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 <t...@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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. >
Block Diagram Adev measurement.pdf
Description: Adobe PDF document
_______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.