H On 2020-03-08 07:38, Poul-Henning Kamp wrote: > -------- > In message <[email protected]>, 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 -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
