[time-nuts] Software Sawtooth correction prerequisites?

[Dr Bruce Griffiths]

Tom Van Baak wrote:
>> This sure sounds like a more complicated measurement than is necessary
>> to me. If you have a 10 MHz oscillator, simply feed it into the "D"
>> input into a latch clocked by the de-sawtoothed GPS 1PPS. The output of
>> the latch is a 0 or 1 depending on the precise phase of the oscillator.
>> You want this latched 0/1 measurement to average to ½ over a long term
>> (seconds). As the statistics deviate from a 50/50 split, you tweak the
>> oscillator. The ~1 nsec of residual noise from the sawtooth corrected
>> GPS rcvr acts a natural dither. No counters, no ramps, no big A/D
>> converter -- it couldn't be simpler! And if the 10MHz (=> 100 nsec phase
>> ambiguity) is too fine for your oscillator, then divide it to 5 MHz
>> (=>200 nsec) or 1 MHz (=> 1µsec). This should be good enough to pull in
>> a xtal that is off by 1:10e6.
>>     
>
> This sounds really simple and irresistible. Have you or Rick
> tried it out? I see instead of a TIC (Time Interval Counter)
> you have a TAC (Time Average Controller ;-)
>
> Not just GPS 1PPS noise but any oscillator noise (jitter), if
> large enough, is also a source of natural dither. Sounds like
> this design would be especially ideal for a low-end GPSDO;
> i.e., one that only needs to be accurate to 10^-9 or 10^-10.
>
> Did you envision that the OCXO EFC would be driven by a
> statistics-collecting microprocessor and a high-resolution
> DAC? Or is there some clever way to tie statistical results
> of the D-latch to the EFC and avoid the DAC too?
>
> /tvb 
>
>
>
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>   
Tom

A D flipflop is a better choice than a transparent latch.

If the PPS signal also interrupts the micro, the built in interrupt 
synchroniser will ensure sufficient delay that when the output of the D 
flipflop is sampled the probability of its output being in a metastable 
state will be extremely low particularly if a 74AC74 or faster flipflop 
is employed.

Bruce