[time-nuts] Best Rubidium Frequency Standard

Bob Camp kb8tq at n1k.org
Sun Mar 13 10:44:32 EDT 2016


Hi

> On Mar 13, 2016, at 10:22 AM, KA2WEU at aol.com wrote:
> 
> Good morning ,
>  
> what is wrong with   the Standford Research  Rubidium standard with a 1 sec sync pulls form a GPS satellite ?” 

The second to second variation in the pps from a normal GPS receiver is in the > 20 ns range. If you lock directly to it with a short time constant loop (say < 500 seconds) your Rb probably can’t even keep up with the changes. 

The daily swing in the GPS signal at L1 is in the 10 to >50 ns range (sunrise and sunset). That’s worse than the stability of these Rb’s over the same time period. In order to average things out you need a loop with a time constant out in the “many days” range. 

The normal way to take care of all this in a GPSDO is to track the lock in process and adjust the loop as it progresses (or does not progress).  The unit may switch between two or three filter settings even after it has fully “settled in”. Some days will be >50 ns days, other days will be 10 ns days ….

Bob


>  
> Ulrich
>  
> In a message dated 3/13/2016 10:16:11 A.M. Eastern Daylight Time, kb8tq at n1k.org writes:
> Hi
> 
> With “real” (who knows how real) Rb based GPSDO’s selling below $250, it’s not clear that running an Rb in a lash up that makes it look like a TBolt is a worthwhile exercise. Unless you can get the time constants out into the “several days” range, a manual adjust is a much better way to go. Since we are talking about a “I just want to plug it all in” sort of approach here, anything more than “set and forget” appears to be out of the question. 
> 
> Bob
> 
> 
> > On Mar 12, 2016, at 10:48 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> > 
> > 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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