Steve Rooke wrote:
2009/8/7 Magnus Danielson <mag...@rubidium.dyndns.org>:
The Hadamard processing (Dev, ModDev or Tot) gives another B term, which
considering that B is fairly small gives a drift-gain. As the series
progresses, the drift derivate dies away faster (1/t^2 rather than 1/t) than
for Allan processing so the longer time sequence used, the better
suppression of the drift mechanism (which is true for Allan processing too).
So, in this context Hadamard is better... but it still does not nail it.
It may be sufficient however. Estimating A and B and remove the trend from
the data isn't too hard.
To a certain extent it depends a lot on the oscillator that is being
measured. Allen Deviation is considered as being the best tool for
this type of work but it does suffer when the subject under test has
significant drift.
Allan Deviation isn't qualitatively the best tool, just the most known.
Look at TotDev or TheoDev as they converge quicker and produce better
confidence interval for the same amount of data.
For the quality of xtals and sources that some
time-nuts are testing, this is unlikely to be a huge problem but for
lesser sources this is a real factor hence my suggestion for the use
of Hadamard Deviation.
Having done the exercise on a fellow time-nuts measurements I beg to
differ. It became clear that estimating the drift as a linear static
component and then calculate ADEV with raw samples and ADEV with drift
compensated samples it became clear that raw samples ADEV was infact
drift compensated as it leveled out on the drift value, which is expected.
You can't make the above assumption unless you know what the drift is
and know that the ADEV is above your drift level for the intended tau
range. The reason I keep pointing this thing out is that after having it
pointed out in several sources relating to how one does real
measurements I have done the exercise and been able to remove the
limiting drift component.
It's not advanced processing, so just do it rather than argue against it.
For practical purposes though, the xtals we use
are generally embedded as part of a GPSDO which will compensate for
the drift in the oscillator but cannot practically compensate for a
noisy xtal and HDEV would make comparing one source with another easy.
The part of the noise being at taus longer than the loop will be
replaced with the GPS-receivers output noise. Measuring loop-locked
oscillators isn't the same as stand alone oscillators, so if that is a
way to remove drift, it will give you false readings if you beleive you
are measuring the oscillator itself.
Cheers,
Magnus
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