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

Mike Monett XDE-L2G3 at myamail.com
Sat Oct 4 02:28:13 UTC 2008


  Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

  >  Mike

  > They actually  use an augmented form of GPS common view  for which
  > the GPS PPS signal and its timing variations are largely common to
  > both locations  and   thus   largely   cancel  when  comparing the
  > frequencies at the customer site with the standards at NIST.

  > With a  good timing receiver (and antenna  location)  the sawtooth
  > corrected PPS signal timing noise can be as low as a few nanosec.

  Is that  like the CNSC02-O1 High Performance PPS system?  From their
  web site, they state:

  "Provides for  dynamic hardware correction of the  1PPS quantization
  ("sawtooth") error.  This reduces the noise on the 1PPS  pulses from
  (typically) +/-27  nsec  (15 nsec 1-sigma) to +/-  11  nsec  (3 nsec
  1-sigma). More importantly, eliminates periods of bias  error caused
  by the quanization error going through a "zero beat" period that can
  last one to two minutes about every 10 minutes or so."

  http://www.cnssys.com/cnsclock/CNSClockII.html

  That paragraph  is  very  interesting.   The  "zero  beat"  error is
  particularly nasty.  I tried to get more information, but  it wasn't
  really clear  how their correction method fixes this  problem. Could
  you explain it a bit more?

  I am working on a new technique that might give one or two orders of
  magnitude improvement  locking  to the 1PPS  signal.  It  might give
  sub-nanosecond locking in a very low cost system. But the  zero beat
  is a  serious  problem,  so  I am interested  to  learn  as  much as
  possible how  other  solutions work. Maybe the simplest  is  to just
  apply some jitter to the GPS crystal oscillator to keep it  off zero
  beat.

  > The "all in view" technique will reduce the noise  contribution to
  > the comparison somewhat.

  That term is a bit confusing. Does it mean using all satellites that
  are in  view, or does it mean the NIST receivers see  the  same ones
  that the user sees?

  > The timestamp  resolution of better than 30ps or  so  ensures that
  > time stamp quantisation noise is negligible.

  > It also  allows, in principle at least, standalone 3  cornered hat
  > comparisons of  the  frequency   instabilities  of  the  3 sources
  > connected to the customer instrument.

  Another term I must study and learn how to do: "3 cornered  hat". It
  sounds like  a very powerful technique, ideally suited for  a fairly
  simple program and some data logging.

  > For more detail see:

  >  http://tf.nist.gov/timefreq/service/fms.htm

  Thanks very  much  for the link. It is curious  they  don't  seem to
  spend much  effort on correcting the user's  frequency  errors. They
  just want to report how much they are off.

  Why is that? You'd think they would perform a more  valuable service
  by applying  advanced techniques to adjust the  user's  equipment to
  minimize the  error, then report and certify the  actual  result. Is
  there some  reason   they   want   to   leave  the  user's equipment
  free-running?

  > Only carrier  phase  GPS  techniques  are  potentially  capable of
  > picosecond noise levels.

  Now we are getting very interesting. How do you do that?

  > However there are a large number of effects that have to  be taken
  > into account and data reduction and correction is very complex.

  What if  we just want to stabilize an oscillator  frequency, without
  caring much  what the exact phase offset is from the USNO?  The high
  carrier frequency  should make it much easier to lock a  rubidium or
  crystal oscillator, and it should give much lower phase noise.

  Of course, auroras and other disturbances would be more significant,
  but here  I'm mainly interested in getting a good  lab  reference to
  measure and compare the performance of other commercial oscillators.

  This is  a  separate  topic, but as long  as  we  are  talking about
  precision signals,  do you happen to know what kind  of distribution
  amplifiers are used at the USNO to distribute the signals from their
  cesium and hydrogen standards?

  www.usno.navy.mil/

  >  Bruce

  Thanks very  much  for  your  help. It  might  seem  like  a  lot of
  questions, but  I'm  pretty sure you will be very  pleased  with the
  results. I think I can beat just about all the other  methods except
  carrier phase techniques.

  Best Regards,

  Mike Monett




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