Steve Rooke wrote:
2009/8/7 Magnus Danielson <[email protected]>:
Steve Rooke wrote:
2009/8/6 Magnus Danielson <[email protected]>:
Ulrich Bangert wrote:
...
Well, stability over time is what exacly is displayed in a
tau-sigma-diagram
of an oscillator. Since only a few words before he is saying that he is
NOT
intersted into Allan Deviation plots, then he is perhaps interested into
something else?
Yes. Sigma-Tau plots of the Allan Deviation fame (with friends) addresses
the instability of the noise part of things. For crystal oscillators and
other non-atomic oscillators "linear" factors in frequency drift is not
best
specified, described or measured using that method, which was invented
purely to be able to handle the phase noise side of things, not the slow
frequency drift.
For these sorts of measurements on drifting oscillators would it not
be prudent to use the Hadamard Deviation?
Hadamard Deviation does not fully cancel the non-stable drift.
From the flow of the discussion I gathered, perhaps incorrectly, that
we were talking about the fairly linear drift in crystal oscillators
for which HD handles. If there is a non-stable drift, it's going to be
an interesting exercise to be able to process that out of the data
easily. Agreed if the drift can be characterized to fit some form
function but I would call that a stable drift anyway. I also had the
impression, and please correct me if I'm wrong, that the use of the
additional adjacent data point in HD calculations resolved the issues
with drift at any point in the data stream.
As been investigated before, linear drift is an approximation and better
models have been found to match the data better, and also been shown to
connect to the various "aging" mechanisms known to exist even if
production techniques can reduce many of them to low levels.
HDEV does a better, but not perfect, job at canceling the non-constant
drift. If you want ADEV or similar, then using methods described will
get you better results.
I found a nice little graphical tool that let's you pull data in, match
against one of several models and then see the produced result. Nice
tool to get started. Can't recall the name right now.
Cheers,
Magnus
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