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

Mike Monett XDE-L2G3 at myamail.com
Thu Oct 9 08:09:08 UTC 2008


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

  > Note another equivalent technique is to use two serial  ports; one
  > for the  receiver  (record  sawtooth corrections)  and  one  for a
  > 53132A-style counter  (record  TI measurements)  and  then  do the
  > calculations in  software. TAC32 does this. I think  TBoltMon also
  > allows it. Roughly, it's a trade-off in equipment and complexity.

  > They give essentially the same performance result.

  I have to learn more about how you do your measurements. A 53132A is
  way out  of  my price range at the moment. But I  do  have  a 53310A
  which should give comparable results.

  > There are  still  many  other sources of  noise,  both  short- and
  > long-term in  an OEM GPS receiver/antenna system. That's  why even
  > if there  were  zero  quantization error, you  would  still  see a
  > couple of ns rms error in the 1PPS output.

  I am interested in learning more about these errors. Is there a list
  somewhere that describes the sources of error, and what can  be done
  to reduce it? I keep coming across hints of doing this  in software,
  but it would seem that has to be done in the signal  processing part
  of the receiver. But we don't have access to that. Is there anything
  else we can do to help remove some of these errors?

  >> Thanks. I  had not found that page yet. Just judging  by eyeball,
  >> the pink  and  yellow traces don't seem to track  very  well. Any
  >> reason?

  > On that  plot,  the various runs weren't concurrent  so  you can't
  > look for  common-mode  effects.   Separate  runs,  showing typical
  > levels of jitter, all plotted on the same x-y scale. You've  got a
  > good eye.

  >> And what's  the blue trace for? I can't seem to  find  the parent
  >> page, so I don't know if you have already explained it.

  > This will help a bit more:

  >http://www.febo.com/pipermail/time-nuts/2005-April/018114.html

  Thanks - I'll have to study that a bit more. Right now I  can't make
  heads or  tails  of it - maybe I'm too tired and  need  to  get some
  sleep. Sure  - I just looked at the clock. It's  4am.  That explains
  it.

  >> Overall, I  was  a  little   disappointed  to  find  the sawtooth
  >> correction only gives about a factor of 3 or so improvement.

  > That's partly  because the sawtooth of the M12+ is small  to begin
  > with, at  least  compared to the earlier VP. So  there  isn't much
  > room for  sawtooth  correction to have a  massive  gain.  Also the
  > granularity of the quantization message from the receiver is 1 ns.

  That is very interesting. What do you mean by granularity? I  have a
  file somewhere  that  talks about the algorithm to  select  the next
  1PPS pulse.  It added 500ps to the rounding, so the result  would be
  on 1 ns boundaries. Is that what you meant?

  > And there's  an internal delay of one or two seconds  which  has a
  > slight effect on the quality of the correction.

  That is getting very subtle. How much effect does it have, and where
  on earth do you learn all this stuff?

  > Another way to look at it is this. If a perfect M12+ had, say, 2.5
  > ns of  jitter, and you saw 10 ns rms  without  sawtooth correction
  > and 3 ns rms with sawtooth correction you could say you achieved a
  > 15x improvement!  (10-2.5)/(3-2.5). Or if you didn't  say  it, the
  > marketing department certainly would.

  Don't you have to use the difference of squares in working  with rms
  values?

  > The closer you get to a few nanoseconds of jitter anyway, the more
  > all the  other  errors  in a typical cheap  OEM  GPS  receiver and
  > antenna system come into play. So once you average beyond a couple
  > of minutes the wander you see in the 1PPS has less and less  to do
  > with sawtooth and more to do with the sum of all the  other subtle
  > errors.

  That's what I need to learn more about.

  > Expensive geodetic  or  timing   receivers  use  an  assortment of
  > techniques to reduce the effect of these remaining  error sources,
  > to the point where you'll hear of millimeters and picoseconds (and
  > priced accordingly).

  Where can I learn more about these techniques?

  >> obtained with  my method have given over two orders  of magnitude
  >> improvement in noise reduction. That's with no optimization.

  > Averaging over  multiple samples, of course, gives a  reduction in
  > noise. You can see this in the ADEV plots at the end of:

  >http://www.leapsecond.com/pages/58503-cns2/

  That is  really  outstanding work. You have a  magic  skill  to take
  complex data  and  present  it in such a way to  make  it  clear and
  understandable.

  But to  be honest, all the noisy traces hurt my eyes. I am  now used
  to seeing  signals  with all the noise removed, so you  can  look at
  more subtle things in the waveforms.

  > On the  other  hand,  if  you found a way  to  get  two  orders of
  > magnitude better  performance  out of an M12+ without  the  use of
  > averaging then I'm all ears.

  This is  a  technique I discovered last century. It  is  really very
  simple, but  some people may have a difficult time believing  it can
  be done. So rather than waste everyone's time trying to  explain it,
  a demonstration with a credible and unbiased person such as yourself
  would probably work a lot better.

  I have  some  stuff on order, and plan to  make  two  simple systems
  using an  Oncore  UT  and a VCXO. It will take  about  two  or three
  months to  get  everything through Canadian  customs  and  build the
  prototype. It will be hand-wired, but I have developed a  new method
  of making  very rugged low-noise prototypes  suited  for frequencies
  well into the microwave region.

  When it is working, I'll make a copy and send it to you. Then if you
  would be  kind enough to make your magical measurements, I  would be
  very pleased to see the results.

  If it is what I think it will be, probably others would like  to see
  the results also.

  If you  are  interested,  I'll let you know my  progress  so  we can
  schedule a time that is convenient for you.

  >/tvb

  Best Regards,

  Mike Monett



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