Hi As far as I know there is no “closed form” solution to tuning a GPSDO. It is very much a measure / tweak / measure / tweak sort of thing.
That said, there are a lot of basic design constraints that are pretty well known. Your DAC needs to have a pretty small LSB. Updates are normally done once a second ( = each time the PPS is measured). Bob > On Mar 7, 2020, at 5:07 AM, Gilles Clement <[email protected]> wrote: > > Hi, > How do you optimally tune the control loop time constant ? > (Mine gets quite unstable when the update rate is slow - and the amplitude of > the change step low - enough not to degrade the OCXO performance ) > Is there a method described somewhere (like the Ziegler–Nichols method for > PID) ? > Thx, > Gilles. > >> Le 3 mars 2020 à 18:28, Attila Kinali <[email protected]> a écrit : >> >> On Tue, 3 Mar 2020 12:14:37 -0500 >> Jim Harman <[email protected]> wrote: >> >>> I don't understand why you say the DAC should have a resolution of 24-30 >>> bits. I can see that the loop time constant affects the precision needed in >>> the filter calculations, but what does the time constant have to do with >>> the needed DAC resolution? We don't have to wait for the whole time >>> constant before changing the DAC, we can update the filter calculations and >>> look at its output every second and adjust the DAC whenever the PI filtered >>> phase error is one DAC step or more. >> >> You do wait the whole time before updating the DAC value.. kind of ish. >> The control loop's time constant is exactly that: The time it takes >> the control loop for a change in the input to affect the output (very >> loosely speaking). Yes, the sample rate at which the loop runs is >> much higher, but that doesn't change the fact that the loop is slow >> to react. And you want it to be slow to react, as otherwise the high >> noise of GPS degrades the performance of your OCXO. >> >>> If the OCXO has a tuning range of 1 ppm and we want frequency control >>> of 1e-12, wouldn't that require a DAC with 1e6 steps or 20 bits, >>> assuming the DAC covers the full tuning range of the oscillator? >> >> Yes. There is a calculation mistake in there. I corrected it in >> the next mail: >> http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2019-October/097963.html >> >> Attila Kinali >> >> -- >> <JaberWorky> The bad part of Zurich is where the degenerates >> throw DARK chocolate at you. >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
