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

John Miles john at miles.io
Mon Aug 6 21:39:01 EDT 2018

> Again, this is analogous to the 5071A having 1 MHz
> spurs of about -90 dBc, yet the ADEV is not much
> different than an open loop 10811.  How high would
> the 1 MHz spurs have to be to affect ADEV?

With ADEV, you have to consider the measurement bandwidth before you can ask
questions like this, much less answer them. :)  1 MHz spurs are far outside
the restricted measurement bandwidth that would be desirable when measuring
a high-performance source like a 5071A.  I suppose there might be some
influence if you use a DC-to-daylight sampling process such as a counter,
but the instrument noise floor will probably obscure any interferers in the
-90 dBc range.  

With a TimePod or similar device, a general rule of thumb is that a spur a
few Hz away from a 10 MHz carrier at -120 dBc will cause ripple in the 1E-11
to 1E-12 range.  This coincides with typical levels of coupling between
nearby RG-58 cables -- see page 38 of
http://www.miles.io/TimePod_5330A_user_manual.pdf for instance. That
particular situation wouldn't have shown up on a traditional counter-based
measurement, but others might.  

For instance, in your 1 MHz example, I doubt you'd see any effect at exactly
1 MHz, but if your spurs are at 1.000001 MHz and your counter is capable of
1-ns single-shot resolution, I can imagine that there will be some
corruption.  The exact numbers will depend on so many factors  that it's
almost easier to set up a test and  observe the effects firsthand.  

-- john, KE5FX
Miles Design LLC

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