Hi Magnus, Quite interesting. Could you point us to one or rwo pages developing please ? Thx. Gilles.
> Le 8 mars 2020 à 10:54, Magnus Danielson <mag...@rubidium.se> a écrit : > > H > >> On 2020-03-08 07:38, Poul-Henning Kamp wrote: >> -------- >> In message <5583e434-4c72-4a4f-a60b-75a4204ef...@n1k.org>, Bob kb8tq writes: >> >>> Backing up a bit …. the objective is not to minimize overshoot or >>> keep the loop from oscillating. The issue here is optimizing the noise >>> output of the combination of GPS + OCXO when combined via >>> the control loop. It’s a very different objective …. >> Yes, but a PI loop is still the best mathematical tool for it, >> you just need the PI loop to have adjustable parameters. >> >> Adjusting those parameters after the initial capture is the hard >> part, because the signal you are looking for is in the "wander" >> domain. > > First, a PID PLL degenerates into a PI loop. The P and D steering ends > up achieving the same thing as P in the PI, do there is no benefit of D. > I have shown such derivations in the past. Not too hard to do. > > Second, a PI loop has trivial dimensioning from damping factor and > frequency, and the damping factor we know we need to keep high enough, > so say 3 or higher, so we end up only having the loop frequency, which > is the reciprocal of time-constant. These rules is easy to do, a page of > paper is enough or a whiteboard. > > So, these basic facts is just the rule of the game. > >> The best I have come up with, is to average the measured phase error >> to get rid of the GPS jitter/sawtooth, and adjust the PI loop >> parameters based on the time between sign-changes of that averaged >> signal. > Third, the averaging filter needs a limitation on how low it frequency > can go, before it starts to affect the pole-pair of the PI-loop, at > which time stability cannot be guaranteed. Corrections needs to be > performed to ensure stability and performance as it comes closer. >> If you plot the histogram of the time between sign-changes, you want >> the peak below the supposed "allan-intercept" and if you get time >> intervals more than double the "allan-intercept" you have probably >> tightend too much. > > The Allan intercept is really where the cut-over from reference Allan > plot to the steered oscillator plot. The concept of Allan intercept is > actually not perfect science, but a concept. The actual physics would > make the cut-over analysis on the phase-noise plots make more sense, but > for the time-constants we talk, that's where the Allan deviation plot > has taken over typically. Actually doing the cut-over in Allan deviation > form carries with it biases values, making the Allan intercept value > biased. It gets you to the right neighborhood, sure, but do expect a few > trims for optimum stability. > > Cheers, > Magnus > > > > > _______________________________________________ > 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. _______________________________________________ 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.