Joe,
> I assume 'holdover' to be when the OCXO EFC voltage is just held fixed?
Holdover is when there is no GPS reception and your GPSDO runs blind.
EFC may or may not be fixed depending on how clever the s/w tries to be.
But the main thing with holdover is that there is no GPS/1PPS present
and so no TIC readings either.
The "discipline-disable" mode is when GPS/1PPS is still valid and the
TIC is still giving you valid readings. The only thing you don't do is
change the EFC. The net result is that your GPSDO is measuring your OCXO
without adjusting your OCXO. Make sense? Try it out. Add a h/w switch or
comment out the line of code where you update the DAC. Watch your TIC
readings start to drift. And plot with TimeLab.
The TBolt user manual is a good read. [1]
/tvb
[1] http://leapsecond.com/pages/tbolt/manual.htm
On 10/26/2020 11:19 PM, Joe & Gisela Noci 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 <[email protected]> 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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