[time-nuts] Question about effect of spurious frequency modulation on Allan Deviation

Ralph Devoe rgdevoe at gmail.com
Tue Aug 7 09:12:48 EDT 2018


     There's a simple way to put an upper bound on the ADEV due to a spur,
by treating the spur as random noise. Assume that the carrier is 1 volt at
10 MHz and the spur is 80 db down. Then the spur has 10(-4) volt amplitude.
The slope of the carrier is 1/(2 pi f) or 16 ns/volt.  Then 10(-4) volts of
noise can shift the zero-crossing by 1.6 ps, which amounts to an ADEV of
1.6 x 10(-12) at a tau of 1 sec. This drops linearly with tau.
       The effect can be smaller than this, of course, since it can average
away, like tvb's plot show.

Ralph DeVoe


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