[time-nuts] Z3805 incident

Dennis Ferguson dennis.c.ferguson at gmail.com
Thu Nov 29 23:14:40 UTC 2012


On 29 Nov, 2012, at 02:32 , Charles P. Steinmetz <charles_steinmetz at lavabit.com> wrote:

>> This is a classic crystal jump. The crystal changed its frequency magically from one second to the next and the software compensated for it
> 
> Here is another example of a 3805 having a bad moment.  For just about two minutes, it reported a phase jump of nearly 3 uS and then immediately fell back nearly to its previous baseline, settling to the baseline in about an hour and not requiring any longer-term change of the EFC voltage.  This does not look like a typical crystal frequency shift to me, but I cannot rule that out.  It looks more like what I'd expect to see if I set the cable delay to 3 uS for 2 minutes, then back to 0.

I think I would be more likely to call this one, where the crystal jumps
to another frequency for a while and then jumps back to about what it was,
a "classic crystal jump".  I've seen this before, though not as large as the
change you show.  I hear these raise hell when they try to use PTP to transmit
telecom-quality timing over asynchronous ethernet because it is hard to run
a PTP control loop tight enough (i.e. at a high enough data rate) to correct
that before it does damage.

I think the other problem, with the crystal jumping to another frequency and
apparently staying there (I'm assuming it hasn't jumped back), could have a
broader range of causes.

Dennis Ferguson



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