[time-nuts] GPSDO recovery from holdover
lists at rtty.us
Sat Dec 1 03:02:12 UTC 2012
On Nov 30, 2012, at 9:49 PM, Bob Camp <lists at rtty.us> wrote:
> 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.
That should have been milliseconds of error. A microsecond is a ppm off for a second, that you commonly see corrected with a frequency slew.
> 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
>> 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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