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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jan 21 04:01:13 EST 2009

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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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.


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