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

jimlux jimlux at earthlink.net
Mon Aug 6 09:17:20 EDT 2018


On 8/5/18 11:22 PM, 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.
<snip>
> 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.
> 



What about doing some sort of fit to the measurement data before 
calculating the ADEV? Similar to removing a linear ramp.

basically you'd solve for the three sine parameters (f, phase, 
amp/deviation), then remove that from the data, then run the ADEV 
calculation.




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