[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:

http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm
http://www.leapsecond.com/pages/gpsdo/
http://www.febo.com/pages/oscillators/g3ruh_gpsdo/

/tvb




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