[time-nuts] Newbie question: GPSDO filtering principles
Ulrich Bangert
df6jb at ulrich-bangert.de
Sun Apr 22 09:58:05 EDT 2007
Peter,
are these GPSDOs homebuilt units following the concept as published in
the German ham radio magazine "cq-DL" some years ago?
Best regrds
Ulrich, DF6JB
> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Peter Vince
> Gesendet: Sonntag, 22. April 2007 15:09
> An: time-nuts at febo.com
> Betreff: [time-nuts] Newbie question: GPSDO filtering principles
>
>
> Hello all,
>
> The recent talk about the ideal GPSDO has reminded this
> newbie of a question I hope someone might be able to simply
> answer: we have a couple of Rubidium GPSDOs which track in
> the long term, but their phase drifts by several hundreds of
> nanoseconds with an irregular, but roughly daily, pattern.
> The units have a log output which shows the changes to the
> EFC DAC, and this changes usually after a period of 8192
> seconds - sometimes longer, but rarely less. I understand
> the need to average the GPS reading over this sort of period,
> but by only changing the EFC at this period leads to
> comparatively wild frequency swings - parts (plural!) in 10^11!
>
> I was wondering if a "sliding window" scheme would be
> better - average over a period of, say, 8192 seconds, but
> recalculate this average more often - say every hundred
> seconds. The problem would be that having applied a
> correction to your oscillator, subsequent averages would be
> difficult as the slope of the readings before and after the
> correction would be different. Perhaps you could process the
> old data to remove the slope of your recent correction?
> Another option would be to use a DDS, and then readings
> wouldn't be affected by your corrections, and so long term
> averages could be taken over any period. But the
> disadvantage would be that you would have no feedback, and
> hence not know if you applied an appropriate amount of correction.
>
> Hmmm, the brain is spinning with all the possibilities
> and problems! Can some provide a quick explanation of the
> way it is (properly) done, or is there a website that discusses it?
>
> Thanks,
>
> Peter
>
>
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