[time-nuts] Re: GPSDO Control loop autotuning

[Poul-Henning Kamp]

--------
Matthias Welwarsky writes:

> > 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.
>
> That's an interesting metric, but just as well you could record the phase 
> error over a certain time and feed it into a linear regression solver.

Not even close.  That will only give the (least-square) average, you
*really* want to look at the curve-shape.

> give you the drift in s/s and you want it to be 0 (or, like, close). You'll 
> know if your PLL is coupled too loosely if you see a drift.
> Or, just look at the ADEV of the phase error. If the curve is going up instead 
> of down, you know something's not quite right ;)

Yes, but neither way are suitable metrics as input to an autotuning PLL.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.