[time-nuts] TIC resolution impact on GPSDO's performance

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Dec 25 19:28:10 EST 2006


Poul-Henning Kamp wrote:
> In message <45905B32.4000301 at xtra.co.nz>, Dr Bruce Griffiths writes:
>
>   
>>> As far as I can see on TVB's site, the 8607 is about 5e-13 at 1000 sec,
>>> so 1ns/1h sounds perfectly good to me.
>>>   
>>>       
>> Only true when one only considers the data available on Tom's site.
>> Quartzlock have a GPS disciplined BVA OCXO that achieves considerably 
>> better stability, admittedly their carrier phase tracking GPS receiver 
>> has a resolution of a few picoseconds for an integration time of 1 second.
>>     
>
> I'm not sure what your point is here...  Obviously if you discipline
> something with an Oncore, you have to choose a PLL timeconstant that
> is optimum for the oscillator you are using.  If your oscillator is
> very good, the oncore may not be good enough at any timeconstant.
>
> But the 1ns resolution of the Oncore is a fact of life and for all
> the "amateur" stuff we are talking about here, 1ns is plenty fine.
>
>   
>>>> Its not the coherence of the sawtooth but the coherence between the 
>>>> oscillator clocking the timer used to position the PPS signal and the 
>>>> frequency of the PPS signal itself.
>>>>         
>>> Yes, those are non-coherent, that is why we need the negative sawtooth
>>> in the first place.  If they were coherent, we could just have
>>> used a formula.
>>>   
>>>       
>> But they are not sufficiently incoherent as hanging bridges and the 
>> quasi periodic form of the sawtooth error amply demonstrate.
>>     
>
> Your choice of the word "coherence" does not help the discussion
> here because it implies that the signals are the same frequency,
> which is not the case.
>
> But if you insist on using the word coherence, then at least
> use it correctly:  The hanging bridges happen because the two
> signals are not coherent.
>
>   
Wrong
The hanging bridges occur as the oscillator frequency closely approaches 
a harmonic of 1Hz and then drifts away again.
Thus some degree of coherence (at least for the duration of the 
"bridge") occurs flattening out the phase error versus time plot.

If the two signals were sufficiently incoherent, hanging bridges would 
not occur.

Poul-Henning

You are using a different meaning for coherence than the standard one.
The standard meaning is something like:
Two signals or waves are said to be */coherent/ *if their behaviour at 
various times/places is linked in a deterministic way.

Which allows statements like:
Harmonics are coherent with the fundamental.
Two signals do not have to have the same frequency to be coherent, they 
can be harmonically related.

Bruce



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