[time-nuts] [Fwd: Re: What is the best counter for a Time Nuts?]

Mike Monett XDE-L2G3 at myamail.com
Fri Oct 17 00:58:07 UTC 2008


  >> hasweb at has.org.nz wrote:

  >> There is a 3575A manual for downloading at the Agilent ftp link.

  >> If you read it you will find that the specified error  with 10MHz
  >> inputs is much larger: 10-20 degrees or so.

  The calculation I gave is an upper bound on the stability. It relies
  on the  resolution.  The accuracy is basically  irrelevant  for this
  measurement. Once  the rate of change exceeds about 1/10  degree per
  second, it is impractical to record the data manually, and  you have
  to go to a different method.

  [...]

  >> The basic  principle is sound. It should be very easy  to  make a
  >> simple PWM  circuit  to measure phase  accurately  at  10MHz. You
  >> mentioned the  same thing earlier. Why not post the  schematic so
  >> we can take a look?

  >> Mike Monett

  >Mike

  > I did some time ago, for some examples (other  implementations are
  > also possible) see:

  > http://www.ko4bb.com/~bruce/LinearPhaseComparators.html

  Those are  all  variations  of an XOR.  This  is  perhaps  the worst
  possible phase  detector to use for precision  applications.  It has
  very high ripple at lock, which generates unwanted sidebands  in the
  PLL oscillator output.

  It requires  signals  90 degrees out of phase, so  you  have  to add
  extra logic to lock to signals that are in phase. This adds jitter.

  The output is not referenced to ground, but to some  floating level.
  This means very small offsets and drift will change the lock point.

  It is  also unsuitable for measuring phase angle. The  slope  of the
  transfer curve for the basic XOR reverses at +/- 90 degrees.  So you
  can't use it to measure phase angle.

  The AD9901  hase  all  the above problems but it  rails  at  +/- 180
  degrees instead. It cannot be used to measure phase angle either.

  The standard phase/frequency detector (PFD) is better  for precision
  PLL's since  the output signal is zero at lock.  This  means minimum
  ripple to generate sidebands. It can be reset to a known state so it
  can be  started  in  phase with  an  incoming  signal.  This greatly
  reduces the lock time.

  It wraps  at 360 degrees, so it is also better suited  for measuring
  phase angle.

  These are only some of the issues in analyzing a phase  detector for
  a precision  pll.  Here is a quickie  LTspice  analysis  showing the
  phase detector transfer curves for the three detectors. I  posted it
  so people would not get their mailbox clogged with attachments.

  http://silversol.net63.net/spice/phasedet/phasedet.htm

  [...]

  > The quickest and easiest is to use the AD9901 phase detector as it
  > only has  a few ns of non linearity at the end of  the  range. For
  > higher resolution just add a sigma delta ADC.

  The AD9901  is a very bad design. Probably the poorest  I  have ever
  seen. It  is  unsuitable for measuring phase  angle.  Adding  a high
  resolution ADC will do nothing to help.

  I notice  there is very little in the way of analysis on  your site.
  This means it is impossible to distinguish one circuit from another.

  What you  really  need  to do is go through  and  put  everything in
  LTspice so people can look at the circuit behavior in detail.

  Then you need to do a tolerance and sensitivity analysis to show how
  stable the circuits are over long periods.

  Then you  need to do a noise analysis to show why some  circuits are
  better than others.

  This would make your information much more valuable and interesting.

  One of my big problems right now is trying to understand why the XOR
  works so well in one of the GPSDO's in Tom's Allan variance pages. I
  can't find it at the moment, but I need to take a very close look at
  this design and see what is going on.

  [...]

  >Bruce

  Best Regards,

  Mike Monett



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