DIY Low offset Phase Noise Analyzer (Erik Kaashoek)
To Erik:
Hi. The PNA block diagram I posted earlier had two errors. I
corrected them in PNA V2.0, as shown in PMA2.PNG
The first error was a missing connection to the Phase-Frequency
Detector (PFD). This was a trivial error and was due to issues with
the image viewer dropping lines. Do not mess with the Irfanview
viewer. I switched to the XNview instead, which gave good
performance. So if you find problems with missing lines in the PNG
files, try a different viewer.
The second error was a missing 90 degree coupler to the Double
Balanced Mixer (DBM), which you generate with discrete components.
If you are interested, Minicircuits makes 90 degree hybrids for many
different frequency ranges.
For example, the https://www.minicircuits.com/pdfs/JSPQ-65W+.pdf is
useful over the 5 MHz to 65 MHz range, and appears to be dead on 90
degrees at 10 MHz.
When you get up to 100 MHz, the
https://www.minicircuits.com/pdfs/PSCQ-2-120+.pdf gives about 0.5
degree error at 100 MHz.
The advantage of a 90 degree hybrid is very low insertion loss and a
50 ohm environment. This eliminates the noise generated by resistors
and op amps.
Speaking of op amp noise, the lowest noise op amp I know of is the
LT1028. The datasheet is shown at
https://www.analog.com/media/en/technical-documentation/data-sheets/1028fd.pdf
It has 0.85nV/vHz typ at 1kHz. However, if you look at the Voltage
Noise vs Frequency curve, you find the noise increases drastically
below 10 Hz.
One way to get around this problem is a zero drift chopamp. An
example is the ADA4523, shown at
https://www.analog.com/media/en/technical-documentation/data-sheets/ada4523-1.pdf
As shown in Fig. 2, the noise is below 5nV/(sqrtHz) from 0.02 Hz to
10 KHz. This might be useful in some designs.
Another item that might be of interest is the PFD. The Hittite
HMC984LP4E has -231 dBc/root(Hz) of noise, which is quite low. The
datasheet is at
https://www.analog.com/media/en/technical-documentation/data-sheets/hmc984.pdf
I noticed in your phase noise plots numerous spikes starting at 50
Hz. These clearly show you are living in Europe. What a wonderful
place. I lived in Metz, France for a number of years while I was
with NATO. Thanks for the memories.
However, the noise spikes show another problem. They may be due to
noise on the VCC supply to the oscillators. This can be a serious
problem, as John Ackermann showed in his page at
https://www.febo.com/pages/oscillators/wenzel_uln/supply.html
You can reduce this with a simple ripple filter, shown in
2N3906S.PNG. This reduces the noise by 80 dB at 10 Hz, and 120 dB at
100 Hz, as shown in 2N3906G.PNG.
If you are interested in modifying the design, the schematic entry
for LTspice is shown in 54E696BA.ASC. The corresponding waveform
plot file is 54E696BA.PLT
The advantage of this configuration is easy to filter negative
voltages by switching to a NPN transistor, such as a 2N3904.
The disadvantage is this filter is only capable of supplying limited
current. It cannot supply the current needed by the OCXO heaters, so
if this is a problem, you might consider switching to VCXO
oscillators.
I am searching for ways to increase the maximum current, but it is
very difficult to keep the attenuation numbers. I have found a
MOSFET that gives good attenuation, but it can only handle 1.4 Amps.
It is the Onsemi FDC2512, and the datasheet is at
https://www.onsemi.com/pdf/datasheet/fdc2512-d.pdf
It might be necessary to use two separate filters, but this might be
an advantage by keeping crosstalk low between the oscillators.
If you would like to eliminate the problem of quadrature lock, the
Hittite HMC984LP4E PFD might be of interest. The -231 dBc/Hz of
noise is very low and might be hard to reach with a DBM.
If you are interested in following up on phase-frequency detectors
to eliminate the narrow lock range of double-balanced mixers, I can
supply you with a wealth of information on the design, implementation,
and testing. Just let me know if this would help.
Mike
HI
On Jul 8, 2022, at 7:44 PM, Mike Monett via time-nuts time-nuts@lists.febo.com wrote:
To Erik:
…….
Another item that might be of interest is the PFD. The Hittite
HMC984LP4E has -231 dBc/root(Hz) of noise, which is quite low. The
datasheet is at
https://www.analog.com/media/en/technical-documentation/data-sheets/hmc984.pdf
…...
If you would like to eliminate the problem of quadrature lock, the
Hittite HMC984LP4E PFD might be of interest. The -231 dBc/Hz of
noise is very low and might be hard to reach with a DBM.
If you are interested in following up on phase-frequency detectors
to eliminate the narrow lock range of double-balanced mixers, I can
supply you with a wealth of information on the design, implementation,
and testing. Just let me know if this would help.
Mike
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Ok, since this keeps coming up ….
The chip guys rate their phase detectors in a somewhat unique way. It works
for their product so that’s fine. However you can’t just toss out their number
and quickly compare it to another number from an entirely different approach.
You need to do the math.
The -231 dbc / Hz number quoted above is a “normalized to one hertz carrier”
number. They call it a FOM or “Figure of Merit” due to the normalization. Other
data sheets phrase things slightly differently when referring to the same number.
The first hint you get that there’s something going on with the > 200 db noise
number is in figures 11,12, and 13 where they show actual performance at
a couple of frequencies. The noise at “phase noise test set” sort of offsets isn’t
making it past 120 dbc / Hz on those plots.
The quick and dirty explanation is that you translate the “FOM” number by
10 Log F to get the noise at the operating frequency. So, if you are at 100 MHz,
you add 80 db to the magic 231 db. That gets you to -151. That still sounds ok ….
but … this is the broadband FOM and not the close in number. It gets worse as
you go closer to carrier ….
So no, that’s not going to beat a typical RPD-1 based setup.
Bob
Getting the simple PNA to lock was a bit difficult due to the overly
simplistic translation of the mixer output to the Vtune of the OCXO
To get some more flexibility I added a summing opamp that summed the mixer
output with the output of the coarse tuning potmeter. As the summing causes
inversion one extra inverting opamp was added. This made the loop gain
constant
To ensure the mixer is in quadrature another opamp was added that amplified
the mixer output into two LEDs. One LED on when below zero ouput from
mixer, the other on when above zero and both dim when zero output. This
made tuning the coarse frequency simple. Turn till the blinking stops and
both LED's light up dim. The fine frequency potmeter was no longer needed
and the frequency counter is also no longer needed to get into lock
With the summing opamp it is also possible to add an integrator but this
has not been done yet.
Shielding is now the biggest problem as any nearby coax connected to a
10MHz source will cause a huge amount of spurs when not at exactly the same
10MHz
Ultra low noise opamps have been ordered to hopefully reduce the internal
noise of the PNA but the reference OCXO may already be the limiting factor.
The REF voltage output of the OCXO turned out to be rather clean. Much
cleaner than a 8705 voltage regulator
Erik
Hi
(see below)
On Jul 9, 2022, at 12:06 PM, Erik Kaashoek erik@kaashoek.com wrote:
Getting the simple PNA to lock was a bit difficult due to the overly simplistic translation of the mixer output to the Vtune of the OCXO
To get some more flexibility I added a summing opamp that summed the mixer output with the output of the coarse tuning potmeter. As the summing causes inversion one extra inverting opamp was added. This made the loop gain constant
Putting some sort of “gain switch” on the summing amp can help in getting the loop gain to the point it is usable.
To ensure the mixer is in quadrature another opamp was added that amplified the mixer output into two LEDs. One LED on when below zero ouput from mixer, the other on when above zero and both dim when zero output. This made tuning the coarse frequency simple. Turn till the blinking stops and both LED's light up dim. The fine frequency potmeter was no longer needed and the frequency counter is also no longer needed to get into lock
That sounds right.
With the summing opamp it is also possible to add an integrator but this has not been done yet.
Typically a simple roll off cap on the feedback R is about all that is done.
Shielding is now the biggest problem as any nearby coax connected to a 10MHz source will cause a huge amount of spurs when not at exactly the same 10MHz
Terminating unused devices “at the socket” is often the only way to keep things reasonable.
Ultra low noise opamps have been ordered to hopefully reduce the internal noise of the PNA but the reference OCXO may already be the limiting factor.
Even a “not so fancy” op amp should do pretty well. The big deal is to get to dual supplies
with a fairly high voltage on the first stage.
The REF voltage output of the OCXO turned out to be rather clean. Much cleaner than a 8705 voltage regulator
The Ref voltage likely also supplies the oscillator. It can be 20 to 40 db “noisier” than the phase
detector output and have little or no impact on the oscillator performance. Yes, there likely is some
filtering between the regulator and the oscillator ….
Bob
Erik
Erik,
On 7/9/22 22:06, Erik Kaashoek via time-nuts wrote:
Getting the simple PNA to lock was a bit difficult due to the overly
simplistic translation of the mixer output to the Vtune of the OCXO
To get some more flexibility I added a summing opamp that summed the mixer
output with the output of the coarse tuning potmeter. As the summing causes
inversion one extra inverting opamp was added. This made the loop gain
constant
To ensure the mixer is in quadrature another opamp was added that amplified
the mixer output into two LEDs. One LED on when below zero ouput from
mixer, the other on when above zero and both dim when zero output. This
made tuning the coarse frequency simple. Turn till the blinking stops and
both LED's light up dim. The fine frequency potmeter was no longer needed
and the frequency counter is also no longer needed to get into lock
With the summing opamp it is also possible to add an integrator but this
has not been done yet.
So, this is where you should attempt the PI loop.
In theory, you have one proportional path P and one integrating path I
that sums to form the EFC. You can imagine this as two op-amps having
inverted gain and then a summing amp to sum these two up. Thus, you have
for the P path a resistor in the negative feedback path and for the I
path a capacitor in the negative feedback path.
Such a setup is nice for testing, but a bit excessive as one progresses.
One can actually reduce this to a single op-amp with the resistor and
capacitor of the negative feedback to be in series, having a common
input resistor.
The integrator part will hold the state that ends up being the DC part
of EFC. The proportional path will provide the AC path and set the
damping factor for the PLL, you want it well damped.
This would replace your normal loop filter. You would still want a
filter to reject the sum-frequency out of the mixer.
The P gain is proportional to the PLL bandwidth time damping factor.
The I gain is proportional to the PLL bandwidth squared.
The capture range is for all practical purposes infiinte (it's wide
enough). The capture time depends to the cube on the PLL bandwidth, so
altering the PLL bandwidth between unlocked and locked conditions have
proven very useful approach to speed things up if one has a need for
larger lock-in frequencies. Rough-tuning with a trimmer can reduce it
significantly. The lock-detection is very simple detection of the
presence of beat-notes or not, that AC component dies away as it locks.
Anyway, the benefit of the PI loop filter is that you can be rather
brutal with parameters, it will lock. So, it can be worth experimenting
with it. I've found that one can ball-park things fairly quickly knowing
how to change the P and I for wished PLL bandwidth and damping. Very
experimentally friendly.
I should advice you that any PLL will provide a low-pass filter of the
reference input, and a high-pass filter on the noise inside the loop,
which includes that of the oscillator. This can help you identify likely
sources of disturbances as per their frequency in relation to the PLL
loop bandwidth.
Cheers,
Magnus
Shielding is now the biggest problem as any nearby coax connected to a
10MHz source will cause a huge amount of spurs when not at exactly the same
10MHz
Ultra low noise opamps have been ordered to hopefully reduce the internal
noise of the PNA but the reference OCXO may already be the limiting factor.
The REF voltage output of the OCXO turned out to be rather clean. Much
cleaner than a 8705 voltage regulator
Erik
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Am 2022-07-09 22:06, schrieb Erik Kaashoek via time-nuts:
Ultra low noise opamps have been ordered to hopefully reduce the
internal
noise of the PNA but the reference OCXO may already be the limiting
factor.
The REF voltage output of the OCXO turned out to be rather clean. Much
cleaner than a 7805 voltage regulator
The existence of my own ultra-low noise amplifiers was originally
triggered
by this problem but has turned into a sport of it's own. Don't yield to
the
temptation of driving this too far. A single AD797, LT1028, or ADA4898-2
all deliver an input noise density of abt. 1nV/rtHz which is the thermal
noise of a 60 Ohm resistor. ADA4898 has goof price/performnce.
The diodes in the mixer can easily feature RS = 20 Ohms, and the 2
conducting
diodes then show 40 Ohms, which is not much less than the 60 Ohm equiv
of the opamps.
RS is ohmic resistance of silicon and contacts, not the differential
slope resistance of the diode which is only half-thermal IIRC.
High level mixers often have additional resistors in series to the
diodes.
It's no wonder then that high level mixers are usually not the winners
in
dynamic range. Maybe an array of low-level mixers that are Wilkinsoned
together on RF and LO, with the IF ports in series would give good
results.
- Stephan R. Kurtz, Watkins-Johnson: Mixers as Phase Detectors
- Bert C. Henderson, W-J: Mixers: Part 2 Theory and Technology
Copyright © 1981 Watkins-Johnson Company
Vol. 8 No. 3 May/June 1981
Revised and reprinted © 2001 WJ Communications, Inc.
cheers, Gerhard
I've updated the schematic to include the latest additions and added
some new measurements
Schematic: http://athome.kaashoek.com/time-nuts/PNA/Simple_PNA.pdf
The resistor values (many 18k) are a bit weird but I happen to have a
big box of 18k resistors.
The value of the low pas filter after the mixer (C2,C3,L1) are probably
wrong. Calculate yourself for the corner frequency you want.
The elco's in the PI_controller and the input of the Audio_LNA are
probably going to explode due to reverse polarity.
The output of the REF_Buffer acts as the virtual ground so care was
taken (almost) not to draw any current, except for the input of the
Audio_LNA.
The supply of the opamps is not drawn but its from Ground and Vcc (+12V)
I've tested symmetric supply but the combination of the REF output
voltage from the DOCXO and the REF_Buffer provided the least noise.
The audio_LNA has a gain of 1 for DC and increasing to 100 for for 1Hz
and above
The R/C values around the PI_Controller have not been optimized but they
work.
As the Summer OPAMP inverts to 5-10V the Inverter OPAMP brings it back
to 0-5V for the Vtune of the DOCXO
The LED's provide visual feedback on the tuning. IF both are just on the
PLL is in lock. It may be better to have two LED's in series at each
side to increase the dimming.
Some measurements.:
All indicated levels are 40dBc/Hz higher compared to actual.
The noise floor: http://athome.kaashoek.com/time-nuts/PNA/PN_baseline_3.JPG
This is measured without DUT input.
Rigol signal generator generating 10MHz Phase modulated with 60 degrees
noise at -80dBc/Hz: http://athome.kaashoek.com/time-nuts/PNA/
Rigol signal generator generating 10MHz phase modulated with 0.006
degrees at 220Hz :
http://athome.kaashoek.com/time-nuts/PNA/PN_Rigol_3_0.006.JPG
The 220Hz is under the cursor at -27dBc, at 0.006 degrees modulation it
should be at -88dBc, so there must still be a big mistake somewhere.
AR60 Rubidium reference:
http://athome.kaashoek.com/time-nuts/PNA/PN_Rb_3.JPG
All seems OK, a bit of 50Hz and harmonics.
OCXO : http://athome.kaashoek.com/time-nuts/PNA/PN_OCXO_3.JPG
very weird spurs between 40 and 50 Hz
The famous cheap Chines TCXO:
http://athome.kaashoek.com/time-nuts/PNA/PN_TCXO_3.JPG
Not too bad for offsets of 100Hz and higher but at 10Hz and lower its
20dB worse.
A home designed/build arduino GPSDO:
http://athome.kaashoek.com/time-nuts/PNA/PN_GPSDO_3.JPG
The GPSDO has a good ADEV but is clearly very noisy!
I also measured a Marconi 2022 signal generator and it was possible to
lock but the phase noise was terrible with strong factional PLL spurs.
I also tried to measure the phase noise of an old Philips analog 10Hz to
12MHz signal generator but it was impossible to get a lock because the
generator output is jumping around several Hz at 10MHz output.
The noise floor of the simple PNA leaves a lot to improve (from
-140dBc/Hz at 10kHz to -180dBc/Hz with better OCXO, LNA and correlation)
but it proved to be able to do a first assessment of some not too good
oscillator performance.
Feedback welcome as these are my first baby steps on phase noise nuttery.
Erik.
Hi
Pretty much the best mixer to use for this in a basement / DIY basis
is a Mini Circuits RPD-1 or one of it’s siblings. It has a 500 ohm
output on the mix port instead of 50 ohms. Yes, you open circuit
terminate it ( so 5K ) but as noted, it’s the Zout of the mixer that likely
sets what the op amp sees. With it’s higher output impedance, you
are even less driven to nutty low noise op amps and 4 ohm feedback
resistors. The good old OP-27 / OP-37 sitting in the dusty back of your
parts drawer from back in 1993 will do just fine.
Yes, this all gets back to being nutty as you get close to carrier. If you
are after -150 dbc / Hz at 1 Hz offset, you will need go a bit crazy. If
you head this way, there are a lot of posts back in the archives leading
you down various paths to get it done.
While others have indeed fried expensive setups while loosing a supply
leg, I’ve never run into that problem. It most certainly can happen. I’ve
never taken any special precautions and have yet to “get bit” by the issue.
As a rough guess, I’d say I’ve powered up various implementations these
setups a couple thousand times over the years.
Bob
On Jul 10, 2022, at 6:32 AM, Gerhard Hoffmann via time-nuts time-nuts@lists.febo.com wrote:
Am 2022-07-09 22:06, schrieb Erik Kaashoek via time-nuts:
Ultra low noise opamps have been ordered to hopefully reduce the internal
noise of the PNA but the reference OCXO may already be the limiting factor.
The REF voltage output of the OCXO turned out to be rather clean. Much
cleaner than a 7805 voltage regulator
The existence of my own ultra-low noise amplifiers was originally triggered
by this problem but has turned into a sport of it's own. Don't yield to the
temptation of driving this too far. A single AD797, LT1028, or ADA4898-2
all deliver an input noise density of abt. 1nV/rtHz which is the thermal
noise of a 60 Ohm resistor. ADA4898 has goof price/performnce.
The diodes in the mixer can easily feature RS = 20 Ohms, and the 2 conducting
diodes then show 40 Ohms, which is not much less than the 60 Ohm equiv of the opamps.
RS is ohmic resistance of silicon and contacts, not the differential
slope resistance of the diode which is only half-thermal IIRC.
High level mixers often have additional resistors in series to the diodes.
It's no wonder then that high level mixers are usually not the winners in
dynamic range. Maybe an array of low-level mixers that are Wilkinsoned
together on RF and LO, with the IF ports in series would give good results.
- Stephan R. Kurtz, Watkins-Johnson: Mixers as Phase Detectors
- Bert C. Henderson, W-J: Mixers: Part 2 Theory and Technology
Copyright © 1981 Watkins-Johnson Company
Vol. 8 No. 3 May/June 1981
Revised and reprinted © 2001 WJ Communications, Inc.
cheers, Gerhard
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Hi
There’s really no need to use the Vref out of the OCXO at all.
Since many devices don’t have one, you will need a “replacement”
at some point. Simply pulling the “set reference” off of a cleaned
up output of your main supply(s) is typically how it is done.
The most basic reason to not hard wire a specific device into the
circuit is to allow a A to B / B to C / C to A swap process to be done.
That is about the only way to get close to working out the numbers
on this or that reference. Without that data, you are flying blind as
you get close to the limits of the reference.
Given the characteristics of the mixer and the other stuff involved,
with a roughly +7 dbm input, anything past -174 dbc / Hz is suspect.
-180 dbc / Hz is significantly better than what you likely can do with
this approach. Indeed, it also is quite a bit better than what you will
find your signal sources doing so that’s not a major constraint.
Yes there is a wonderful “bet a beer” / after work conversation to be
had about the ultimate phase noise of a +7 dbm signal. More or less,
the bet is won by postulating a 1 ohm source impedance. For real
world sources …. not so much ….
Bob
On Jul 10, 2022, at 7:52 AM, Erik Kaashoek via time-nuts time-nuts@lists.febo.com wrote:
I've updated the schematic to include the latest additions and added some new measurements
Schematic: http://athome.kaashoek.com/time-nuts/PNA/Simple_PNA.pdf
The resistor values (many 18k) are a bit weird but I happen to have a big box of 18k resistors.
The value of the low pas filter after the mixer (C2,C3,L1) are probably wrong. Calculate yourself for the corner frequency you want.
The elco's in the PI_controller and the input of the Audio_LNA are probably going to explode due to reverse polarity.
The output of the REF_Buffer acts as the virtual ground so care was taken (almost) not to draw any current, except for the input of the Audio_LNA.
The supply of the opamps is not drawn but its from Ground and Vcc (+12V)
I've tested symmetric supply but the combination of the REF output voltage from the DOCXO and the REF_Buffer provided the least noise.
The audio_LNA has a gain of 1 for DC and increasing to 100 for for 1Hz and above
The R/C values around the PI_Controller have not been optimized but they work.
As the Summer OPAMP inverts to 5-10V the Inverter OPAMP brings it back to 0-5V for the Vtune of the DOCXO
The LED's provide visual feedback on the tuning. IF both are just on the PLL is in lock. It may be better to have two LED's in series at each side to increase the dimming.
Some measurements.:
All indicated levels are 40dBc/Hz higher compared to actual.
The noise floor: http://athome.kaashoek.com/time-nuts/PNA/PN_baseline_3.JPG
This is measured without DUT input.
Rigol signal generator generating 10MHz Phase modulated with 60 degrees noise at -80dBc/Hz: http://athome.kaashoek.com/time-nuts/PNA/
Rigol signal generator generating 10MHz phase modulated with 0.006 degrees at 220Hz : http://athome.kaashoek.com/time-nuts/PNA/PN_Rigol_3_0.006.JPG
The 220Hz is under the cursor at -27dBc, at 0.006 degrees modulation it should be at -88dBc, so there must still be a big mistake somewhere.
AR60 Rubidium reference: http://athome.kaashoek.com/time-nuts/PNA/PN_Rb_3.JPG
All seems OK, a bit of 50Hz and harmonics.
OCXO : http://athome.kaashoek.com/time-nuts/PNA/PN_OCXO_3.JPG
very weird spurs between 40 and 50 Hz
The famous cheap Chines TCXO: http://athome.kaashoek.com/time-nuts/PNA/PN_TCXO_3.JPG
Not too bad for offsets of 100Hz and higher but at 10Hz and lower its 20dB worse.
A home designed/build arduino GPSDO: http://athome.kaashoek.com/time-nuts/PNA/PN_GPSDO_3.JPG
The GPSDO has a good ADEV but is clearly very noisy!
I also measured a Marconi 2022 signal generator and it was possible to lock but the phase noise was terrible with strong factional PLL spurs.
I also tried to measure the phase noise of an old Philips analog 10Hz to 12MHz signal generator but it was impossible to get a lock because the generator output is jumping around several Hz at 10MHz output.
The noise floor of the simple PNA leaves a lot to improve (from -140dBc/Hz at 10kHz to -180dBc/Hz with better OCXO, LNA and correlation) but it proved to be able to do a first assessment of some not too good oscillator performance.
Feedback welcome as these are my first baby steps on phase noise nuttery.
Erik.
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Hi Erik,
On 7/10/22 17:52, Erik Kaashoek via time-nuts wrote:
I've updated the schematic to include the latest additions and added
some new measurements
Schematic: http://athome.kaashoek.com/time-nuts/PNA/Simple_PNA.pdf
The resistor values (many 18k) are a bit weird but I happen to have a
big box of 18k resistors.
The value of the low pas filter after the mixer (C2,C3,L1) are
probably wrong. Calculate yourself for the corner frequency you want.
I get 22,5 kHz which isn't completely off the charts. Sure helps to eat
the 20 MHz and higher, as well as stray 10 MHz. For the 20 MHz it will
in ideal have -180 dB damping, but in practice it will leak over but
probably not too bad.
The elco's in the PI_controller and the input of the Audio_LNA are
probably going to explode due to reverse polarity.
You want the resistor and capacitor to be in series and not in parallel
in that negative feedback.
As you put a resistor in parallel you will drain the state of the
capacitor and loose performance.
You can choose to either locate a 1 uF non-polar cap, or shift the
values a bit to get into plastic caps such as polypropylene. 100 nF and
220 nF should be easy enough to get hold off. You could even put a pair
of 470 nF in parallel.
A generic note: Most if not all op-amps tends to operate better in terms
of offset behavior as they see about the same resistance DC on both +
and - inputs.
The output of the REF_Buffer acts as the virtual ground so care was
taken (almost) not to draw any current, except for the input of the
Audio_LNA.
The supply of the opamps is not drawn but its from Ground and Vcc (+12V)
I've tested symmetric supply but the combination of the REF output
voltage from the DOCXO and the REF_Buffer provided the least noise.
The audio_LNA has a gain of 1 for DC and increasing to 100 for for 1Hz
and above
The R/C values around the PI_Controller have not been optimized but
they work.
As the Summer OPAMP inverts to 5-10V the Inverter OPAMP brings it back
to 0-5V for the Vtune of the DOCXO
You could do away with the Summer and Inverter op-amps if you fed the
TUNING into the + input of the inverter. By skewing the PI-controller
balance the output will be suitably offset. The benefit will be that you
avoid noise contribution from two op-amps and their resistors.
The LED's provide visual feedback on the tuning. IF both are just on
the PLL is in lock. It may be better to have two LED's in series at
each side to increase the dimming.
I would advice moving those LEDs off-board. Let that run on separate
"dirty" power. I love the direct observation aspect, but I fear it just
add noise to the measurement.
Keep up the good work!
Cheers,
Magnus
Some measurements.:
All indicated levels are 40dBc/Hz higher compared to actual.
The noise floor:
http://athome.kaashoek.com/time-nuts/PNA/PN_baseline_3.JPG
This is measured without DUT input.
Rigol signal generator generating 10MHz Phase modulated with 60
degrees noise at -80dBc/Hz: http://athome.kaashoek.com/time-nuts/PNA/
Rigol signal generator generating 10MHz phase modulated with 0.006
degrees at 220Hz :
http://athome.kaashoek.com/time-nuts/PNA/PN_Rigol_3_0.006.JPG
The 220Hz is under the cursor at -27dBc, at 0.006 degrees modulation
it should be at -88dBc, so there must still be a big mistake somewhere.
AR60 Rubidium reference:
http://athome.kaashoek.com/time-nuts/PNA/PN_Rb_3.JPG
All seems OK, a bit of 50Hz and harmonics.
OCXO : http://athome.kaashoek.com/time-nuts/PNA/PN_OCXO_3.JPG
very weird spurs between 40 and 50 Hz
The famous cheap Chines TCXO:
http://athome.kaashoek.com/time-nuts/PNA/PN_TCXO_3.JPG
Not too bad for offsets of 100Hz and higher but at 10Hz and lower its
20dB worse.
A home designed/build arduino GPSDO:
http://athome.kaashoek.com/time-nuts/PNA/PN_GPSDO_3.JPG
The GPSDO has a good ADEV but is clearly very noisy!
I also measured a Marconi 2022 signal generator and it was possible to
lock but the phase noise was terrible with strong factional PLL spurs.
I also tried to measure the phase noise of an old Philips analog 10Hz
to 12MHz signal generator but it was impossible to get a lock because
the generator output is jumping around several Hz at 10MHz output.
The noise floor of the simple PNA leaves a lot to improve (from
-140dBc/Hz at 10kHz to -180dBc/Hz with better OCXO, LNA and
correlation) but it proved to be able to do a first assessment of some
not too good oscillator performance.
Feedback welcome as these are my first baby steps on phase noise nuttery.
Erik.
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