> 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 _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
