[time-nuts] GPSDO recovery from holdover
SAIDJACK at aol.com
SAIDJACK at aol.com
Sun Dec 2 01:13:00 UTC 2012
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!
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
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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