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

[Joe & Gisela Noci]

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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