John,
On 01/20/2015 01:55 AM, John Miles wrote:
John,
Your new three-corner hat feature is really cool. Already tried it, even
if I did not spend quality time on setting source labels correctly.
Whenever I have a setup capable of running it, I will test-spin it again.
I suspect that when you make separate measurements, the noise of the
individual sources does not match up perfectly which may reduce quality,
especially systematics like hum.
True, and Bill Riley's documents emphasize that simultaneous measurements are
best. That's easy to do with the SMA jacks on the TimePod but more of a
challenge with counters, unless you buy several of them and run them from
separate GPIB adapters or serial dongles.
IMO, if you can keep correlation, tonal artifacts, and other systematic errors
under control and make long-enough measurements to generate a lot of data
points in the tau range of interest, then the technique should work well with
non-simultaneous acquisitions. At the end of the day it's no better or worse
than the quality of the individual ADEV measurements. You have to be wary of
measurements at taus where any of the traces have large error bars or are
wandering up and down over time, or where some of the traces are suspiciously
close to each other.
Agree.
For oscillators, they should have been turned on long enough such that
any drift is negligible. Alternatively you process out the quadratic
trend out of it. The later should be accompanied by some quality measure
of how much remaining systematics there is (see Jim Barnes PTTI paper on
Drift Estimators).
Also, identifying systematic noises (such as in phase-noise plot) and
notching them out before processing should improve the quality of the
3/N-cornered hat processing, since it really is for solving power-sums
of random noise, but not for systematic noise.
Until such processing of each measurement is in place, we have to make
as clean measurement runs as possible, which is the basic quality of the
ADEV.
Cheers,
Magnus
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