[time-nuts] ADEV noise bandwidth (was LPRO Rubidium Performance)

[Magnus Danielson]

On 02/13/2012 02:19 AM, John Miles wrote:
>> The ADEV difference of about 6 db at 1ms tau can be explained by the fact
>> that if I apply a 500 Hz LP  filter to my 9600 sps raw data, the same
> filter
>> used on the 5120A's  1K sps data, then even our 1ms ADEV answers
>> become very close.
>> I have found that using a 1/2 zero tau BW filter like the 5120A does can
>> falsely lower its tau zero ADEV answer by 3 to 6 dB.
>> The 5120A's use of a 1/2 tau zero LP cutoff filter is why the 5120A ADEV
>> answers are generally not the same at Tau zero when sampled at different
>> tau
>> zero rates.
>
> At least some of that effect comes down to the improvement in the instrument
> floor at lower filter bandwidths, but I agree that not all of it does.  An
> example that bothers me is the BVA-vs-H-maser plot at
> http://www.leapsecond.com/pages/adev-bw/ compared to the BVA J1 vs J2 plot
> right below it.  The latter is mostly determined by the instrument floor,
> since J1 and J2 are driven by the same source through two buffers that are
> presumably very phase-stable.

Even if it is a bit hard to see, that white phase modulation does seem 
to follow the power-laws of

http://en.wikipedia.org/wiki/Allan_variance#Power-law_noise

Notice how the square root of bandwidth will scale the Allan deviation 
result.

PS. I just added the Allan deviation formulas to assist you.

Cheers,
Magnus