[time-nuts] Mentorship needed in learning about Allan Deviation and variation.

[Bob kb8tq]

Hi

What are we trying to measure with ADEV? 

On a frequency source, it’s used to measure the noise of that source. Mostly we 
put up plots of ADEV to show how quiet our source is. (Yes, we might also measure 
noise floor or amplifier contributions ….).

That’s the statistics part. Now for the instrumentation:

There is no device that will directly measure frequency / time / phase to the accuracy
levels we are after. The closest we can get is to measure A against B and look at the
delta. We also could look at A vs B vs C and do some fancy math, that takes a bit 
of setup and has some pretty significant limits. You still are doing a comparison.

With an A to B comparison, you need to know that one or the other device is much
lower noise than the other. Then the plot will (essentially) be the noise of the not so
quiet source. If that’s not true, you have a real tangle. It’s even worse if both our 
sources have the *same* noise in them. 

So how does this apply in your case?

The 1 pps out of your GPS module is *far* from a low noise source close in. Further 
out it will wander more than a little due to ionosphere issues. Depending on how the 
OCXO is locked, there  will be a cross over between “free run” and “follows GPS”. 

The real ADEV of any GPSDO starts out with the free run OCXO noise + loop noise +
GPS noise. Hopefully (but not always) the OCXO noise is the big item close in. As you
get further out, GPS noise becomes the dominant contributor to the output noise. Again
this is a “hopefully” sort of thing. We test our designs because that’s what shows us 
where improvement is needed …..

What to do?

You need an independent source or set of sources to compare against. You need to
feed them into something like a DMTD to measure what’s going on. 

Bob

> On Oct 27, 2020, at 2:19 AM, Joe & Gisela Noci <jgnoci at gmail.com> wrote:
> 
> Bob and Tom,
> Thanks again for your time on this.  I understand a lot better, and have
> just one issue I would like to 'harp' on a little, if you would allow..
> In the simplistic example of a freq counter measuring its own reference, It
> it easy to grasp and understand the incestous nature of the measurement.
> 
> I am having difficulty extending that concept to what I have though.  For
> the purpose of understanding this concept, lets ignore jitter, and all
> other 2nd order effects for now.
> 
>  My OCXO is phase locked to a GPS 1PPS . The same GPS 1PPS is 'locked' to
> a very accurate , very stable  (Cesium?) reference within the SAT
> constellation.
> I would say that if I use the GPS 1PPS ( which is the same as the Cesium
> reference, in my example..) as my reference, then when I measure the phase
> delta between the OCXO and that 1PPS
> I am in fact measuring the phase delta between the OXCO and the accurate
> Cesium reference.
> I realise I am in fact measuring how well the OCXO is phase locked to the
> Reference ( 1PPS , derived from the Cesium reference..) , but that should
> still show what the frequency and phase offset is
> between the OCXO and the Reference.
> I am having difficulty seeing that this is in fact not independent - the
> underlying raw reference for the measurement is the Cesium reference and I
> can't get better than that.
> Substituting a separate, equally good Cesium reference from which I derive
> a 1PPS, is surely no different?
> 
> To simplify my confusion, I have attached a PDF block diagram - this shows
> a 'perfect' 10MHz reference oscillator - perfect in accuracy, drift, phase
> errors, etc - just perfect.
> It is the reference for a PLL with the OCXO being controlled. The perfect
> osc is divided down to present a 1PPS to the TIC. The OCXO is divided down
> to present a 1MHz signal to the TIC.
> The resultant phase delta is logged and used to plot Adev - basically what
> I described above, but a perfect Osc instead of the GPS.
> This surely is comparing the OCXO phase to the perfect osc phase,
> regardless of what is controlling or steering the OCXO?
> 
> 
> Tom, I am not sure what you mean by -
> 
> *The Trimble Thunderbolt (aka TBolt) GPSDO has this
> disable-discipliningfeature. Note it's not "holdover"; that's something
> else entirely.  *
> 
> I assume 'holdover' to be when the OCXO EFC voltage is just held fixed?
> If so, I do not understand how disciplining can be disabled without the EFC
> voltage just being held to a fixed value?
> Can you explain the difference between 'disciplining-disabled' and
> 'holdover' please?
> 
> Chaps, thank you for indulging me on this - the basic concepts are the
> formative grounding for beginning to understand this subject even a little
> and I appreciate your assistance
> and guidance in this!
> Regards
> Joe
> 
> On Tue, Oct 27, 2020 at 1:51 AM Tom Van Baak <tvb at leapsecond.com> wrote:
> 
>> Joe,
>> 
>>> I log the output of the TIC, in nanoseconds, and use that file to
>> generate an ADEV plot.
>> 
>> Good. That's what you need. During normal operation those readings are
>> bounded by the PLL. So it's essentially a measurement of how well the
>> PLL is working, how aggressive the OCXO is steered, etc. ADEV isn't the
>> best way to process that kind of data because it's a boring, even
>> misleading, straight line going down forever.
>> 
>>> Maybe my setup is in fact comparing itself with itself?..!
>> 
>> Yes. Oops. But, here's an idea for you.
>> 
>> One useful technique is to have your GPSDO running fine and then
>> *disable* the disciplining. If you designed the GPSDO you'll know the
>> exact spot in the h/w or s/w to do this. From this point forward your
>> OCXO is still running, your GPS/1PPS receiver is still receiving, the
>> TIC is still comparing, and you are still logging TIC readings every
>> second. But now the DAC is frozen and the OCXO is free-running.
>> 
>> When you plot this data you will see phase slowly wandering away from
>> zero, you may see a slight drift in frequency, and mostly what you will
>> see is the "bathtub" ADEV shape that you were looking for. This method
>> works because as soon as your disable disciplining your OCXO becomes
>> independent of GPS and so the ADEV plot will be a measurement of an
>> oscillator instead of a measurement of a PLL.
>> 
>> This is not something you would do everyday, but especially now that you
>> are understanding how a GPSDO works and playing with Allan statistics
>> it's a educational exercise.
>> 
>> The Trimble Thunderbolt (aka TBolt) GPSDO has this disable-disciplining
>> feature. Note it's not "holdover"; that's something else entirely.
>> 
>> /tvb
>> 
>> 
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