[time-nuts] Time Interval Algebra?

[Tom Van Baak]

Brooke,

If you're handy with C here's some code for Allan Deviation:
  double adev (double phase[], long count)
  {
      long i;
      double sum, y0, y, dy;

      sum = 0.0;
      if (count > 2) {
          for (i = 1; i < count; i += 1) {
              y = phase[i] - phase[i - 1];
              if (i > 1) {
                  dy = y - y0;
                  sum += dy * dy;
              }
              y0 = y;
          }
      }
      return sqrt(sum / (2.0 * (count - 1)));
  }
/tvb

----- Original Message ----- 
From: "Brooke Clarke" <brooke at pacific.net>
To: <jra at febo.com>; <time-nuts at febo.com>
Sent: Sunday, December 19, 2004 16:07
Subject: Re: [time-nuts] Time Interval Algebra?


> Hi John:
> 
> I would think you would want to use your most stable (lowest Allan 
> variance at 1 second) 10 Mhz source as the clock for your time interval 
> counter.
> 
> It's all relative as to what's being measured.  Just think that you are 
> measuring the GPS 1 PPS (which in my view is the more correct point of 
> view since the short term (1 second) variation on the GPS is way larger 
> that that on the Rb if you want to see the GPS 1 second variation.).
> 
> If you assume the Rb is perfect then whatever the variation you see can 
> all be attributed to the GPS system.  This is probably correct for a 1 
> second Allan variation.  From the other point of view if you are looking 
> at very long term drift then it's certain to all be in the Rb source.  
> This would be the case for a 10,000 second Allan variation.
> 
> Some TI counters can make a new measurement once per second, but most 
> can not.  As an example of the latter case suppose that at time 0 
> there's a start signal, then at time 0.999 seconds that there's a stop 
> signal. The counter displays 0.999 but has not yet armed in time to 
> respond to the next start trigger at 1.000 seconds and so waits until 
> the next start signal at 2.000 seconds and then stops at 2.999 seconds 
> for another display of 0.999.
> 
> I'd like to see how the math works when using my SR620 counter so that I 
> could write my own Allan Variance software.
> 
> Have Fun,
> 
> Brooke Clarke, N6GCE
> 
> -- 
> w/Java http://www.PRC68.com
> w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
> http://www.precisionclock.com
> 
> 
> 
> 
> 
> 
> John Ackermann N8UR wrote:
> 
> > I have what may be a dumb question but I can't get my head around it 
> > (having lousy high school algebra grades is not a good omen for a 
> > future time-nut).
> >
> > I'm doing a time-interval measurement of Rb vs. GPS, using 1pps 
> > signals.  For convenience, I'm using the Rb as the reference for the 
> > counter.  The Rb 1pps is going to the counter "start" input, and the 
> > GPS 1pps from a UT+/TAC is going to the "stop" input.
> >
> > Over many days, the phase record indicates about a -1x10e-12 frequency 
> > offset.
> >
> > My confusion stems from the fact that the counter is clocked by the 
> > device under test (the Rb), not the real reference (GPS).  Does that 
> > mean that the measured phase is actually twice the actual drift, so my 
> > -1x10e-12 is actually -5x10e-13?  I think so, but I don't have a lot 
> > of confidence in that conclusion.
> >
> > Once that question is resolved, next is what impact, if any, this has 
> > on the AVAR calculation.  Is it the equivalent of measuring two 
> > identical units, so you'd divide AVAR by sqrt(2)?  (This I'm not so 
> > sure about, since true "identical units" would have independent noise, 
> > while here the "two" devices would be walking together.)
> >
> > I suppose the real answer is to use a GPSDO as the counter reference 
> > to effectively have zero offset against GPS, but I didn't think of 
> > that in time :-).
> >
> > Thanks for any enlightenment...
> >
> > John
> >
> >
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> >
> >
> 
> 
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