[time-nuts] Software Sawtooth correction prerequisites?

Tom Van Baak tvb at LeapSecond.com
Mon Jul 23 21:36:36 EDT 2007

Just to clarify, credit for the clever D-latch idea goes to a posting
by the other Tom (Dr Tom Clark), not me.

Tom Clark, K3IO wrote (on May 11, 2007):
> 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.

Tom Van Baak (tvb) replied:
> 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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