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

[Magnus Danielson]

Hi,

On 08/06/2018 08:22 AM, Poul-Henning Kamp wrote:
> --------
> 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.
> 

One type of FM detector is to run it through a limiter, which kills most
of the AM, just as you do for frequency measurements, so yes. The moment
the transitions is where they should be is the through zero moment.

So yes, we expect to see the FM creep into ADEV just fine.

Cheers,
Magnus