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

[Bruce Griffiths]

Surely all that's required is a simplistic worst case analysis.
Just assume that the value of Tau is always the worst possible so the full effect of the modulation is always seen.

If the worst case phase modulation is say phi

then the worst case ADEV (Tau) will be proportional to phi*T/Tau
where T is the nominal signal period.

i.e. ADEV(Tau) < constant*phi*T/Tau

Bruce
> On 07 August 2018 at 10:37 "Richard (Rick) Karlquist" <richard at karlquist.com> wrote:
> 
> 
> The discussion has gotten off track.
> I probably didn't make myself clear.
> 
> When I talked about cleaning up the signal, I meant
> exactly that, cleaning up the signal using a filter.
> As opposed to cleaning up the measurement after the
> fact.  I just need to know how well I need to filter
> the signal to meet a certain ADEV measurement level.
> 
> The FM I have is not "slow".  The rate
> is in the MHz range.  Can Stable 32 simulate that?
> 
> 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?
> 
> Thanks.
> 
> Rick
> 
> On 8/6/2018 1:56 PM, Tom Van Baak wrote:
> >> 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.
> > 
> > /tvb
> > 
> > 
> > 
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