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.
