[time-nuts] GPSDO recovery from holdover
Bob Camp
lists at rtty.us
Sun Dec 2 02:54:59 UTC 2012
Hi
On Dec 1, 2012, at 8:13 PM, SAIDJACK at aol.com wrote:
> Hi Magnus,
>
> yup, at the levels we are interested in, a prefix or two sometimes doesn't
> make any real difference :)
>
> Most of the time typical GPSDO's won't ever drift out of a say +/-100ns
> window. If they do, then the antenna must have been shot off by someone, or
> something else must have gone horribly wrong.
>
> Just for fun I attached two phase correction examples from a FireFly-IIA,
> and a CSAC GPSDO. Both were essentially brand new and not yet calibrated
> when turned on, and thus you can see a large EFC variation over the first 15
> minutes or so as the frequency stabilized.
>
> Then you can see the phase stabilize slowly, this takes about 1.2 hours for
> the FF-IIA with a much more aggressive loop setting, and about 3 hours for
> the CSAC GPSDO.
>
> The most perplexing fact for me is that while you can clearly see the exact
> point at which the phase has stabilized, you cannot really see any
> corresponding change in EFC behavior at that time. You can see a large EFC voltage
> change initially as the frequency stabilizes after power-on, but then it
> goes into the noise floor. This shows that the EFC corrections for phase
> error are essentially smaller than the proportional noise floor of the loop!
>
Driving an integrator is never an easy thing. Watching EFC and looking at phase indeed watching the loop drive an integrator.
> The maximum phase error in these plots was about 100ns for the CSAC, and
> 230ns for the FF-IIA. Here we can see that the FF-IIA has a much more
> aggressive loop approach (~5x more gain on the phase correction). Since the CSAC
> is an atomic clock we can increase the time constant quite a bit and make
> the loop much less aggressive.
>
> bye,
> Said
>
Bob
>
> In a message dated 12/1/2012 14:39:58 Pacific Standard Time,
> magnus at rubidium.dyndns.org writes:
>
> One can also wonder if the limit is relevant, as you are about to
> resolve a rather catastrophic situation where you already cause
> interference, so moving out of it quickly should be first priority and
> only when back to reasonable time-error would it be relevant to obey
> frequency error limits.
>
> The transmitters and the recievers would be able to follow, as they have
> large enough bandwidth for it.
>
>> But if you set the loop parameters more aggressively to 1ns/s as in your
>> example, it would take less than 20 minutes to correct 1us.. Not 12hrs.
>> Unless you meant to say ms?
>
> What's a off by one prefix among friends?
>
> But still, one has to be careful.
>
> Cheers,
> Magnus
>
>
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