DIY Low offset Phase Noise Analyzer (Erik Kaashoek)
You stated:
Mike,
The phase detector is an ADE-1 mixer, the IF output of the mixer goes
into a loop filter that has a corner frequency of about 0.2Hz to enable
Phase noise measurements down to 1Hz offset
That is your problem. A double balanced mixer is an exclusive-or phase
detector. The lock range is determined by the loop bandwidth, as you have
found.
The phase-frequency detector is completely different. It will lock to any
signal in the lock range, independent of loop bandwidth. You can have a
bandwidth of 0.001 Hz, and it will still lock. Think of what this could do
for your phase measurements.
Mike.
One concern I have with active components as mixer is noise. For an SA I
designed only a passive DB diode mixer had low enough output noise. Would a
PF detector as being an active component, not create more noise as output?
Erik
On Tue, Jul 5, 2022, 18:20 Mike Monett via time-nuts <
time-nuts@lists.febo.com> wrote:
You stated:
Mike,
The phase detector is an ADE-1 mixer, the IF output of the mixer goes
into a loop filter that has a corner frequency of about 0.2Hz to enable
Phase noise measurements down to 1Hz offset
That is your problem. A double balanced mixer is an exclusive-or phase
detector. The lock range is determined by the loop bandwidth, as you have
found.
The phase-frequency detector is completely different. It will lock to any
signal in the lock range, independent of loop bandwidth. You can have a
bandwidth of 0.001 Hz, and it will still lock. Think of what this could do
for your phase measurements.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Hi
On Jul 5, 2022, at 9:00 AM, Erik Kaashoek via time-nuts time-nuts@lists.febo.com wrote:
Mike.
One concern I have with active components as mixer is noise. For an SA I
designed only a passive DB diode mixer had low enough output noise. Would a
PF detector as being an active component, not create more noise as output?
Erik
Yes, you are correct. The only thing with a low enough noise floor for good
phase noise measurements (via the quadrature technique) is some sort of mixer.
Normal digital phase detectors have way to high a noise floor.
Bob
On Tue, Jul 5, 2022, 18:20 Mike Monett via time-nuts <
time-nuts@lists.febo.com> wrote:
You stated:
Mike,
The phase detector is an ADE-1 mixer, the IF output of the mixer goes
into a loop filter that has a corner frequency of about 0.2Hz to enable
Phase noise measurements down to 1Hz offset
That is your problem. A double balanced mixer is an exclusive-or phase
detector. The lock range is determined by the loop bandwidth, as you have
found.
The phase-frequency detector is completely different. It will lock to any
signal in the lock range, independent of loop bandwidth. You can have a
bandwidth of 0.001 Hz, and it will still lock. Think of what this could do
for your phase measurements.
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
Hi,
On 2022-07-05 12:13, Mike Monett via time-nuts wrote:
You stated:
Mike,
The phase detector is an ADE-1 mixer, the IF output of the mixer goes
into a loop filter that has a corner frequency of about 0.2Hz to enable
Phase noise measurements down to 1Hz offset
That is your problem. A double balanced mixer is an exclusive-or phase
detector. The lock range is determined by the loop bandwidth, as you have
found.
The phase-frequency detector is completely different. It will lock to any
signal in the lock range, independent of loop bandwidth. You can have a
bandwidth of 0.001 Hz, and it will still lock. Think of what this could do
for your phase measurements.
Actually, there is two schools here.
There is the school of stateless phase-detectors (such as mixers) and
the school of stateful phase-detectors (such as three-state mixers).
Now, in the school of stateless phase-detectors, mixers, XOR-gates,
samplers etc. the capture range becomes dependent on the loop gain.
For passive lag filters, you will need a significant static
phase-difference on the input to provide the state of the EFC to
compensate on the frequency. It's very simply that the DC volt
difference coming out of the detectors, through the DC gain of the
filter is then what becomes the EFC.
In active lag filters, you add additional gain, and this requires lower
phase detector voltage to support the same EFC error.
Both these actually have an implicit state in the phase detector to
compensate the lack of state elsewhere. It is just not that the phase
detector holds explicit state.
In PI filters, the state of the frequency error is moved from the phase
detector to the filter. The integrator has close to infinity in DC gain
(naturally limited in practice, but for many purposes we can assume it
being infinite) such that it drives the DC phase offset out of the phase
detector to zero and builds up the needed EFC state in the integrator
capacitor. This have the benefit that capture range is in theory
unlimited, but even if the actual range is in practice limited, it is so
wide that we can treat it as infinite for most cases. The PI loop those
do not need any form of aiding to lock up. However, aiding it can
increase lock-up time. You could either pre-trim the EFC or you could
increase the PLL bandwidth to achieve quick lockup. The later is
actually very simple and has very huge impact.
The thing people do wrong with PI filters is to scale the bandwidth on
the output side of the integrator. This is wrong, as one then needs to
scale the output to maintain the acquired state to match the needed EFC.
The right way to do it is to scale it on the input side. That way the
scaling to EFC is maintained and no state-scaling is needed.
As one scales the bandwidth through I one needs to scale P accordingly
to maintain good damping properties.
Fairly simple PI-loop setups allow for good lockup and stability properties.
Cheers,
Magnus
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.
Hi
Yes, you do need to know the system gain. Since we are talking about
gain at audio, measuring the gain directly is not a crazy thing to do. One
of the things that makes audio spectrum analyzers a nice tool for this that
they eliminate the “variable gain to the sound card” issue.
Some sound card setups are a lot easier to work with than others. If you
are restricted to the sound input on your motherboard things can get a bit
crazy. It is not unusual for folks to dig up a “pro” (whatever that means
on a sound card ) card that has better drivers and more access to this and
that.
Given how fast the PC world changes, the board that was a wonderful thing
last time somebody dove in, likely is long out of production by now. The drivers
that made it work so well may have been “improved” and it no longer gives
you the control it once did. This makes for a bit of trial and error to get it all
going.
Bob
On Jul 7, 2022, at 2:47 AM, Erik Kaashoek erik@kaashoek.com 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.
On 7/7/22 8:55 AM, Bob kb8tq via time-nuts wrote:
Hi
Yes, you do need to know the system gain. Since we are talking about
gain at audio, measuring the gain directly is not a crazy thing to do. One
of the things that makes audio spectrum analyzers a nice tool for this that
they eliminate the “variable gain to the sound card” issue.
Some sound card setups are a lot easier to work with than others. If you
are restricted to the sound input on your motherboard things can get a bit
crazy. It is not unusual for folks to dig up a “pro” (whatever that means
on a sound card ) card that has better drivers and more access to this and
that.
Given how fast the PC world changes, the board that was a wonderful thing
last time somebody dove in, likely is long out of production by now. The drivers
that made it work so well may have been “improved” and it no longer gives
you the control it once did. This makes for a bit of trial and error to get it all
going.
Bob
Rather than a sound card, it might be better to pick a small singleboard
like a Teensy that has a decent ADC, and make a "sampling engine" with a
USB interface.
Or, in general, going to a USB interface sound interface might be good.
You can get them with a lot of channels (at least 8) and they sample
simultaneously, so the uncertainty in USB latency won't bite you.
Google for things like the Focusrite Scarlett
I've not tried it for this kind of application, but it is likely to have
better noise properties than a "inside the PC" card. Typically 24 bit
converters and 192kHz sample rates.
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
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
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.
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.
When you reduce the modulation depth with a factor 10 the measured
output voltage should decrease with 20dB.
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?
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