[time-nuts] Best Rubidium Frequency Standard

Charles Steinmetz csteinmetz at yandex.com
Sat Mar 12 22:48:19 EST 2016


I wrote:

>With the right settings, a PRS10 *does* work extremely well with the 
>PPS input from a GPS.  They do generally take several days or more 
>to lock, because of the long time constants involved.

Bob replied:

>I would call having to wait a few days for it to lock a bit of a 
>disadvantage. Even more so for those with an antenna challenged 
>environment that gives them dropouts every few hours.

Well, one is certainly free to use shorter time constants to achieve 
lock faster.  The very long TCs simply allow one to exploit the 
exemplary stability of the PRS10 for performance much better than 
what an OCXO-based GPSDO can deliver.  If you are content with the 
stability of, say, a TBolt, you can replicate that performance by 
adjusting the PRS10 loop to match TBolt dynamics -- in which case it 
will lock in a comparable time.

WRT holdover, the long TC and the inherent stability of the PRS10 
mean that it will stay very close to the GPS PPS even over long 
holdover periods, so re-acquiring lock does not take nearly as long 
as acquiring it initially.  For the same reason, a PRS10 set up for 
maximum stability can acquire lock even if there are holdover periods 
during the acquisition process (in both cases, assuming that the GPS 
does not output "bad" PPS pulses when it is not locked to GPS).

In principle, one might be able to begin the process by setting the 
PRS10 loop "tighter," then changing the loop constants in one or more 
steps after it achieves initial lock.  I have not tried this, and do 
not know if changing the loop programming on the fly upsets the PRS10 
phase.  If not, it should work (and one could even program a BBB, 
'uino, or other small processor to do it automatically).

Best regards,

Charles




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