[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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