[time-nuts] New Subscriber, DIY GPSDO project (yes, another one)

[Poul-Henning Kamp]

--------
In message <5583E434-4C72-4A4F-A60B-75A4204EF2D5 at 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.

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.

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.

With good modern dual-band GPS receivers, you can probably also
steer after the magnitude of the averaged signal, in a sense
putting a PI loop around the PI loop.

-- 
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.