[time-nuts] GPSDO recovery from holdover

Charles P. Steinmetz charles_steinmetz at lavabit.com
Sat Dec 1 10:17:33 UTC 2012

Hal wrote:

>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.

Both the TBolt and HP38xx default to the second method you describe, 
and both can be manually forced to jump to the nearest cycle (TBolt = 
"jam sync," HP = ":SYNChronization:IMMediate"), which gets the PPS 
within 50 nS.  At that point, they revert to the first method and do 
the last <=50 nS by adjusting the oscillator frequency.

The TBolt allows you to program a "maximum frequency offset," which 
seems as if it should establish a limit on how fast it can correct 
the PPS, but I have never seen one come anywhere near the default 
maximum offset.  The TBolt also allows you to set a "jam sync 
threshold," which seems as if it ought to make the unit jam sync 
automatically when the threshold is exceeded -- but I've never seen 
one do that, either, even when set to "fast recovery."  So far as I 
have seen, jam sync only occurs manually.

Best regards,


More information about the time-nuts mailing list