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

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Aug 6 02:22:07 EDT 2018

In message <84a802ff-88f1-5f50-1f79-71d8ba3c42c6 at rubidium.dyndns.org>, Magnus D
anielson writes:

>What does exists is a formula for how a single sine spur would produce
>ADEV. A FM deviation with low enough modulation index creates two
>side-bands of opposite sign but same amplitude.

I find the easiest way to wrap my head around this is to think
about measuring Adev by timing zero-crossings.

If we had a perfect zero-crossing detector, perfect AM would not
matter, because perfect AM does not move the zero-crossings.

But in theory at least one electron, and in practice many, must run
the opposite direction before we can detect the zero-crossing, so
even perfect AM matters in practice.  (Interesting detail: Measuring
at high impedance may be smarter than at 50 Ohm impedance)

For FM or PM there is no loophole: They move the zero-crossings and that's that.

Depending on the modulation signal, there may be moments where the
zero-crossing is "where it should be", for instance if the modulation
is sine or triangular, but not if it is a signed square wave.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
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Never attribute to malice what can adequately be explained by incompetence.

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