[time-nuts] Re: GPSDO Control loop autotuning

[Poul-Henning Kamp]

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Markus Kleinhenz via time-nuts writes:

> But I think i got some pointers that can put you on your path.

One (always?) overlooked metric, is the frequency spectrum of the
zero-crossings, of the phase difference input to the PLL.

(NB: This obviously require good resolution of the phase error,
including application of the "negative sawtooth" correction,
otherwise the "hanging bridges" noise will swamp the signal
you are looking for.)

If your loop is (too) tight, the PLL will chase every wiggle in the
GPS signal and your phase difference input to the PLL will change
sign all the time.

If your loop is (too) loose, the PLL will not follow the GPS signal
as close as it should, and the phase difference will only rarely
change sign, if your OCXO/Rb drifts, possibly not ever.

My heuristic is that when you plot the histogram of the time between
zero-crossings, you want the "bulge" on the low side of, but close to,
the "allan-intercept" (Ie: where the OCXO and GPS allan curves cross.)

If you are working with 3rd order PLLs (predicting frequency drift),
plotting separate histograms of the duration of positive and negative
phase inputs helps:  The more they overlap, the better your drift estimate.

I have tried my hand at the math of this phenomena, but it disappears
into dark corners of math text-books, where I cannot follow its trail.

Poul-Henning

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