[time-nuts] 1PPS for the beginner

Poul-Henning Kamp phk at phk.freebsd.dk
Tue Aug 14 03:45:14 EDT 2018

In message <F1F265C0-DF79-469E-BB29-9B061F759392 at sfr.fr>, Mike Cook writes:

> I don’t think that you are buying much with disciplining the PRS10
> with a GPSDO 1PPS. Do you have any TIC measurements in this config
> to compare with a direct GPS 1PPS feed?

There is a large footnote about "hanging bridges" here.

If you feed the PRS10 from a GPS which produces "hanging bridges",
it will affect resultat PRS10 stability, as these bridges can be
quite long (any GPS) and tall (depending on model).

The 256 exponential average you can enable in the PRS10 is not a
good solution for this problem.

The best solution is if your GPS emits a "negative sawtooth correction"
and you grab that and feed it into the PRS10 and have it DTRT,
(provided its firmware is new enough).  That gets rid of some
of the "GPS-allan-ski-jump"

I would expect running the GPS through a GPSXO and into the PRS10
and selecting a correspondingly longer timesconstant in the PRS10
might also work, but I have not tried it.

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.

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