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

Mike Monett XDE-L2G3 at myamail.com
Tue Oct 14 03:54:44 UTC 2008


  >> So where did the 1ns granularity come in?

  > For example, Motorola receivers output the sawtooth  correction as
  > an 8-bit  signed  binary field in the @@En/Hn  TRAIM  message. The
  > range of  said byte is -128 to +127; the  scale,  the granularity,
  > the units of that field are 1 ns. Make sense now?

  Thanks, but I still can't make the connection. I have  studied every
  site I can find on GPS architecture. Some are very good, but I can't
  find anything that relates a 1 second GPS message to the 1PPS pulse.

  I can  understand how the sawtooth is generated from a  crystal that
  doesn't have a convenient division factor to get to 1 Hz.  I believe
  similar pulse-swallowing  techniques  are  used  in  synthesizers to
  generate arbitrary  division ratios. Unfortunately  these  have poor
  phase noise.

  In any  event,  my system ignores all  the  timing  and quantization
  errors in the 1PPS signal. What I am really interested in is how the
  GPS receiver  can  identify  the  correct  time  messages  from many
  different satellites  at wildly different distances,  with  huge and
  changing doppler  shift, and old satellites  constantly  leaving and
  new ones appearing. That is a complete mystery.

  >> 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.

  > The hump  around  1000 seconds is  characteristic  of quartz-based
  > GPSDO. It  is  related   to   the   "cross-over  point"  where the
  > instability of  the XO meets the stability of GPS on an  ADEV plot
  > and to the time-constant of the PLL.

  OK, thanks. That seems similar to the phase noise shelf in PLL loops.

  But why can't the GPS correct the oscillator faster? Why is the loop
  time constant so long?

  I think  that  has to do with the errors in the  1PPS  output pulse,
  such as sawtooth, quantization, and so on.

  And that what I hope my system will fix.

  > Here's another GPSDO related link for you to study.


  Yes. Drool, drool:)

  Did you know your page at


  is referenced by Bob at "Rubidium and GPS DO From China" on eBay?

  To see it, go to eBay and search

  RUBIDIUM DATUM LPRO 10Mhz Oscillator longer Lamp life

  The graph at the bottom of the page is what convinced me to  get the
  LPRO instead  of  the cheaper rubidium version on  eBay.  Your graph
  shows the  LPRO is just as good as the PRS10 for averaging  times of
  one second or longer. That is fantastic! Thanks very much.

  BTW, were  you going to send me the log20696.txt file shown  on your
  web page? Or maybe link to it so others can se it also?


  Thanks for all your help.

  Best Regards

  Mike Monett

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