MD
Magnus Danielson
Fri, Jul 8, 2022 1:21 PM
Hi,
Well, both amplitudes can be measured. The method I refer to is one of
several out of NIST, so it's not one of my own invention. See their AM
and PM Calibration material.
Using multiple methods you can evaluate how well the method functions.
The side-tone method generates known PM with the uncertainty in relative
amplitude. It can be easier to validate than a phase modulator approach,
as it needs calibration.
Cheers,
Magnus
On 2022-07-08 03:57, Bob kb8tq wrote:
Hi
One consideration:
If you do signal injection for calibration, you have the amplitude uncertainties on
both the “carrier” and injected signals. The slope at zero on the beat note is likely
to be much more accurate ( even if gain measurement at audio gets thrown in …)
Bob
On Jul 7, 2022, at 5:19 PM, Magnus Danielson via time-nuts time-nuts@lists.febo.com wrote:
Hi,
A well established method is to use a separate offset RF generator that you can steer frequency to form suitable offset and amplitude to form known level. You can now inject this ontop of a signal to measure. Consider that you steer your offset frequency to be +1 kHz of the carrier you measure, and you set the amplitude to be -57 dB from the carrier. This now becomes equivalent to having a -60 dBc phase modulation at 1 kHz.
The RF generator does not have to be ultra-clean in phase noise just reasonably steerable in frequency and amplitude.
Cheers,
Magnus
On 2022-07-07 12:47, Erik Kaashoek via time-nuts wrote:
Bob, others.
It has been explained that for the best phase noise level calibration on should use a signal with one radian phase modulation and measure the output voltage.
The problem with this approach is the unknown gain of the path into the PC. And due to the gain one can not modulate with one radian as this saturates the whole path.
An alternative method for phase noise level calibration could be to create an oscillator so bad its phase noise can be measured using a spectrum analyzer. To make such a bad oscillator a 10MHz signal was phase modulated with noise. The phase noise became visible on the spectrum analyzer just above 20 degrees of modulation. The phase noise level saturated between 55 and 60 degrees which is consistent with one radian (57 degrees). The spectrum analyzer could measure the phase noise at a flat -80dbc/Hz ( yes Bob, I better use the right dimensions)
The simple phase noise analyzer also measured the phase noise at -80dBc providing evidence the level calibration was done correctly.
I also tried to increase the DUT drive into the mixer further above saturation so see if this made any change in the measured level but once above 0dBm I did not observe any change up to +10dBm drive. Any higher levels felt too dangerous.
There is still a lot of work to be done to further increase accuracy.
Erik.
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To unsubscribe send an email to time-nuts-leave@lists.febo.com
Hi,
Well, both amplitudes can be measured. The method I refer to is one of
several out of NIST, so it's not one of my own invention. See their AM
and PM Calibration material.
Using multiple methods you can evaluate how well the method functions.
The side-tone method generates known PM with the uncertainty in relative
amplitude. It can be easier to validate than a phase modulator approach,
as it needs calibration.
Cheers,
Magnus
On 2022-07-08 03:57, Bob kb8tq wrote:
> Hi
>
> One consideration:
>
> If you do signal injection for calibration, you have the amplitude uncertainties on
> both the “carrier” and injected signals. The slope at zero on the beat note is likely
> to be *much* more accurate ( even if gain measurement at audio gets thrown in …)
>
> Bob
>
>> On Jul 7, 2022, at 5:19 PM, Magnus Danielson via time-nuts <time-nuts@lists.febo.com> wrote:
>>
>> Hi,
>>
>> A well established method is to use a separate offset RF generator that you can steer frequency to form suitable offset and amplitude to form known level. You can now inject this ontop of a signal to measure. Consider that you steer your offset frequency to be +1 kHz of the carrier you measure, and you set the amplitude to be -57 dB from the carrier. This now becomes equivalent to having a -60 dBc phase modulation at 1 kHz.
>>
>> The RF generator does not have to be ultra-clean in phase noise just reasonably steerable in frequency and amplitude.
>>
>> Cheers,
>> Magnus
>>
>> On 2022-07-07 12:47, Erik Kaashoek via time-nuts wrote:
>>> Bob, others.
>>> It has been explained that for the best phase noise level calibration on should use a signal with one radian phase modulation and measure the output voltage.
>>> The problem with this approach is the unknown gain of the path into the PC. And due to the gain one can not modulate with one radian as this saturates the whole path.
>>> An alternative method for phase noise level calibration could be to create an oscillator so bad its phase noise can be measured using a spectrum analyzer. To make such a bad oscillator a 10MHz signal was phase modulated with noise. The phase noise became visible on the spectrum analyzer just above 20 degrees of modulation. The phase noise level saturated between 55 and 60 degrees which is consistent with one radian (57 degrees). The spectrum analyzer could measure the phase noise at a flat -80dbc/Hz ( yes Bob, I better use the right dimensions)
>>> The simple phase noise analyzer also measured the phase noise at -80dBc providing evidence the level calibration was done correctly.
>>> I also tried to increase the DUT drive into the mixer further above saturation so see if this made any change in the measured level but once above 0dBm I did not observe any change up to +10dBm drive. Any higher levels felt too dangerous.
>>> There is still a lot of work to be done to further increase accuracy.
>>> Erik.
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts@lists.febo.com
>>> To unsubscribe send an email to time-nuts-leave@lists.febo.com
>> _______________________________________________
>> time-nuts mailing list -- time-nuts@lists.febo.com
>> To unsubscribe send an email to time-nuts-leave@lists.febo.com
EK
Erik Kaashoek
Fri, Jul 8, 2022 3:12 PM
Not something I want to implement on short notice but maybe for the future.
The biggest limitation in this DIY PNA is the phase noise of the
reference OCXO and the noise of the opamp amplifying the output of the
mixer.
So I was wondering if it would make sense to do the following
1: Split the output of the DUT into two completely separate PNA's
2: Feed the output of the two PNA's into the PC left/right audio inputs
where the noise of both ADC's gets added.
3: Do a cross correlation of the two inputs.
This should (as far as I have understood the feedback) eliminate both
the phase noise of the two independent OCXO's used as reference and
eliminate the noise of the opamps in the two PNA's and the ADC's, given
enough time to do the correlation.
Erik.
Not something I want to implement on short notice but maybe for the future.
The biggest limitation in this DIY PNA is the phase noise of the
reference OCXO and the noise of the opamp amplifying the output of the
mixer.
So I was wondering if it would make sense to do the following
1: Split the output of the DUT into two completely separate PNA's
2: Feed the output of the two PNA's into the PC left/right audio inputs
where the noise of both ADC's gets added.
3: Do a cross correlation of the two inputs.
This should (as far as I have understood the feedback) eliminate both
the phase noise of the two independent OCXO's used as reference and
eliminate the noise of the opamps in the two PNA's and the ADC's, given
enough time to do the correlation.
Erik.
BK
Bob kb8tq
Fri, Jul 8, 2022 4:12 PM
On Jul 7, 2022, at 10:10 PM, Erik Kaashoek erik@kaashoek.com wrote:
Bob,
You may have explained this before but I still do not understand.
Does the phase modulation slope at the detector depend on the depth of the phase modulation? I think not.
The “phase modulation” you are looking at when observing the slope with a beat note
is a full 2-pi radians of modulation for every cycle of the beat note. Since that’s guaranteed
with no further effort, it makes a nice standard to use. There is no modulation being done
to either signal in this case.
With 57 degrees one should get an output voltage that is to be regarded as the 0dBc level but this can not be measured due to the high gain in the audio path.
Which is why you want a two op amp approach. This also gets you a nice path to use
for the DC feed for lock.
When you reduce the modulation depth with a factor 10 the measured output voltage should decrease with 20dB.
Except you didn’t start out modulating either signal. You simply unlocked them and got
a result that happens to provide 2 pi radians of signal at the output of the mixer.
Modern digital signal generators are supposed to provide phase modulation with at least 0.01 degree accuracy.
So it could be possible to measure the phase detector slope with 0.57 phase modulation depth by measuring what should be -40dBc
Or, if the gain is very high, less accurate with 0.06 phase modulation.
Or am I making a mistake in my reasoning?
The calibration of the system changes ( or can change ) each and every time you swap
out signal sources. The levels are not going to be consistent setup to setup. Thus you
calibrate each and every time you change out either device.
Since signal generators are not likely to get you to the same sort of noise levels as a
very good stand alone source, you very much do not typically want a signal generator
involved in a real measurement. Yes, there are always exceptions to any blanket
statement …
Bob
Erik.
On 8-7-2022 3:57, Bob kb8tq via time-nuts wrote:
Hi
One consideration:
If you do signal injection for calibration, you have the amplitude uncertainties on
both the “carrier” and injected signals. The slope at zero on the beat note is likely
to be much more accurate ( even if gain measurement at audio gets thrown in …)
Bob
Hi
(see below)
> On Jul 7, 2022, at 10:10 PM, Erik Kaashoek <erik@kaashoek.com> wrote:
>
> Bob,
> You may have explained this before but I still do not understand.
> Does the phase modulation slope at the detector depend on the depth of the phase modulation? I think not.
The “phase modulation” you are looking at when observing the slope with a beat note
is a full 2-pi radians of modulation for every cycle of the beat note. Since that’s guaranteed
with no further effort, it makes a nice standard to use. There *is* no modulation being done
to either signal in this case.
> With 57 degrees one should get an output voltage that is to be regarded as the 0dBc level but this can not be measured due to the high gain in the audio path.
Which is why you want a two op amp approach. This also gets you a nice path to use
for the DC feed for lock.
> When you reduce the modulation depth with a factor 10 the measured output voltage should decrease with 20dB.
Except you didn’t start out modulating either signal. You simply unlocked them and got
a result that happens to provide 2 pi radians of signal at the output of the mixer.
> Modern digital signal generators are supposed to provide phase modulation with at least 0.01 degree accuracy.
> So it could be possible to measure the phase detector slope with 0.57 phase modulation depth by measuring what should be -40dBc
> Or, if the gain is very high, less accurate with 0.06 phase modulation.
> Or am I making a mistake in my reasoning?
The calibration of the system changes ( or can change ) each and every time you swap
out signal sources. The levels are not going to be consistent setup to setup. Thus you
calibrate each and every time you change out either device.
Since signal generators are not likely to get you to the same sort of noise levels as a
very good stand alone source, you very much do not typically want a signal generator
involved in a real measurement. Yes, there are always exceptions to any blanket
statement …
Bob
> Erik.
>
>
> On 8-7-2022 3:57, Bob kb8tq via time-nuts wrote:
>> Hi
>>
>> One consideration:
>>
>> If you do signal injection for calibration, you have the amplitude uncertainties on
>> both the “carrier” and injected signals. The slope at zero on the beat note is likely
>> to be *much* more accurate ( even if gain measurement at audio gets thrown in …)
>>
>> Bob
>>
>
EK
Erik Kaashoek
Fri, Jul 8, 2022 4:32 PM
The calibration of the system changes ( or can change ) each and every time you swap
out signal sources. The levels are not going to be consistent setup to setup. Thus you
calibrate each and every time you change out either device.
Assuming each source is saturating the mixer sufficiently (to be
confirmed by measuring the output level of the source into 50 ohm) I do
not understand how changing a source can change the calibration. Can you
explain what is happening?
Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would
already be great.
Erik.
Bob
This confuses me.
> The calibration of the system changes ( or can change ) each and every time you swap
> out signal sources. The levels are not going to be consistent setup to setup. Thus you
> calibrate each and every time you change out either device.
Assuming each source is saturating the mixer sufficiently (to be
confirmed by measuring the output level of the source into 50 ohm) I do
not understand how changing a source can change the calibration. Can you
explain what is happening?
Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would
already be great.
Erik.
BK
Bob kb8tq
Fri, Jul 8, 2022 4:58 PM
Hi
Like it or not, the mixer is a non-linear load. It also has a frequency
dependence. Even with “saturation” levels, the slope can and does
change. That’s the short list, as you dive into it, things get even more
complex in terms of “might be” sort of issues.
How can you be in saturation and have the slope change ( it does sound
unreasonable) ? The fundamental is not changing much (so you are
in saturation). The harmonics of the fundamental are changing. Since
the output is actually a triangle wave with rounded “corners” there are
indeed harmonics very much present.
The flat parts of the triangle wave are a “good thing” in this case. It
makes the device linear over a bit wider range than a sine wave would
provide. This gets you out of all sorts of nutty analysis concerning the
noise being “to much” to measure with the device. It also relaxes the
needed accuracy of the DC lock part of things. ( = slope of a sine wave
changes quickly ….).
You never really get away from the “to much noise” question. The
common definition of phase noise is that it’s more than 60 db below
carrier. That is really just the commonly used limit for “you may need
to think about FM sidebands”. Yes, that’s another rabbit hole to wander
down ….
Bob
On Jul 8, 2022, at 8:32 AM, Erik Kaashoek erik@kaashoek.com wrote:
Bob
This confuses me.
The calibration of the system changes ( or can change ) each and every time you swap
out signal sources. The levels are not going to be consistent setup to setup. Thus you
calibrate each and every time you change out either device.
Assuming each source is saturating the mixer sufficiently (to be confirmed by measuring the output level of the source into 50 ohm) I do not understand how changing a source can change the calibration. Can you explain what is happening?
Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would already be great.
Erik.
Hi
Like it or not, the mixer is a non-linear load. It also has a frequency
dependence. Even with “saturation” levels, the slope can and does
change. That’s the short list, as you dive into it, things get even more
complex in terms of “might be” sort of issues.
How can you be in saturation and have the slope change ( it does sound
unreasonable) ? The fundamental is not changing much (so you are
in saturation). The harmonics of the fundamental are changing. Since
the output is actually a triangle wave with rounded “corners” there are
indeed harmonics very much present.
The flat parts of the triangle wave are a “good thing” in this case. It
makes the device linear over a bit wider range than a sine wave would
provide. This gets you out of all sorts of nutty analysis concerning the
noise being “to much” to measure with the device. It also relaxes the
needed accuracy of the DC lock part of things. ( = slope of a sine wave
changes quickly ….).
You never really get away from the “to much noise” question. The
common definition of phase noise is that it’s more than 60 db below
carrier. That is really just the commonly used limit for “you may need
to think about FM sidebands”. Yes, that’s another rabbit hole to wander
down ….
Bob
> On Jul 8, 2022, at 8:32 AM, Erik Kaashoek <erik@kaashoek.com> wrote:
>
> Bob
> This confuses me.
>> The calibration of the system changes ( or can change ) each and every time you swap
>> out signal sources. The levels are not going to be consistent setup to setup. Thus you
>> calibrate each and every time you change out either device.
> Assuming each source is saturating the mixer sufficiently (to be confirmed by measuring the output level of the source into 50 ohm) I do not understand how changing a source can change the calibration. Can you explain what is happening?
> Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would already be great.
> Erik.
EK
Erik Kaashoek
Fri, Jul 8, 2022 5:16 PM
Bob,
Clear, you have a lot more experience and knowledge. For me this is
typical a case of "If you don't know about something it must be simple"
So best would be to make it possible in the simple PNA to switch off the
opamp gain, without changing the impedance the mixer sees, so the
offset tuned signal can be used to calibrate the slope.
I found this picture very helpful to understand the relation between
phase modulation depth and the strength of the side bands
http://athome.kaashoek.com/time-nuts/PM_Sidebands.JPG
It shows that below 0.2 radian peak phase modulation you can simplify to
narrowband FM as only the 1st sideband has relevant power (certainly for
the accuracy I am after)
The whole presentation including the calculation can be found here:
http://athome.kaashoek.com/time-nuts/Measuring_phase_modulation.pdf
Written by Bob Nelson from Keysight.
Very helpful presentation for people (like me) that are new to all this.
Erik.
On 8-7-2022 18:58, Bob kb8tq wrote:
Hi
Like it or not, the mixer is a non-linear load. It also has a frequency
dependence. Even with “saturation” levels, the slope can and does
change. That’s the short list, as you dive into it, things get even more
complex in terms of “might be” sort of issues.
How can you be in saturation and have the slope change ( it does sound
unreasonable) ? The fundamental is not changing much (so you are
in saturation). The harmonics of the fundamental are changing. Since
the output is actually a triangle wave with rounded “corners” there are
indeed harmonics very much present.
The flat parts of the triangle wave are a “good thing” in this case. It
makes the device linear over a bit wider range than a sine wave would
provide. This gets you out of all sorts of nutty analysis concerning the
noise being “to much” to measure with the device. It also relaxes the
needed accuracy of the DC lock part of things. ( = slope of a sine wave
changes quickly ….).
You never really get away from the “to much noise” question. The
common definition of phase noise is that it’s more than 60 db below
carrier. That is really just the commonly used limit for “you may need
to think about FM sidebands”. Yes, that’s another rabbit hole to wander
down ….
Bob
On Jul 8, 2022, at 8:32 AM, Erik Kaashoek erik@kaashoek.com wrote:
Bob
This confuses me.
The calibration of the system changes ( or can change ) each and every time you swap
out signal sources. The levels are not going to be consistent setup to setup. Thus you
calibrate each and every time you change out either device.
Assuming each source is saturating the mixer sufficiently (to be confirmed by measuring the output level of the source into 50 ohm) I do not understand how changing a source can change the calibration. Can you explain what is happening?
Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would already be great.
Erik.
Bob,
Clear, you have a lot more experience and knowledge. For me this is
typical a case of "If you don't know about something it must be simple"
So best would be to make it possible in the simple PNA to switch off the
opamp gain, without changing the impedance the mixer sees, so the
offset tuned signal can be used to calibrate the slope.
I found this picture very helpful to understand the relation between
phase modulation depth and the strength of the side bands
http://athome.kaashoek.com/time-nuts/PM_Sidebands.JPG
It shows that below 0.2 radian peak phase modulation you can simplify to
narrowband FM as only the 1st sideband has relevant power (certainly for
the accuracy I am after)
The whole presentation including the calculation can be found here:
http://athome.kaashoek.com/time-nuts/Measuring_phase_modulation.pdf
Written by Bob Nelson from Keysight.
Very helpful presentation for people (like me) that are new to all this.
Erik.
On 8-7-2022 18:58, Bob kb8tq wrote:
> Hi
>
> Like it or not, the mixer is a non-linear load. It also has a frequency
> dependence. Even with “saturation” levels, the slope can and does
> change. That’s the short list, as you dive into it, things get even more
> complex in terms of “might be” sort of issues.
>
> How can you be in saturation and have the slope change ( it does sound
> unreasonable) ? The fundamental is not changing much (so you are
> in saturation). The harmonics of the fundamental are changing. Since
> the output is actually a triangle wave with rounded “corners” there are
> indeed harmonics very much present.
>
> The flat parts of the triangle wave are a “good thing” in this case. It
> makes the device linear over a bit wider range than a sine wave would
> provide. This gets you out of all sorts of nutty analysis concerning the
> noise being “to much” to measure with the device. It also relaxes the
> needed accuracy of the DC lock part of things. ( = slope of a sine wave
> changes quickly ….).
>
> You never really get away from the “to much noise” question. The
> common definition of phase noise is that it’s more than 60 db below
> carrier. That is really just the commonly used limit for “you may need
> to think about FM sidebands”. Yes, that’s another rabbit hole to wander
> down ….
>
> Bob
>
>> On Jul 8, 2022, at 8:32 AM, Erik Kaashoek <erik@kaashoek.com> wrote:
>>
>> Bob
>> This confuses me.
>>> The calibration of the system changes ( or can change ) each and every time you swap
>>> out signal sources. The levels are not going to be consistent setup to setup. Thus you
>>> calibrate each and every time you change out either device.
>> Assuming each source is saturating the mixer sufficiently (to be confirmed by measuring the output level of the source into 50 ohm) I do not understand how changing a source can change the calibration. Can you explain what is happening?
>> Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would already be great.
>> Erik.
BK
Bob kb8tq
Fri, Jul 8, 2022 9:03 PM
Hi
Indeed you can switch the gain of the amp. You still need to provide
a low gain output to feed the EFC input on your reference. The chain
to feed the sound card will be crazy high gain for the typical TCXO or
OCXO. Don’t even think of running that sort of gain into a VCO ….
Bob
On Jul 8, 2022, at 9:16 AM, Erik Kaashoek erik@kaashoek.com wrote:
Bob,
Clear, you have a lot more experience and knowledge. For me this is typical a case of "If you don't know about something it must be simple"
So best would be to make it possible in the simple PNA to switch off the opamp gain, without changing the impedance the mixer sees, so the offset tuned signal can be used to calibrate the slope.
I found this picture very helpful to understand the relation between phase modulation depth and the strength of the side bands
http://athome.kaashoek.com/time-nuts/PM_Sidebands.JPG
It shows that below 0.2 radian peak phase modulation you can simplify to narrowband FM as only the 1st sideband has relevant power (certainly for the accuracy I am after)
The whole presentation including the calculation can be found here:
http://athome.kaashoek.com/time-nuts/Measuring_phase_modulation.pdf
Written by Bob Nelson from Keysight.
Very helpful presentation for people (like me) that are new to all this.
Erik.
On 8-7-2022 18:58, Bob kb8tq wrote:
Hi
Like it or not, the mixer is a non-linear load. It also has a frequency
dependence. Even with “saturation” levels, the slope can and does
change. That’s the short list, as you dive into it, things get even more
complex in terms of “might be” sort of issues.
How can you be in saturation and have the slope change ( it does sound
unreasonable) ? The fundamental is not changing much (so you are
in saturation). The harmonics of the fundamental are changing. Since
the output is actually a triangle wave with rounded “corners” there are
indeed harmonics very much present.
The flat parts of the triangle wave are a “good thing” in this case. It
makes the device linear over a bit wider range than a sine wave would
provide. This gets you out of all sorts of nutty analysis concerning the
noise being “to much” to measure with the device. It also relaxes the
needed accuracy of the DC lock part of things. ( = slope of a sine wave
changes quickly ….).
You never really get away from the “to much noise” question. The
common definition of phase noise is that it’s more than 60 db below
carrier. That is really just the commonly used limit for “you may need
to think about FM sidebands”. Yes, that’s another rabbit hole to wander
down ….
Bob
On Jul 8, 2022, at 8:32 AM, Erik Kaashoek erik@kaashoek.com wrote:
Bob
This confuses me.
The calibration of the system changes ( or can change ) each and every time you swap
out signal sources. The levels are not going to be consistent setup to setup. Thus you
calibrate each and every time you change out either device.
Assuming each source is saturating the mixer sufficiently (to be confirmed by measuring the output level of the source into 50 ohm) I do not understand how changing a source can change the calibration. Can you explain what is happening?
Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would already be great.
Erik.
Hi
Indeed you can switch the gain of the amp. You still need to provide
a low gain output to feed the EFC input on your reference. The chain
to feed the sound card will be crazy high gain for the typical TCXO or
OCXO. Don’t even think of running that sort of gain into a VCO ….
Bob
> On Jul 8, 2022, at 9:16 AM, Erik Kaashoek <erik@kaashoek.com> wrote:
>
> Bob,
> Clear, you have a lot more experience and knowledge. For me this is typical a case of "If you don't know about something it must be simple"
> So best would be to make it possible in the simple PNA to switch off the opamp gain, without changing the impedance the mixer sees, so the offset tuned signal can be used to calibrate the slope.
> I found this picture very helpful to understand the relation between phase modulation depth and the strength of the side bands
> http://athome.kaashoek.com/time-nuts/PM_Sidebands.JPG
> It shows that below 0.2 radian peak phase modulation you can simplify to narrowband FM as only the 1st sideband has relevant power (certainly for the accuracy I am after)
> The whole presentation including the calculation can be found here:
> http://athome.kaashoek.com/time-nuts/Measuring_phase_modulation.pdf
> Written by Bob Nelson from Keysight.
> Very helpful presentation for people (like me) that are new to all this.
> Erik.
>
> On 8-7-2022 18:58, Bob kb8tq wrote:
>> Hi
>>
>> Like it or not, the mixer is a non-linear load. It also has a frequency
>> dependence. Even with “saturation” levels, the slope can and does
>> change. That’s the short list, as you dive into it, things get even more
>> complex in terms of “might be” sort of issues.
>>
>> How can you be in saturation and have the slope change ( it does sound
>> unreasonable) ? The fundamental is not changing much (so you are
>> in saturation). The harmonics of the fundamental are changing. Since
>> the output is actually a triangle wave with rounded “corners” there are
>> indeed harmonics very much present.
>>
>> The flat parts of the triangle wave are a “good thing” in this case. It
>> makes the device linear over a bit wider range than a sine wave would
>> provide. This gets you out of all sorts of nutty analysis concerning the
>> noise being “to much” to measure with the device. It also relaxes the
>> needed accuracy of the DC lock part of things. ( = slope of a sine wave
>> changes quickly ….).
>>
>> You never really get away from the “to much noise” question. The
>> common definition of phase noise is that it’s more than 60 db below
>> carrier. That is really just the commonly used limit for “you may need
>> to think about FM sidebands”. Yes, that’s another rabbit hole to wander
>> down ….
>>
>> Bob
>>
>>> On Jul 8, 2022, at 8:32 AM, Erik Kaashoek <erik@kaashoek.com> wrote:
>>>
>>> Bob
>>> This confuses me.
>>>> The calibration of the system changes ( or can change ) each and every time you swap
>>>> out signal sources. The levels are not going to be consistent setup to setup. Thus you
>>>> calibrate each and every time you change out either device.
>>> Assuming each source is saturating the mixer sufficiently (to be confirmed by measuring the output level of the source into 50 ohm) I do not understand how changing a source can change the calibration. Can you explain what is happening?
>>> Please keep in mind I'm not after 0.1dBc/Hz accuracy, +/- 5dBc/Hz would already be great.
>>> Erik.
>
MD
Magnus Danielson
Sun, Jul 10, 2022 12:02 AM
Hi Erik,
On 7/8/22 17:12, Erik Kaashoek via time-nuts wrote:
Not something I want to implement on short notice but maybe for the
future.
The biggest limitation in this DIY PNA is the phase noise of the
reference OCXO and the noise of the opamp amplifying the output of the
mixer.
So I was wondering if it would make sense to do the following
1: Split the output of the DUT into two completely separate PNA's
2: Feed the output of the two PNA's into the PC left/right audio
inputs where the noise of both ADC's gets added.
3: Do a cross correlation of the two inputs.
This should (as far as I have understood the feedback) eliminate both
the phase noise of the two independent OCXO's used as reference and
eliminate the noise of the opamps in the two PNA's and the ADC's,
given enough time to do the correlation.
This makes perfect sense. You will not remove the noise of the two
channels, but you will get a direct benefit and as you average the
complex output of successive FFT-cross-correlations you will suppress
the measurement noise even further.
Have you attempted doing a PI-loop as I've suggested?
However, you benefit greatly at optimizing the performance of a single
channel first before going to the cross-correlation. Bob's many good
suggestions should provide you directions enough. Cross-correlation is
not a replacement for doing the homework well, it's to get the icing on
the cake.
Cheers,
Magnus
Hi Erik,
On 7/8/22 17:12, Erik Kaashoek via time-nuts wrote:
> Not something I want to implement on short notice but maybe for the
> future.
> The biggest limitation in this DIY PNA is the phase noise of the
> reference OCXO and the noise of the opamp amplifying the output of the
> mixer.
> So I was wondering if it would make sense to do the following
> 1: Split the output of the DUT into two completely separate PNA's
> 2: Feed the output of the two PNA's into the PC left/right audio
> inputs where the noise of both ADC's gets added.
> 3: Do a cross correlation of the two inputs.
> This should (as far as I have understood the feedback) eliminate both
> the phase noise of the two independent OCXO's used as reference and
> eliminate the noise of the opamps in the two PNA's and the ADC's,
> given enough time to do the correlation.
This makes perfect sense. You will not remove the noise of the two
channels, but you will get a direct benefit and as you average the
complex output of successive FFT-cross-correlations you will suppress
the measurement noise even further.
Have you attempted doing a PI-loop as I've suggested?
However, you benefit greatly at optimizing the performance of a single
channel first before going to the cross-correlation. Bob's many good
suggestions should provide you directions enough. Cross-correlation is
not a replacement for doing the homework well, it's to get the icing on
the cake.
Cheers,
Magnus
EK
Erik Kaashoek
Sun, Jul 10, 2022 7:11 AM
Hi Magnus,
Yes, and it works very well, locking is easier as once locked it nicely
stay's in lock, , even with a slow drift of either the DUT or the
reference. As I could not find a bipolar capacitor the tuning potmeter
has to be kept at the low side to avoid blowing the integration
capacitor. Maybe a back to back series capacitor with pull down resistor
is safer to use.
Will need to update the schematic to show the small improvements.
@Bob,
You mentioned "dual supplies with high voltage" for the first gain
opamp. How much impact would dual voltage bring as its a pain to implement.
I understand everything gets ground reference and you loose the noise of
the buffer opamp but as the first gain opamp is in differential mode for
its input it does not see the noise of the buffer opamp. Or am I making
a mistake?
On 10-7-2022 2:02, Magnus Danielson via time-nuts wrote:
Have you attempted doing a PI-loop as I've suggested?
Hi Magnus,
Yes, and it works very well, locking is easier as once locked it nicely
stay's in lock, , even with a slow drift of either the DUT or the
reference. As I could not find a bipolar capacitor the tuning potmeter
has to be kept at the low side to avoid blowing the integration
capacitor. Maybe a back to back series capacitor with pull down resistor
is safer to use.
Will need to update the schematic to show the small improvements.
@Bob,
You mentioned "dual supplies with high voltage" for the first gain
opamp. How much impact would dual voltage bring as its a pain to implement.
I understand everything gets ground reference and you loose the noise of
the buffer opamp but as the first gain opamp is in differential mode for
its input it does not see the noise of the buffer opamp. Or am I making
a mistake?
On 10-7-2022 2:02, Magnus Danielson via time-nuts wrote:
> Have you attempted doing a PI-loop as I've suggested?
G
ghf@hoffmann-hochfrequenz.de
Sun, Jul 10, 2022 3:17 PM
Am 2022-07-10 9:11, schrieb Erik Kaashoek via time-nuts:
@Bob,
You mentioned "dual supplies with high voltage" for the first gain
opamp. How much impact would dual voltage bring as its a pain to
implement.
I think is was Rubiola who wrote that he exploded a costly microwave
mixer
with a LT1028 that had lost one of its power rails.
If you go AC coupling, don't forget that the input capacitor must not be
selected for f-3dB but that it must be much bigger to short the thermal
noise
of the bias network to pV levels through the low impedance source.
Otherwise you see a noise rise towards 0 Hz that is MUCH steeper than
1/f.
Scott Wurzer (designer of ad797) saw that immediately on my 20 * ada4898
220pV/rtHz amplifier. Wish he was more explicit. It took me some time to
get it. :-)
I had 10K/100u foil, ended up with 10k/4700uF wet tantalum, which opens
another can of worms.
I have converted to FETs now. A few pVrtHz more, but much less noise
current.
In cross correlation setups, the noise current of both amplifiers
produces
a common voltage drop in the (common) source resistance, and that does
not
average away. (May apply only to voltage measurements from a single
source.)
Gerhard
Am 2022-07-10 9:11, schrieb Erik Kaashoek via time-nuts:
> @Bob,
> You mentioned "dual supplies with high voltage" for the first gain
> opamp. How much impact would dual voltage bring as its a pain to
> implement.
I think is was Rubiola who wrote that he exploded a costly microwave
mixer
with a LT1028 that had lost one of its power rails.
If you go AC coupling, don't forget that the input capacitor must not be
selected for f-3dB but that it must be much bigger to short the thermal
noise
of the bias network to pV levels through the low impedance source.
Otherwise you see a noise rise towards 0 Hz that is MUCH steeper than
1/f.
Scott Wurzer (designer of ad797) saw that immediately on my 20 * ada4898
220pV/rtHz amplifier. Wish he was more explicit. It took me some time to
get it. :-)
I had 10K/100u foil, ended up with 10k/4700uF wet tantalum, which opens
another can of worms.
I have converted to FETs now. A few pVrtHz more, but much less noise
current.
In cross correlation setups, the noise current of both amplifiers
produces
a common voltage drop in the (common) source resistance, and that does
not
average away. (May apply only to voltage measurements from a single
source.)
Gerhard
