[time-nuts] Some long-term data
Tom Clark, K3IO
K3IO at verizon.net
Sun Dec 24 05:26:21 UTC 2006
As a comment on this dialogue:
> > Such as temperature and humidity. One of the Cesiums seemed to have a rathe
r
> > large frequency offset. Did you drift-compensate your ADEV measures or not?
>
> No, I ran the ADEV in Stable32 without removing drift. My understanding
> is that linear drift shouldn't affect the calculation. I was also a
> little concerned whether drift removal would work very well given that
> both Cesiums had periods where the offset changed for quite a long
> period (CS1 near the end of the data, CS2 at the beginning) and I wasn't
> sure what impact that might have.
The straight ADEV will certainly be affected if there is a drift. The
ADEV for delta time=T is simply the RMS difference between the clock
phase at time t and time t+T. Any rate offset will appear in this
calculation.
If you are using STABLE32, you might want to fit out a smooth
polynomial with the tools that are supplied. Or as an alternative, use
the Modified ADEV; this computes the difference to both earlier and
later times and implicitly includes the removal of a simple offset.
John -- you can get info on STABLE32 at [1]http://www.wriley.com/.
Even if you don't get the software, the manual is a good bible and can
be fetched at [2]http://www.wriley.com/Manual146.pdf. Pay special
attention in the manual to pages 31-49 for a discussion of the types
of variances that are of use in the community. FYI -- The folks who
maintain the Rb and Cs clocks on the GPS satellites are sold on the
Hadamard variance as a long-term performance measure.
To learn how to read an interpret the slopes on an ADEV plot, refer to
pages 77-83. On page 79, you will see that the Modified ADEV gives you
more information in the short-term (i.e. the area where the VCXO is
the principal contributor) part of the curve for White vs Flicker
phase modulation.
I highly recommend STABLE32 -- many of us have found it well worth the
$400 price that Bill Riley charges for his labor of love!
73, Tom
References
1. http://www.wriley.com/
2. http://www.wriley.com/Manual146.pdf
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