[time-nuts] Noise types and Allan Deviation

[Bruce Griffiths]

Tom Van Baak wrote:
>> The tau range for which ADEV etc are sensibly independent of measurement
>> system bandwidth should differ significantly when comparing a pair of
>> hydrogen masers or a pair of low noise OCXOs (OSA8607, FTS1200 etc).
>>     
>
> I'm not sure what "significantly" is, but as you suggested,
> I do see more of a difference at low tau with two ultra low
> noise references than with two normal OCXO. The effect
> is zero to a couple of dB. See plots below.
>
>   
>> The lower limit for an 8607 or a good FTS1200 is probably in the 1 to
>> 10s range.
>> That is around the value of tau for which ADEV vs tau first approximates
>> a horizontal straight line or reaches a minimum.
>>
>> Bruce
>>     
>
> Right you are. It's 2 seconds for the pair of 8607's and near 10
> seconds for a 8607 vs. active H-maser. Compare these:
> http://www.leapsecond.com/pages/adev-bw/tbolt-tbolt-bw.gif
> http://www.leapsecond.com/pages/adev-bw/8607-8607-bw.gif
> http://www.leapsecond.com/pages/adev-bw/8607-hm75-bw.gif
>
> I'll add a maser-maser plot if I have it.
>
> /tvb
>
>
>   
Tom

Nice to see that my guesses weren't too far off.

The really interesting part is that the source of the phase noise
components whose contribution to ADEV, MDEV, etc is dependent on the
measurement system noise bandwidth is actually the buffer amplifier
chain and not the oscillator itself.

Thus in a poorly designed oscillator the minimum value of tau for which
ADEV, MDEV etc are independent of the measurement system noise bandwidth
could vary significantly from the results found with the sources you
have tested.

Bruce