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

Tom Van Baak tvb at LeapSecond.com
Mon Aug 6 16:56:09 EDT 2018

> If you know the *source* of the bias you're trying to remove (i.e. you 
> know it's a sinusoidal frequency modulation), I don't know that it's any 
> different than removing long term drift.

The TimeLab 'n' command (apply notch filter to phase records) is specifically for this purpose. JohnM added it when he ran into an H-maser which suffered from some sort of consistent periodic modulation. It spoiled the ADEV plots, but it did so in a deterministic manner. In a case like this you either debug the root cause of the h/w problem and make the repair, or just "repair it" in s/w. Like you say, it's the same concept (and danger) as removing linear drift or other deterministic / model-able effects.

To explore this for yourself, you can use Stable32 to generate synthetic phase/frequency data with your choice of noise and modulation. Then use TimeLab to view the raw data and to apply the notch filter.

I've written up some quick examples here: www.leapsecond.com/pages/adev-fm/

The plots dramatically show what effect slow FM can have on an ADEV plot. It also shows how well the TimeLab notch filter works. If you don't have time to look at that page, I've attached one plot to whet your appetite.

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