[time-nuts] Testing frequency using NTP Bruce GPS ps

[Mike Monett]

  "Tom Van Baak" <tvb at LeapSecond.com> wrote:

  >> 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.

  So where did the 1ns granularity come in?

  >> 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/

  Thanks, already studied and analyzed. He uses an XOR phase detector,
  but the  funny hump at 1,000 seconds bothers me. I can't see  a good
  reason for  that.  The drop starting at 1e4 is  interesting,  and it
  would have been nice to see what happens further out.

  His results also show that some other methods  using phase/frequency
  detectors are  badly implemented. That is to be expected.  PLL's are
  tough for people not used to working with noise and crosstalk.

  >/tvb

  Best Regards,

  Mike Monett