[time-nuts] Z3801A with other GPS receiver

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri May 18 23:11:04 EDT 2007

Didier Juges wrote:
> Bruce,
> Not knowing exactly what that entails, in spite of the volume of useful 
> information you and others have dispensed on this list on that subject, 
> but always the hopeful engineer that I am :-), I wholeheartedly agree 
> with you.
> Another approach I am interested in pursuing is the Thunderbolt 
> approach, where the 10 MHz is used (directly or via PLL or multiplier) 
> as the processor clock. That eliminates the sawtooth issue entirely, but 
> would limit the GPS engine to one that can be operated that way (with 
> the right clock frequency).
If the GPS receiver uses the OCXO as its local oscillator reference, 
then this is what the carrier phase (with code phase assistance) 
disciplining technique does.
No additional phase detector, no sawtooth, only a DAC or a DDS is 
required to adjust the OCXO frequency.
Using older (well aged) OCXOs that have drifted so much (0.5-1ppm or so) 
that they cannot be adjusted back to their nominal frequency is also 
In effect a  small additional "doppler shift" from the frequency offset 
is of little consequence.
The new Trimble GPSDOCXO probably incorporates some elements of this 
scheme, however the data sheet specs give no frequency stability data.
> Either way, it sounds more fun than a serial converter.
> However, it would be a fairly long term project for me, and for the 
> foreseeable future, I probably won't be able to dedicate enough time to 
> get such an ambitious project going.
> Oh well, there is always retirement...
> Didier


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