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
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