[time-nuts] The VE2ZAZ GPSDO

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Oct 21 20:20:25 EDT 2006

Tim Shoppa wrote:
> "John Miles" <jmiles at pop.net> wrote:
>> 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.
> Having seen some bad vibes unfold through the channels of the
> letters column and other communications with editors, I think that
> BY FAR the most useful thing to do would be to contribute an
> improvement (preferably with schematic) rather than a lengthy
> textual or mathematical criticism.
> Tim.
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Since the disciplining technique is far from optimum, this may only 
serve to further sidetrack the less knowledgeable from the best 
engineering solution which can achieve a far better performance for a 
similar cost. A comparison of the performance of this method with the 
optimum method using similar receivers and crystal oscillators would 
have been helpful in assisting newcomers in selecting a more appropriate 

Perhaps someone should set up a webpage detailing how one should go 
about disciplining a high stability crystal oscillator.

Allan deviation vs tau for various GPS receivers
Allan deviation vs tau for various crystal oscillators.
Various phase measurement techniques and associated tradeoffs
Effect of various phase measurement techniques on the system Allan 
deviation vs tau.
Filtering phase measurements and discarding outliers.
Choosing the appropriate loop time constant.

Use of synchronisers to reduce probability of generating runt pulses and 
metastability problems.
Don't use same IC to buffer different frequencies
Resynchronising divider chain outputs to reference frequency clock to 
reduce jitter.

DAC requirements and tradeoffs
Short term stability
Cost etc.

Monotonic high resolution DAC techniques
Multibit Audio DACs


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