[time-nuts] GPSDO recovery from holdover

Azelio Boriani azelio.boriani at screen.it
Sun Dec 2 13:40:18 UTC 2012


As usual I cannot refrain to make my mistake: seconds for minutes... yes,
it is 13 minutes not 13 hours.

On Sun, Dec 2, 2012 at 3:54 AM, Bob Camp <lists at rtty.us> wrote:

> 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
> >
> >
> > <phase_corrections.zip>_______________________________________________
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>
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