[time-nuts] Noise types and Allan Deviation

[Bruce Griffiths]

Tom Van Baak wrote:
>> A table indicating the dependence of the ADEV of these phase noise types
>> on the measurement system bandwidth would also be helpful.
>>     
>
> Bruce,
>
> I've never bothered with the details of measurement system
> bandwidth as you describe. A couple of reasons.
>
> 1) Almost none of the thousand ADEV plots I've ever seen in
> commercial and academic literature ever mentions it. That's
> my first clue it might be a "Bruce thing" ;-)
>
> 2) It looks like it's a pain to compute, and if it doesn't end up
> making any difference then it's not worth the bother.
>   
The TSC5120A calculates it for you.
It can be difficult to do when using a time interval counter in some
systems.
However the noise bandwidth of a classical dual mixer system is well
defined and easy to calculate.
> 3) If you take NEQ BW into account you can see when it
> makes a difference and, as important, when it doesn't.
>
> Here's a nice example from last week:
>
> http://www.leapsecond.com/pages/tbolt-tc/tbolt-tbolt-neq-1.gif
>
> It's the ADEV of two non-optimized TBolt's against each other
> on a TSC 5120A (which allows you a selection of NEQ BW)
>
> The good news for you is that there are data points where the
> measurement bandwidth makes a difference. See tau 0.01
> and 0.02 for example.
>
> The better news is that for the rest of the plot, all the way from
> tau 0.1 to 100 000 seconds, it makes no difference at all, not
> even one pixel.
>
> /tvb
>
>
>
>   
Tom

When white phase modulation and flicker phase modulation are not the
dominant phase noise processes and white FM, flicker FM or random walk
FM dominate then the measured ADEV, MDEV will be independent of the
measurement system bandwidth.

This has implications for constructing an ADEV standard.
If a white phase noise source is used the resultant ADEV etc
measurements will depend on the measurement system noise bandwidth.
However if white FM phase noise is added the resultant ADEV etc
measurements will be independent of the measurement system bandwidth,
however this is somewhat more difficult to implement.

Bruce