[time-nuts] Newbie question: GPSDO filtering principles

Peter Vince pvince at theiet.org
Sun Apr 22 09:08:48 EDT 2007

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

     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?



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