[time-nuts] GPSDO recovery from holdover

Bob Camp lists at rtty.us
Sat Dec 1 02:49:58 UTC 2012


A few more possibilities:

1) Slip the clock by one cycle per second rather than 10 at once.
2) Take the pps off of a 100 MHz source sync'd to the 10 MHz and slip by less than 100 ns per step
3) Take the pps off of a DDS and fine tune the slip however slowly you might wish.

In practice, the approach is "get back to +/- 100 ns as soon as you can". The system majority of specs care about being at the right time, and not so much how you get there. There are a few specs that prefer a frequency slew to bring the pps in from microseconds of error, but not many.


On Nov 30, 2012, at 9:10 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> Does anybody know what happens in a TBolt or Z3801?  (or any other boxes?)
> Suppose your system goes into holdover for long enough to be interesting.  
> Suppose for discussion that the clock drifts so that the PPS if off by a 
> mircosecond.
> I can see two ways to recover.  One is to jump the 10 MHz clock by 10 cycles. 
> The other is to adjust the frequency so that the PPS slews back to on-time.
> The first approach gives you a second with the wrong number of cycles.  The 
> second approach has your clock frequency off for a while with a trade off 
> between how far off and how long it's off.
> Are there any other approaches?
> -- 
> These are my opinions.  I hate spam.
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