Seeking advice: Is this the right way to check very short term (below 1s) stability?
As the collection of frequency sources and counters in my home lab is
growing I'd like to understand the performance of the frequency sources.
Two different GPSDO do help to check long term stability.
But the Rubidium frequency standard I have (Accubeat AR60A) is fairly
unknown and seemingly not of good reputation, more specifically its
(very) short term stability is doubted.
So how best to check very short term (below 1s) frequency stability.
The frequency counters available loose resolution quickly when the gate
time is reduced below 1 second and high performance phase noise
measurement equipment is not available so google helped with a search
for alternative measurement methods.
What I found was a method using two frequency sources, one of the two
being a VCO, a mixer and some filters and amplifiers.
By weak locking (large time constant) the VCO source using the mixer as
phase detector to the other source, the output of the mixer's IF port
should carry a voltage real time proportionally to the phase difference
and by filtering and amplifying it should be possible to check for
variations in the 1ms-1s range.
Maybe even a scope can see the variations.
When you know the amplification and the full range voltage you can even
do an absolute measurement.
Would this method work?
Any specific concerns to take note of when doing the measurement?
Removing the DC component (or locking the VCO such that there is no DC
component) will be crucial I guess but given the slow speed of the loop
even an ADC->computer->DAC->VCO setup can work.
Any suggestion is welcome.
Erik.
Hi
The AR-60A is a pretty good “telecom” grade Rb. Depending on which one
you have it will have a 1 second ADEV in the 5x10^-12 (rare) to 1.5x10^-11
range.
Once you get inside the 1 second range on any Rb, the ADEV is going to be
dominated by the OCXO in the device. The telecom grade units are rarely
targeted at super duper low phase noise.
In any setup that compares A to B and gives you a result, the numbers will
be dominated by the performance of the worse of the two devices. VCO’s
rarely have good ADEV …..
Bob
On Nov 28, 2021, at 1:19 PM, Erik Kaashoek erik@kaashoek.com wrote:
As the collection of frequency sources and counters in my home lab is growing I'd like to understand the performance of the frequency sources.
Two different GPSDO do help to check long term stability.
But the Rubidium frequency standard I have (Accubeat AR60A) is fairly unknown and seemingly not of good reputation, more specifically its (very) short term stability is doubted.
So how best to check very short term (below 1s) frequency stability. The frequency counters available loose resolution quickly when the gate time is reduced below 1 second and high performance phase noise measurement equipment is not available so google helped with a search for alternative measurement methods.
What I found was a method using two frequency sources, one of the two being a VCO, a mixer and some filters and amplifiers.
By weak locking (large time constant) the VCO source using the mixer as phase detector to the other source, the output of the mixer's IF port should carry a voltage real time proportionally to the phase difference and by filtering and amplifying it should be possible to check for variations in the 1ms-1s range.
Maybe even a scope can see the variations.
When you know the amplification and the full range voltage you can even do an absolute measurement.
Would this method work?
Any specific concerns to take note of when doing the measurement?
Removing the DC component (or locking the VCO such that there is no DC component) will be crucial I guess but given the slow speed of the loop even an ADC->computer->DAC->VCO setup can work.
Any suggestion is welcome.
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-leave@lists.febo.com
To unsubscribe, go to and follow the instructions there.
The way I look at short term stability is to multiply up to microwave
frequencies then mix the two and look at the resulting beat note. The
popular ADF4351 Fract-N synthesizer is ideal for this. Take two of them,
and programme for two frequencies in the GHz region a 1kHz or so apart when
driven by the two 10MHz freqs that are being compared Apply the two
multiplied signals to a mixer, and look at the resulting IF product with a
PC soundcard input. Using spectral analysis software with a waterfall
display, such as Spectran or Spectrum Lab you can look at the multiplied
frequency instability in real time.
An example of the techniques used on a range of 10MHz reference sources can
be found here (I used a different Fract-N synth with a smaller setting grid
possible than teh ADF4351 can give, but the same idea applies)
Andy
www.g4jnt.com
On Sun, 28 Nov 2021 at 18:19, Erik Kaashoek erik@kaashoek.com wrote:
As the collection of frequency sources and counters in my home lab is
growing I'd like to understand the performance of the frequency sources.
Two different GPSDO do help to check long term stability.
But the Rubidium frequency standard I have (Accubeat AR60A) is fairly
unknown and seemingly not of good reputation, more specifically its
(very) short term stability is doubted.
So how best to check very short term (below 1s) frequency stability.
The frequency counters available loose resolution quickly when the gate
time is reduced below 1 second and high performance phase noise
measurement equipment is not available so google helped with a search
for alternative measurement methods.
What I found was a method using two frequency sources, one of the two
being a VCO, a mixer and some filters and amplifiers.
By weak locking (large time constant) the VCO source using the mixer as
phase detector to the other source, the output of the mixer's IF port
should carry a voltage real time proportionally to the phase difference
and by filtering and amplifying it should be possible to check for
variations in the 1ms-1s range.
Maybe even a scope can see the variations.
When you know the amplification and the full range voltage you can even
do an absolute measurement.
Would this method work?
Any specific concerns to take note of when doing the measurement?
Removing the DC component (or locking the VCO such that there is no DC
component) will be crucial I guess but given the slow speed of the loop
even an ADC->computer->DAC->VCO setup can work.
Any suggestion is welcome.
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send
an email to time-nuts-leave@lists.febo.com
To unsubscribe, go to and follow the instructions there.
I forgot the URL
http://g4jnt.com/10MHzDist.pdf
Andy
www.g4jnt.com
On Sun, 28 Nov 2021 at 18:39, Andy Talbot andy.g4jnt@gmail.com wrote:
The way I look at short term stability is to multiply up to microwave
frequencies then mix the two and look at the resulting beat note. The
popular ADF4351 Fract-N synthesizer is ideal for this. Take two of them,
and programme for two frequencies in the GHz region a 1kHz or so apart when
driven by the two 10MHz freqs that are being compared Apply the two
multiplied signals to a mixer, and look at the resulting IF product with a
PC soundcard input. Using spectral analysis software with a waterfall
display, such as Spectran or Spectrum Lab you can look at the multiplied
frequency instability in real time.
An example of the techniques used on a range of 10MHz reference sources
can be found here (I used a different Fract-N synth with a smaller setting
grid possible than the ADF4351 can give, but the same idea applies)
Andy
www.g4jnt.com
On Sun, 28 Nov 2021 at 18:19, Erik Kaashoek erik@kaashoek.com wrote:
As the collection of frequency sources and counters in my home lab is
growing I'd like to understand the performance of the frequency sources.
Two different GPSDO do help to check long term stability.
But the Rubidium frequency standard I have (Accubeat AR60A) is fairly
unknown and seemingly not of good reputation, more specifically its
(very) short term stability is doubted.
So how best to check very short term (below 1s) frequency stability.
The frequency counters available loose resolution quickly when the gate
time is reduced below 1 second and high performance phase noise
measurement equipment is not available so google helped with a search
for alternative measurement methods.
What I found was a method using two frequency sources, one of the two
being a VCO, a mixer and some filters and amplifiers.
By weak locking (large time constant) the VCO source using the mixer as
phase detector to the other source, the output of the mixer's IF port
should carry a voltage real time proportionally to the phase difference
and by filtering and amplifying it should be possible to check for
variations in the 1ms-1s range.
Maybe even a scope can see the variations.
When you know the amplification and the full range voltage you can even
do an absolute measurement.
Would this method work?
Any specific concerns to take note of when doing the measurement?
Removing the DC component (or locking the VCO such that there is no DC
component) will be crucial I guess but given the slow speed of the loop
even an ADC->computer->DAC->VCO setup can work.
Any suggestion is welcome.
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe
send an email to time-nuts-leave@lists.febo.com
To unsubscribe, go to and follow the instructions there.
Erik,
Den 2021-11-28 kl. 19:19, skrev Erik Kaashoek:
As the collection of frequency sources and counters in my home lab is
growing I'd like to understand the performance of the frequency sources.
Two different GPSDO do help to check long term stability.
But the Rubidium frequency standard I have (Accubeat AR60A) is fairly
unknown and seemingly not of good reputation, more specifically its
(very) short term stability is doubted.
So how best to check very short term (below 1s) frequency stability.
The frequency counters available loose resolution quickly when the
gate time is reduced below 1 second and high performance phase noise
measurement equipment is not available so google helped with a search
for alternative measurement methods.
What I found was a method using two frequency sources, one of the two
being a VCO, a mixer and some filters and amplifiers.
By weak locking (large time constant) the VCO source using the mixer
as phase detector to the other source, the output of the mixer's IF
port should carry a voltage real time proportionally to the phase
difference and by filtering and amplifying it should be possible to
check for variations in the 1ms-1s range.
Maybe even a scope can see the variations.
When you know the amplification and the full range voltage you can
even do an absolute measurement.
Would this method work?
Any specific concerns to take note of when doing the measurement?
Removing the DC component (or locking the VCO such that there is no DC
component) will be crucial I guess but given the slow speed of the
loop even an ADC->computer->DAC->VCO setup can work.
Any suggestion is welcome.
So, in that region one typically transition into measuring phase noise,
as for shorter taus your performance will be dominated by the wideband
white noise, and counters isn't the best tool to analyze that.
The weak locking technique can be used to a limit, but to get good
results, you need to calibrate it. I suggest you set up the loop in the
analog domain and only digitize the residual noise. Then inject using a
synthesizer side-tones to your carrier and know relationship in
amplitude and frequency offsets, for which then the phase-noise should
be known, and use that to build a calibration scale. This is described
in the NIST T&F catalog. You can do that with a varity of sources, but
eventually you will be limited by the noise of the other oscillator.
I use a cross-correlator setup in the form of TimePod most of the times,
with quiet references.
Cheers,
Magnus