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

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Dec 25 17:46:52 EST 2006


Poul-Henning

Poul-Henning Kamp wrote:
> In message <458F1D50.3000707 at xtra.co.nz>, Dr Bruce Griffiths writes:
>
>   
>>> We know that the hardware PPS signal from gps is phasemodulated
>>> with a +/- N ns signal which has a box distribution and upper
>>> frequency limit of 2 Hz and which, subject to temperature stability
>>> and hanging bridges, has no significant frequency components below
>>> < 1/500s.
>>>       
The sawtooth error is certainly not entirely random, there would appear 
to be a strong correlation between the errors for adjacent pulses.
>>>   
>>>       
>> Has this been established by actual measurement?
>>     
>
> With the caveat that I wrote 2Hz instead of 0.5Hz (Nyquist thing)
> this follows directly from signals theory.
>
> Running a FFT over the negative sawtooth values is an interesting
> exercise, in particlar of you collect a weeks worth of data or more.
>
>   
>> Ignoring the combined effects of quantisation and coherence would not 
>> seem to be very sensible.
>>     
>
> A quantum of 1ns is not a concern if the attack point for the PLL
> is on the order of hours.
>
>   
This statement is only true when one is disciplining or monitoring 
relatively with sufficiently large frequency and/or phase instabilities.
When one wishes to discipline an Oscilloquartz 8607 OCXO for example 
every nanosecond or fraction thereof matters.
If the appropriate timing receiver (not an M12+, or M12M) is used an 
Allan variance less than 2E-13 is possible from  1s < tau < IE6 sec for 
the disciplined OCXO.
> I'm not sure what coherence you are talking about here, the
> negative sawtooth signal is not coherent with anything as far
> as we know.
>
>   
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.

Bruce



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