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

Poul-Henning Kamp phk at phk.freebsd.dk
Sat May 5 18:35:28 UTC 2012


In message <CDAA55B4E94149A6972740E6FD0C8C53 at Warcon28Gz>, "WarrenS" writes:
>Magnus wrote:

>This is not a paper about "Don't  GPSD your RB", as the nut subject line 
>suggest.

Actually, that is their point, or maybe more specifically:  Don't GPSDO
it the way we do with OCXOs.

At least as I read the paper.

It chimes pretty well with my own experiments and measurements.

The short term noise and offsets of GPS-PPS signals in most cases
is quite detrimal to disciplining a Rb.

Case in point:  Oncore + PRS10.

If you do not apply the negative sawtooth, there is no guarantee
that the PPS signals average will be coincident with the epoch the
receiver tries to mark.

Tom has some plots of the "hanging bridges" you will experience
and they can have durations so long that it starts to leak through
the PLL steering the Rb, thereby ruining the result.

This is the reason why you need to configure a PRS10 with a
timeconstant a fair bit shorter than theory predicts if you
just hook it up to a GPS-PPS signal.

Interestingly enough, applying the PRS10's 1/256th filter makes
worst case behaviour worse, because the filter makes the PRS10
"latch onto" even shorter hanging bridges.

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 belive that is the same thing the paper advocates, although
they communicate it very badly.


-- 
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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