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

Steve Rooke sar10538 at gmail.com
Fri Oct 17 18:30:14 UTC 2008


2008/10/17 Mike Monett <XDE-L2G3 at myamail.com>:

>  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.

One of the greatest problems in any pll circuit is instability caused
by time delays in the feedback low pass circuit. In a locked system
where the feedback voltage approaches the rails, stability improves
due to the lower error signal. Getting the time constant of a simple
low pass feedback circuit right is not easy especially in high
precision phase locked systems. Ideally the filter should have a very
low frequency cutoff but have very low time delay and the two are
usually at odds with each other in simple designs.

73
Steve
-- 
Steve Rooke - ZL3TUV & G8KVD
Omnium finis imminet




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