[time-nuts] TimeLab and the Adev plot

Hal Murray hmurray at megapathdsl.net
Wed Jan 29 00:16:12 EST 2014


> Whenever I plot an ADEV chart for a given oscillator I see the diagonal line
> descending from upper left to lower right.

> However at near the end of the plot, in this case 3600 sec at 1 sample per
> sec.  the trend begins to reverse at about 1200 sec. in.

> Is this a function of the shorter term noise being averaged out and some
> other drift the major contribution to the ADEV? 

If you collect enough data, it should be a V shaped graph.

On the left side, the error is dominated by measurement noise.  Longer times 
between samples average the noise over a longer time so things look better.

On the right side, the error is dominated by the drift in the clock you are 
measuring.  Longer times between samples give the clock more opportunity to 
drift.

There should be several good URLs out there describing that.  I don't have 
one handy.  It's pretty obvious after you see it.



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