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 > > > > _______________________________________________ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > 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 _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts