[time-nuts] GPS noise reduction

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 5 20:23:44 EDT 2008

Richard H McCorkle wrote:
> Bruce,
> Once again, thanks for the explaination. I am using a common 100M OCXO
> and not independent XOs for the two TICs in the dual design. That is why
> the plots are so similar. What effect does this have on the disciplining
> of the individual oscillators?
> Thanks,
> Richard

When the transitions (PPS and divided down OCXO ouputs) are close to the 
100MHz clock edges the 2 TICs measurements may differ.
If you dont use synchronisers then the differences between the TIC 
readings can occasionally be large and the averages of the TIC outputs 
will be biased.

Sharing a common TIC oscillator will have no effect on the individual 
oscillators although it will tend to increase the correlation between them.

Ideally the TIC resolution should be increased so that the PPS timing 
jitter (after sawtooth correction) is greater than the TIC resolution, 
and the TIC oscillators should then be phase locked to their respective 
OCXOs to improve their stability. The PPS timing noise will then be 
sufficient by itself (if the TIC uses synchronisers) for the averaged 
TIC outputs to be effectively unbiased estimators of the phase errors of 
their respective oscillators.
When using a PPS signal it only remains to determine the optimum 
algorithm for discipling the OCXOs.
The improvement achieved by increasing the TIC resolution further is 
somewhat marginal.

A Kalman filter technique may have some advantages over a PLL 
disciplining technique.

If you are using software to correct the TIC measurements for the PPS 
sawtooth error then you may need (depending on your GPS receiver's PPS 
timing noise) a higher resolution TIC to improve the performance. If you 
are using hardware to correct the PPS signal for  sawtooth error then a 
simpler scheme usng a single D flipflop may suffice together with the 
receiver's TRAIM capability (if any).


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