[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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