To further demonstrate the Diode - R- C- approach, here (hopefully) is a screenshot of the raw DAC output vs time on my Arduino Micro (32u4) based system. For this test the oscillator is free running with an error of about 1 usec per 460 sec or 2.17x10^-9. The horizontal scale is 125 sec/div (1000 sec total) and the vertical is 1024 DAC counts (0-2.56 V) which corresponds to 1 usec of offset between the oscillator and the reference.
You can see that there is some curvature because the capacitor is being charged through a resistor and not a true current source, but as I mentioned earlier this does not affect the system's ability to lock the oscillator to the pps reference. When locked with a time constant of 1000 sec, the phase detector output is almost always less than +/- 100 counts from the setpoint of 500. The noise is due mostly to jitter in my PPS reference, which is generated by an Adafruit GPS module. Presumably it would be less if I had a real timing receiver. . If the inserted image does not come through, I will re-send as an attachment. > > -- --Jim Harman
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