ohms.
-
No attempt was made to set up directional couplers and figure out what the "real" input to the mixers actually is. Ditto on playing with series resistors to improve the match.
So there it is. Anybody else got some data to compare to.
Bob
On Feb 28, 2010, at 3:53 PM, Bruce Griffiths wrote:
My simulations indicate that terminating the Mixer IF port in an RF short (with both RF and LO ports saturated) increases the beat frequency zero crossing slope by more than a factor of 2 (exact value depends on mixer component characteristics) but doesnt significantly increase the beat frequency amplitude over that with a high value resistive termination. To achieve this the IF port termination impedance needs to be high at the beat frequency and its significant harmonics. The value above which the impedance is considered high depends on mixer details such as transformer turns ratio, RF source impedance, diode characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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I got to correspond with a person at NIST today about DMTD - about the
HP 10514B Mixers. They pointed me to a application note at
http://www.thegleam.com/ke5fx/stellex/5952-8217.pdf (hey - thats one of
our time-nuts!) that shows driving the 10514 mixer at +13 dbm for the LO
port and 0 dbm for the RF port.
This application note is a good read. HP recommends terminating the
mixer into a 3900 pF cap and 50 ohms in series for the RF termination.
ANd they use a LC filter for their VNA set for 90 Khz. I also looked
at the schematic of the 8407A were I got the mixers from. They
terminate the IF into a 1100 pf cap in series with a 51 ohm resistor,
and then it passes thru a 2.7 uH choke going into an amplifier - the IF
I beleive is 298 Khz for this unit.
I also did a reconfiguration of my temporary DMTD experiment that has
also improved the noise floor. Drawing attached. Also changed the
drive levels.
For reference the previous configuration showed 2x10-11 for 0.1 sec;
2x10-12 for 1 sec, 5x10-13 for 10 sec, 6x10-14 for 100 sec, 7x10-15 for
1000 sec. for the noise floor.
The current configuration (of the drawing attached) shows 8x10-12 for
0.1 sec; 8x10-13 for 1 sec, 8x10-14 for 10 sec, 8x10-15 for 100 sec and
8x10-16 for 1000 sec. I used a little more attention in sheilding and
spacing to eliminate crosstalk. Note the noise shows some linearity
this time....plus more drive....
Brian - KD4FM
Brian Kirby wrote:
I got to correspond with a person at NIST today about DMTD - about the
HP 10514B Mixers. They pointed me to a application note at
http://www.thegleam.com/ke5fx/stellex/5952-8217.pdf (hey - thats one
of our time-nuts!) that shows driving the 10514 mixer at +13 dbm for
the LO port and 0 dbm for the RF port.
This application note is a good read. HP recommends terminating the
mixer into a 3900 pF cap and 50 ohms in series for the RF
termination. ANd they use a LC filter for their VNA set for 90 Khz.
I also looked at the schematic of the 8407A were I got the mixers
from. They terminate the IF into a 1100 pf cap in series with a 51
ohm resistor, and then it passes thru a 2.7 uH choke going into an
amplifier - the IF I beleive is 298 Khz for this unit.
I also did a reconfiguration of my temporary DMTD experiment that has
also improved the noise floor. Drawing attached. Also changed the
drive levels.
For reference the previous configuration showed 2x10-11 for 0.1 sec;
2x10-12 for 1 sec, 5x10-13 for 10 sec, 6x10-14 for 100 sec, 7x10-15
for 1000 sec. for the noise floor.
The current configuration (of the drawing attached) shows 8x10-12 for
0.1 sec; 8x10-13 for 1 sec, 8x10-14 for 10 sec, 8x10-15 for 100 sec
and 8x10-16 for 1000 sec. I used a little more attention in sheilding
and spacing to eliminate crosstalk. Note the noise shows some
linearity this time....plus more drive....
Brian - KD4FM
NP0/C0G caps are a good idea as they arent voltage dependent and have
lower phase noise than other types.
If you use a choke its a good idea to select one whose first self
resonance lies somewhat above the sum frequency.
Suitable chokes are a little scarce if one uses the 100uH, 220uH or
330uH chokes that NIST have often used.
EPCOS has a range of suitable chokes which consist of a single layer
winding on a ferrite rod core.
These are available from Farnell or their US distributor (Newark, IIRC).
Bruce
Brian Kirby wrote:
I got to correspond with a person at NIST today about DMTD - about the
HP 10514B Mixers. They pointed me to a application note at
http://www.thegleam.com/ke5fx/stellex/5952-8217.pdf (hey - thats one
of our time-nuts!) that shows driving the 10514 mixer at +13 dbm for
the LO port and 0 dbm for the RF port.
This application note is a good read. HP recommends terminating the
mixer into a 3900 pF cap and 50 ohms in series for the RF
termination. ANd they use a LC filter for their VNA set for 90 Khz.
I also looked at the schematic of the 8407A were I got the mixers
from. They terminate the IF into a 1100 pf cap in series with a 51
ohm resistor, and then it passes thru a 2.7 uH choke going into an
amplifier - the IF I beleive is 298 Khz for this unit.
I also did a reconfiguration of my temporary DMTD experiment that has
also improved the noise floor. Drawing attached. Also changed the
drive levels.
For reference the previous configuration showed 2x10-11 for 0.1 sec;
2x10-12 for 1 sec, 5x10-13 for 10 sec, 6x10-14 for 100 sec, 7x10-15
for 1000 sec. for the noise floor.
The current configuration (of the drawing attached) shows 8x10-12 for
0.1 sec; 8x10-13 for 1 sec, 8x10-14 for 10 sec, 8x10-15 for 100 sec
and 8x10-16 for 1000 sec. I used a little more attention in sheilding
and spacing to eliminate crosstalk. Note the noise shows some
linearity this time....plus more drive....
Brian - KD4FM
Is that circuit stable?
The LT1037 is only stable for gains of 5 or more.
When either set of diodes is forward biased the gain drops to a little
above unity so does the opamp then oscillate?
Bruce
Hi
These days you can get (for a price) NPO caps up above .1 uf. That makes super high value coils less important in a low frequency beat note system. If you are trying to measure phase noise at 100 KHz then 0.1 uf to ground at multiple places likely is not a real good idea.
Bob
On Mar 2, 2010, at 11:28 PM, Brian Kirby wrote:
I got to correspond with a person at NIST today about DMTD - about the HP 10514B Mixers. They pointed me to a application note at http://www.thegleam.com/ke5fx/stellex/5952-8217.pdf (hey - thats one of our time-nuts!) that shows driving the 10514 mixer at +13 dbm for the LO port and 0 dbm for the RF port.
This application note is a good read. HP recommends terminating the mixer into a 3900 pF cap and 50 ohms in series for the RF termination. ANd they use a LC filter for their VNA set for 90 Khz. I also looked at the schematic of the 8407A were I got the mixers from. They terminate the IF into a 1100 pf cap in series with a 51 ohm resistor, and then it passes thru a 2.7 uH choke going into an amplifier - the IF I beleive is 298 Khz for this unit.
I also did a reconfiguration of my temporary DMTD experiment that has also improved the noise floor. Drawing attached. Also changed the drive levels.
For reference the previous configuration showed 2x10-11 for 0.1 sec; 2x10-12 for 1 sec, 5x10-13 for 10 sec, 6x10-14 for 100 sec, 7x10-15 for 1000 sec. for the noise floor.
The current configuration (of the drawing attached) shows 8x10-12 for 0.1 sec; 8x10-13 for 1 sec, 8x10-14 for 10 sec, 8x10-15 for 100 sec and 8x10-16 for 1000 sec. I used a little more attention in sheilding and spacing to eliminate crosstalk. Note the noise shows some linearity this time....plus more drive....
Brian - KD4FM
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Bruce Griffiths wrote:
Brian Kirby wrote:
I got to correspond with a person at NIST today about DMTD - about
the HP 10514B Mixers. They pointed me to a application note at
http://www.thegleam.com/ke5fx/stellex/5952-8217.pdf (hey - thats one
of our time-nuts!) that shows driving the 10514 mixer at +13 dbm for
the LO port and 0 dbm for the RF port.
This application note is a good read. HP recommends terminating the
mixer into a 3900 pF cap and 50 ohms in series for the RF
termination. ANd they use a LC filter for their VNA set for 90
Khz. I also looked at the schematic of the 8407A were I got the
mixers from. They terminate the IF into a 1100 pf cap in series with
a 51 ohm resistor, and then it passes thru a 2.7 uH choke going into
an amplifier - the IF I beleive is 298 Khz for this unit.
I also did a reconfiguration of my temporary DMTD experiment that has
also improved the noise floor. Drawing attached. Also changed the
drive levels.
For reference the previous configuration showed 2x10-11 for 0.1 sec;
2x10-12 for 1 sec, 5x10-13 for 10 sec, 6x10-14 for 100 sec, 7x10-15
for 1000 sec. for the noise floor.
The current configuration (of the drawing attached) shows 8x10-12 for
0.1 sec; 8x10-13 for 1 sec, 8x10-14 for 10 sec, 8x10-15 for 100 sec
and 8x10-16 for 1000 sec. I used a little more attention in
sheilding and spacing to eliminate crosstalk. Note the noise shows
some linearity this time....plus more drive....
Brian - KD4FM
Is that circuit stable?
The LT1037 is only stable for gains of 5 or more.
When either set of diodes is forward biased the gain drops to a little
above unity so does the opamp then oscillate?
Bruce
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I am not seeing any signs of oscillation. I have went back to the old
circuit, to see if everything duplicates , for repeatability reasons.
SO running some more test to compare to each other.
Hi
There was a factor of two "oops" in the math. All the numbers are 2X reality.
Bob
On Feb 28, 2010, at 10:58 PM, Bruce Griffiths wrote:
Unless Minicircuits have made significant changes to the RPD-1 there has to be some kind of calibration error or an as yet poorly understood effect.
Did you try the load and filter shown in the attached application note?
Replicating Minicircuits measurements within 10% or so is probably necessary to correctly assess the effect of various termination networks.
Bruce
Bob Camp wrote:
Hi
Ok, RPD-1 #1 puts out 9.97 volts into a 500 ohm resistor to ground termination (no blocking capacitor). That's still well above the catalog spec. I'm running 25% more voltage than their 7 dbm. That still does not fully explain what I'm seeing.
The scope does indeed indicate 15 volts when I hook it to a 15 volt supply. Given the number of broken pieces of test gear I seem to own that was worth checking. ...
Bob
On Feb 28, 2010, at 8:54 PM, Bob Camp wrote:
Hi
The Minicircuits guys claim 800 to 1000 mv / radian. In my units that would be 5 to 6.2 volts per cycle. I believe I'm getting ~ 3 X that mostly from the open circuit termination at audio. It's certainly something I could head back downstairs and check.
The< 10% increase in slope between resistive and capacitive termination has never really been enough with the RPD-1 to make it seem to be worth it. It's certainly worth it with a ZAD-3.
Bob
On Feb 28, 2010, at g8:39 PM, Bruce Griffiths wrote:
The results for the RPD-1's are about what I expected: there's little difference in slope between either a 50 ohm +47nF termination or a 47nF termination.
The slopes are about 6.5x greater than the values given by Minicircuits. (8mv/degree = 2.88V/cycle). I assume that they use 500 ohms connected directly to ground not via a capacitor.
So there's something in NISTs claims of improved slope at least for the RPD-1.
I suspect that NISTs original 50 ohm terminations were actually 50 ohms direct to ground not via a capacitor.
Using a series capacitor increases the termination impedance at the beat frequency substantially over that when the resistor is connected directly to ground.
Since its is also claimed by NIST and others that reactive termination reduces the noise, one also needs to measure the output noise spectral density for the various IF port terminations.
Bruce
Bob Camp wrote:
Hi
Here's some data:
The setup is very simple:
Two oscillators at 10 MHz driving common base / 50 ohm output buffers. The buffers ensure that the source impedance is really 50 ohms. One puts out 9.3 dbm the other 9.5 dbm. They can be tuned for a beat note in the 0 to 100 Hz range.
The basic mixer termination filter is a pair cascaded / identical L networks. Both have 10 uh in the series leg and 0.047 uf to ground in the shunt leg. The "audio end" of the filter hooks straight into a digitizing scope.
The termination options for the mixer are:
- inductive - just running into the 10 uh of the first L network.
- 50 ohms - drop a 57 ohm in series with 0.047 from the mixer output to ground
- Capacitive - 0.047 uf to ground at the mixer output.
The data is computed from the time to cross the center 50% of the output waveform. If the output is 1V p-p then the data would cover the range -.25 to +.25 volts. I've normalized it to "volts / cycle". Divide by 2* pi if you want to get volts / radian.
mixer 50 ohms inductive capacitive
ZAD-3 3.51 2.96 9.98
RPD-1 #1 17.77 10.50 18.85
RPD-1 #2 17.40 10.058 18.53
10514A #1 5.796 4.396 10.31
10514A #2 5.826 4.406 10.33
10534A 5.402 4.078 10.88
ZP3-MH 8.06 5.81 11.28
ZAD-1H 7.73 5.93 9.38
Since not everybody has memorized mixer catalogs:
ZAD-3 typical minicircuits 7 to 10 dbm mixer
RPD-1 500 ohm output phase detector (50 ohms is the "wrong" termination for it)
10514, 10534 HP products from a ways back
ZP3-MH a 13 dbm class mixer, 9 dbm should be under driving it
ZAD-1H a 17 dbm class mixer, should be 8 db under driven.
Bottom line - Capacitive termination helps some parts more than others. The RPD-1 does not get a real big boost, but the ZAD-1 certainly does. There's no real way to know what it's going to do without checking your mixer under your conditions.
A few other notes:
-
The measurement technique slightly under states the slope for the 50 ohm case. Since the beat note is approximately a sine wave in all cases, the true slope at zero is a bit higher than this technique indicates.
-
The Inductive termination gives the widest linear region. The output is very nearly an ideal triangle wave. It would make the best "wide range" phase detector.
-
The terminations are not precise, but they are identical in all cases. A more purely inductive load could be constructed. The parts are just what I had lying around.
-
No strange bumps or peaks were detected in the beat notes of any of the mixers. Never seen one, regardless of what NIST says they have seen.
-
Eventually I'll go back and check the RPD's with 500 ohms. I stuck with 50 simply to keep everything as "same same" as I could.
-
Sweeping the beat note from 100 Hz to 1 Hz showed no change in the output amplitude.
-
Contrary to my previous post the peak-peak output voltages are within 10% for all terminations. Slope and peak to peak are different things.....
-
All mixers are running into essentially an open circuit load at audio. The scope input is> 1 M ohm and the capacitive reactances are>100 K ohms.
-
No attempt was made to set up directional couplers and figure out what the "real" input to the mixers actually is. Ditto on playing with series resistors to improve the match.
So there it is. Anybody else got some data to compare to.
Bob
On Feb 28, 2010, at 3:53 PM, Bruce Griffiths wrote:
My simulations indicate that terminating the Mixer IF port in an RF short (with both RF and LO ports saturated) increases the beat frequency zero crossing slope by more than a factor of 2 (exact value depends on mixer component characteristics) but doesnt significantly increase the beat frequency amplitude over that with a high value resistive termination. To achieve this the IF port termination impedance needs to be high at the beat frequency and its significant harmonics. The value above which the impedance is considered high depends on mixer details such as transformer turns ratio, RF source impedance, diode characteristics and RF input levels, etc.
Bruce
Bob Camp wrote:
Hi
Putting The C on the feedback R in a positive gain setup is only going to take the "roll off" gain down to 1. Doing the same with an inverting amp or using a series R / cap to ground will drop the gain a lot more in the roll off region.
I would worry about any resistor that's marked as 10K and reads 20K. It's likely noisy.
A typical DBM has a loss of 5 to 7 db when not in compression. With a +7 to +10 dbm drive that should give you an output of 0 to 2 dbm . The mixer output should be in the .6 to .8 V p-p range into 50 ohms. You should get about twice that on the beat note running into a load> 500 ohms. A gain of 20 should be plenty. That would give you .6 x 2 x 20 = 24 V p-p out of the amp.
If you "rf short" the output of the mixer you may double the beat note again (total of 4X the 50 ohm value). Net would be a 2.4 to 3.2 V p-p beat note. Anything much over a gain of 10 would be a problem then. This is one of the cases where 2 X 2 probably does not = 4, so measurements are indeed in order.
Bob
On Feb 28, 2010, at 1:01 AM, Brian Kirby wrote:
The values in the schematics are wrong for the op amp gain. The drawing was from an earlier drawing where I made a preamp to start checks on the mixers, and I sent it to you (Bruce G). Thats when you determined I did not have enough gain to get near the noise floor. The THAT1512/1646 ICs were ordered to make a new preamp for the future measurements on the mixers.
When I use the scope and check the outputs of the IC, I have 20 volts peak to peak, sine-wave. I know from previous readings I see about 500 mv p-p out of the mixer.
I went down to the bench and the resistors I used were still there (I bought several taped reels of Dale RN55D resistors when a local business went out). I used 294 ohms and 14.9 kilo-ohms, for a gain of 50 (the power rails are +/- 15 volts). Also not shown on the schematic is a 0.47 uF cap around the 14.9 kilo-ohm resistor. I think I was trying to limit the bandwidth to around 15 hertz.
Also the resistor going between the op amp and the limiting diodes was marked 10K, its 20K. The diodes are 1N4148. Corrected drawing attached.
This is what happens to time nuts who can only play on the weekend and stay up all night....and my employer just thinks I party too hard.....for Monday mornings.
Brian KD4FM
Bruce Griffiths wrote:
The LT1037 is shown with a gain of ~1690x, if this amplifier is used to amplify the beat frequency signal, it will saturate.
Opamp recovery from saturation is poorly documented and may be very slow.
It would be better to use some diodes in the amplifier feedback network to limit the large signal gain to 5x (so that the LT1037 remains stable as it isn't unity gain stable).
This will ensure a somewhat faster recovery from overload as the LT1037 then avoids saturation and the opamp input stage remains in the linear region.
Bruce
Bob Camp wrote:
Hi
Assuming that the junction of the back to back diodes goes trough a chunk of coax to get to the counter:
You are forming a low pass filter with the 10K resistor and the coax capacitance. The LT1037 is quite happy driving a 600 ohm load. You could easily drop the impedance at that point below 300 ohms. That should give you a faster edge into the counter.
You also should check the slew rate performance of the 1037. You don't want the op amp to be slew rate limited.
Bob
On Feb 27, 2010, at 12:41 PM, Brian Kirby wrote:
I am in the process of designing a DMTD system. As an experiment to do basic measurements on the chosen mixer, I used a capacitor (0.01 uF) in series to ground with a 47 ohm metal film resistor. Where the capacitor and resistor meets, another resistor is attached (390 ohms) that goes to ground. The idea is to provide a 50 ohm termination at 20 Mhz and a lighter termination at audio frequencies. I seen this is a NBS note and I can say, its a starting point for my experiments.
This (my) system is designed for 10 Mhz, using a 10 hertz beat. A schematic is attached of what I am experimenting with at the moment. A HP5370B is the recording instrument. The noise floor from 1 days observations show 2x10-11 at 0.1 seconds, 2x10-12 at 1 sec, 5x10-13 at 10 sec, 6x10-14 at 100 sec, 7x10-15 at 1000 sec, and 7x10-16 at 10,000 secs. It will be interesting when the project is completed to see how much improvement there will be.
As I understand (or learning..) mixer performance is the key to the DMTD system. It occurs to me that maybe a capacitor designed for 50 ohms at 20 mhz may be a better termination (for the IF port) for this mixer. A 16 pF capacitor is 50 ohms at 20 mhz, and for comparison at 10 hertz, it would be 100 meg-ohms, which would give maximum amplitude at 10 hertz. As I understand, a capacitor terminated mixer will give a triangle wave output, which is very beneficial to the design - as the end result is to get maximum slope out of the mixer. I would say, unqualified as I am, the capacitor termination matches the 20 mhz signal, and helps attenuates the harmonics of the mixer, and has no , or very little effect on the audio frequencies that we are interested in.
And saying/rambling on... that if maximum slope is needed, its needed on the 10 hertz beat signal - so maybe a capacitive termination on the 10 hertz signal only and something resistive on the 20 mhz signal........another idea use the 16 pF direct off the mixer, then a series resistor for isolation and then a large capacitor on the 10 hertz beat for maximum slope.
At the present, I am awaiting parts to build a low noise preamp base on the THAT1512 so I can make better measurements on the mixer. Bruce has provided a lot of good suggestions and helpful comments on my project and Ulrich has provided me quite a bit of user support on his program, Plotter. Thanks to all.
Comments ? Brian KD4FM
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Whilst your corrected measurements for the 10534 and 10514 are now
within about 10% of those found by NIST for one of their mixers, the
discrepancy with Minicircuits estimates for the RPD-1s is somewhat
larger but their phase detector gain specs seem incompatible with their
max output specs.
The noise of their phase detector output preamp is surprisingly high
(maybe this is relatively unimportant as they use the cross power
spectrum technique to measure phase noise?? ), even my crude single
NJFET input stage preamplifier breadboard is about 3dB quieter.
Bruce
Bob Camp wrote:
Hi
There was a factor of two "oops" in the math. All the numbers are 2X reality.
Bob
On Feb 28, 2010, at 10:58 PM, Bruce Griffiths wrote:
Hi
I checked a few newer RPD-1's (actually MPD-1's, but I believe they are the same thing). The two ones I have here from who knows where have about 20% more output (2.4 V p-p) than a typical unit (2.0 V p-p).
Bob
On Mar 3, 2010, at 10:56 PM, Bruce Griffiths wrote:
Whilst your corrected measurements for the 10534 and 10514 are now within about 10% of those found by NIST for one of their mixers, the discrepancy with Minicircuits estimates for the RPD-1s is somewhat larger but their phase detector gain specs seem incompatible with their max output specs.
The noise of their phase detector output preamp is surprisingly high (maybe this is relatively unimportant as they use the cross power spectrum technique to measure phase noise?? ), even my crude single NJFET input stage preamplifier breadboard is about 3dB quieter.
Bruce
Bob Camp wrote:
Hi
There was a factor of two "oops" in the math. All the numbers are 2X reality.
Bob
On Feb 28, 2010, at 10:58 PM, Bruce Griffiths wrote:
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Perhaps they changed the diodes used in MPD/RPD phase detectors over the
years.
Minicircuits also make a ZP10514 which may use monolithic quad diodes.
These may be noisier at 100Hz offset than the genuine HP10514.
Bruce
Bob Camp wrote:
Hi
I checked a few newer RPD-1's (actually MPD-1's, but I believe they are the same thing). The two ones I have here from who knows where have about 20% more output (2.4 V p-p) than a typical unit (2.0 V p-p).
Bob
On Mar 3, 2010, at 10:56 PM, Bruce Griffiths wrote:
Whilst your corrected measurements for the 10534 and 10514 are now within about 10% of those found by NIST for one of their mixers, the discrepancy with Minicircuits estimates for the RPD-1s is somewhat larger but their phase detector gain specs seem incompatible with their max output specs.
The noise of their phase detector output preamp is surprisingly high (maybe this is relatively unimportant as they use the cross power spectrum technique to measure phase noise?? ), even my crude single NJFET input stage preamplifier breadboard is about 3dB quieter.
Bruce
Bob Camp wrote:
Hi
There was a factor of two "oops" in the math. All the numbers are 2X reality.
Bob
On Feb 28, 2010, at 10:58 PM, Bruce Griffiths wrote:
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time-nuts-bounces@febo.com wrote on 03/03/2010 10:56:46 PM:
From:
Bruce Griffiths bruce.griffiths@xtra.co.nz
To:
Discussion of precise time and frequency measurement
Date:
03/03/2010 11:03 PM
Subject:
Re: [time-nuts] DMTD Mixer Terminations
Sent by:
Whilst your corrected measurements for the 10534 and 10514 are now
within about 10% of those found by NIST for one of their mixers, the
discrepancy with Minicircuits estimates for the RPD-1s is somewhat
larger but their phase detector gain specs seem incompatible with their
max output specs.
The diode ring mixers MiniCircuits offers as phase detectors are usually
designed for and characterized at 500 ohms load, not 50 ohms. Does this
resolve the discrepancy?
Joe
The specifications for the maximum output given on the datasheet are so
wide that the 20% higher than typical beat frequency amplitude may well
be within spec.
The RPD-1's were being over driven by about 2.5dBm or so, which would
also tend to increase the output.
A test made with 500 ohms to ground as shown in a Minicircuits
application note had about the same output as when terminating the IF
port in 50 ohms + 47nF.
The remaining questions are:
1)What effect does the IF port termination have on the phase detector
phase noise spectrum?
- What effect does the IF port termination have on the noise to phase
detector gain ratio?
Bruce
Joseph M Gwinn wrote:
time-nuts-bounces@febo.com wrote on 03/03/2010 10:56:46 PM:
From:
Bruce Griffithsbruce.griffiths@xtra.co.nz
To:
Discussion of precise time and frequency measurement
Date:
03/03/2010 11:03 PM
Subject:
Re: [time-nuts] DMTD Mixer Terminations
Sent by:
Whilst your corrected measurements for the 10534 and 10514 are now
within about 10% of those found by NIST for one of their mixers, the
discrepancy with Minicircuits estimates for the RPD-1s is somewhat
larger but their phase detector gain specs seem incompatible with their
max output specs.
The diode ring mixers MiniCircuits offers as phase detectors are usually
designed for and characterized at 500 ohms load, not 50 ohms. Does this
resolve the discrepancy?
Joe
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HI
In other words the hard part of the task still remains to be done.
Bob
On Mar 4, 2010, at 4:45 PM, Bruce Griffiths wrote:
The specifications for the maximum output given on the datasheet are so wide that the 20% higher than typical beat frequency amplitude may well be within spec.
The RPD-1's were being over driven by about 2.5dBm or so, which would also tend to increase the output.
A test made with 500 ohms to ground as shown in a Minicircuits application note had about the same output as when terminating the IF port in 50 ohms + 47nF.
The remaining questions are:
1)What effect does the IF port termination have on the phase detector phase noise spectrum?
- What effect does the IF port termination have on the noise to phase detector gain ratio?
Bruce
Joseph M Gwinn wrote:
time-nuts-bounces@febo.com wrote on 03/03/2010 10:56:46 PM:
From:
Bruce Griffithsbruce.griffiths@xtra.co.nz
To:
Discussion of precise time and frequency measurement
Date:
03/03/2010 11:03 PM
Subject:
Re: [time-nuts] DMTD Mixer Terminations
Sent by:
Whilst your corrected measurements for the 10534 and 10514 are now
within about 10% of those found by NIST for one of their mixers, the
discrepancy with Minicircuits estimates for the RPD-1s is somewhat
larger but their phase detector gain specs seem incompatible with their
max output specs.
The diode ring mixers MiniCircuits offers as phase detectors are usually
designed for and characterized at 500 ohms load, not 50 ohms. Does this
resolve the discrepancy?
Joe
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