[time-nuts] The VE2ZAZ GPSDO

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Oct 21 18:39:18 EDT 2006


John Miles wrote:
> I think it'd be great to see Dr. Griffiths' list of issues in a letter to
> QEX, sans a couple of the more, er, "editorial" comments.  This would give
> the author a chance to respond, and alert other builders of opportunities
> for improvement.
>
> Some of the concerns may not be major issues (e.g., it seems safe to say
> that the FLL action will correct for drift caused by the 7805's tempco,
> unless you plan to dump a can of R134a on it), but others are worth bringing
> to the magazine's attention.  If you'll forward your comments to Doug Smith
> at kf6dx (at) arrl.org, he'll most likely print them in the Letters column.
>
> -- john, KE5FX
>
>   
>> Well said, Randy.
>>  It wasn't my intention when I asked for comments to trash anybody or
>> anything. In the somewhat 'rarefied atmosphere' of this forum I doubt
>> that any comments made were intended to be anything other than
>> objective.
>>
>>  I guess the ultimate question is whether or not the claimed
>> performance objective can be met with the hardware/firmware detailed
>> in the article, regardless of some of the apparent shortcomings
>> already noted.   If the answer is yes, then I believe the discussion
>> here should be interpreted as no more than suggested improvements that
>> could be implemented.
>>
>> Regards
>> DaveB, NZ
>>
>>
>>     
>
>
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>   
A frequency lock loop is not the optimum technique for disciplining a 
frequency standard as for periods of less than a few hours measurement 
system white phase noise (PPS timing jitter) dominates.
The optimum phase lock technique requires a similar number of components.
Experiments are fine but why try and sell the result of an ill conceived 
one?
This "solution" represents a step backwards from existing solutions of 
similar cost but better performance.

Since a correctly designed system will have a time constant of around an 
hour or so (with a good OCXO), the temperature coefficient of the power 
supply is significant as temperature fluctuations with a period less 
than this will affect the EFC voltage and hence cause the frequency to 
fluctuate. The frequency correction loop will be unable to fully correct 
these variations unless the time constant is drastically reduced in 
which case the frequency fluctuations due to the GPS PPS pulse jitter 
will increase substantially.


Bruce



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