[time-nuts] Software Sawtooth correction prerequisites?

Tom Van Baak tvb at LeapSecond.com
Sat May 12 13:54:55 EDT 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.

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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