[time-nuts] Interesting paper: Don't GPSD' your Rb...

Poul-Henning Kamp phk at phk.freebsd.dk
Sun May 6 06:47:01 UTC 2012


In message <20120506021212.EC21A800073 at ip-64-139-1-69.sjc.megapath.net>, Hal Murray writes:
>
>phk at phk.freebsd.dk said:
>> If you cannot apply the negative sawtooth, you will get better results by
>> disciplining almost any random quartz xtal, ovenized or not to the GPS,
>> divide it down to PPS and then discipline the PRS10 to that. 
>
>I don't understand that.  What am I missing?

You are missing that the average of the 1PPS pulse only can be trusted
to be zero over a timescale of many hours.

This is an error-source distinct from GPS reception, caused by the 
picking a preexisting flank nearest to the epoch, with no attempt
to keep the average of the resulting error zero.

Imagine that the GPS receivers clock happens to run on perfect
frequency for a while:  That means that the flank used to generate
the PPS will have a fixed location relative to the epoch, for instance
always 12 nanoseconds early or late.

I belive that some GPS receivers have deliberately de-tuned Xtals
for this very reason, but unfortunately that is only a partial
fix, as the problem is a modulus-issue, so not only is perfect
frequency bad, but "perfect +/- n Hz" is equally bad.

The hanging bridges Tom has plots of on leapsecond.com, arises when
the frequency of the GPS xtal changes.

At one point I tried putting a 1W resister close to the xtal and
feed it with a very slow sine-wave to see if "jittering" it would
get me an average of zero of shorter timespans.  My experiment
was inconclusive, but the idea is not unsound.


-- 
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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