[time-nuts] Noise types and Allan Deviation

[Magnus Danielson]

Bruce,

Bruce Griffiths skrev:
> Tom Van Baak wrote:
>> A number of you have asked about Allan deviation recently so
>> I thought I'd share this handy one-page chart that shows both
>> ADEV and MDEV for the following noise types:
>>
>> - white phase
>> - flicker phase
>> - white frequency
>> - flicker frequency
>> - random walk frequency
>>
>> http://www.leapsecond.com/notes/Exploring_Allan_Deviation.pdf
>>
>> It's in high resolution PDF format so if it looks blurry on your
>> screen either zoom by 2x or just print it on a laser printer.
>>
>> /tvb
>>
>>
>>
>>   
> Tom
> 
> A table indicating the dependence of the ADEV of these phase noise types
> on the measurement system bandwidth would also be helpful.
> 
> There seems to be some general confusion about this.
> In general, the measured ADEV, MDEV, etc., depends both on the phase
> noise characteristics of the 2 oscillators being compared and on the
> measurement system characteristics.
> 
> Thus without correction for the effect of the measurement system noise
> bandwidth, 2 different measurement systems measuring the same oscillator
> pair will produce different results even though both measurement systems
> are working correctly.

There is yeat more effects which may compromise the measurement. Length 
of measurement is certainly an issue. Stefano Bregni have made alot of 
analysis on this as apparent in his book and in his articles. ITU-T 
specifies that the measurement windown needs to be at least 12 times 
longer than the highest tau being measured. Similarly, there is a 
requirement on lower end of the tau scale. The number of samples used in 
the gliding window will affect the values computed compared to expected 
value. The use of drift rate compensation is yeat another issue.

So the lack of annotated bandwidth is just one of several omissions that 
will tweak data of the chart. There is more issues to be included for 
real measures. I would point out some other peculiarities in the data.

Cheers,
Magnus