[time-nuts] GPSDO Question

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Sep 3 20:06:03 UTC 2007


In message <c03.175890b6.340db548 at aol.com>, SAIDJACK at aol.com writes:
>); SAEximRunCond expanded to false

>This brings up another question: how good are true PID loops that also make  
>use of the differential term (e.g. correcting for rate of change of the 
>phase)?  The literature talks about the differential term being hardly used in the  
>industry because it can add noise and instability to a system...

I tried it and found that you could indeed et faster convergence, but
I did it with a HP5370B as phase comparator to minimize the noise.

With more realistic phase comparators I don't think it is sensible.

I personally prefer to start with a very short tempered PLL and then
increase the timeconstant as things get better.

The hard part is knowing when to stop increasing the time constant,
and I have found that keeping track of phase error zero crossings
is the best indicator:  If you don't see any (for too long time),
your PLL has too long timeconstant.

Poul-Henning

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Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
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