[time-nuts] Setting loop TC using thunderbolt internal data.

Magnus Danielson magnus at rubidium.dyndns.org
Wed Jan 21 16:05:58 EST 2009


Bruce Griffiths skrev:
> Magnus Danielson wrote:
>> Tom,
>>
>> Tom Van Baak skrev:
>>   
>>> Ah, OK. So you keep the antenna connected and you keep the
>>> GPS receiver in position hold mode, still receiving fixes.
>>>
>>> All you're doing is disabling the software disciplining algorithm.
>>> That sounds like a good test. Let me try this too and see if I
>>> agree with your conclusions.
>>>     
>> We discussed this a little while back. I proposed a three-cornered hat 
>> using two receivers and one counter... as the Thunderbolts has a builtin 
>> TIC to GPS which can be logged. We also discussed just running a single 
>> Thunderbolt in holdover. Others had already tried that approach. Bruce 
>> and I discussed to some degree the effect of steering on the result.
>>
>>   
>>> I'll try to simultaneously measure the 1PPS or 10 MHz outputs
>>> as you suggest.
>>>
>>> Has anyone hacked a TBolt yet to find which internal pin has
>>> a raw 1PPS from the GPS engine (as opposed to the 1PPS
>>> divided down from the OCXO)?
>>>     
>> Hmm... I have not even "repaired" (in the sense, that they are broken 
>> until opened and "repaired", i.e. just looking under the hood for sake 
>> of curiosity). any of mine...
>>
>> Cheers,
>> Magnus
>>
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>>   
> Magnus
> 
> There may not even be a real PPS signal produced by the GPS engine which
> is compared with the PPS generated by dividing down the OCXO output.
> With the GPS engine LO derived from the OCXO, having a real PPS signal
> generated by the GPS engine for such a comparison isnt necessary in
> order to measure the phase error of the PPS signal generated from the
> OCXO output.

Actually, the PPS out may very well be the PPS output of the engine...

But yes, I agree. There is no point in realizing the PPS twice when you 
dicipline the OCXO anyways, and the time-difference is never measured by 
a TIC, the GPS solution gives the time-error and the PPS only need 
re-assignment when it is too big.

Cheers,
Magnus




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