[time-nuts] Testing frequency using NTP Bruce GPS ps
Tom Van Baak
tvb at LeapSecond.com
Mon Oct 13 04:22:18 UTC 2008
> Now I am confused. I thought it used the internal crystal clock for
> the timing, and selected which pulse to output for the 1PPS, and
> this is what causes the sawtooth. If so, the timing is determined by
> the crystal.
Correct. For example, if the xtal were 20.00 MHz then the hardware
timing pulse could be off by up to +/- 25.00 ns from the nominal
value. But the firmware knows approximately how far off, and so
it outputs this value as a correction message. The correction is
not perfect, but if used, is always better than hardware alone.
> I don't use software to correct the sawtooth. It is a whole new way
> of locking to the 1PPS.
You might want to check out James Miller's excellent GPSDO.
His simple design also does not use sawtooth correction and
the results are very good. See:
More information about the time-nuts