[time-nuts] GPSDO recovery from holdover

Bob Camp lists at rtty.us
Sat Dec 1 03:02:12 UTC 2012


Hi

Typo…

Sorry

Bob

On Nov 30, 2012, at 9:49 PM, Bob Camp <lists at rtty.us> wrote:

> Hi
> 
> 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.


> Bob
> 
> 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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> 
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