[time-nuts] fleabay cesium
magnus at rubidium.dyndns.org
Sat May 9 09:17:32 UTC 2009
> Well Dick,
> Assuming it is setup and properly adjusted, it is absolute as a
> frequency reference by "definition." It does have a given resolution
> as stated in the manufacturer's manual for the instrument. This
> resolution [or noise floor as I see it] is due to a number of factors,
> not least of which is the condition of the physics package (the actual
> Cesium tube). They do ware out !
Actually, Cesiums can both be offset in frequency and have drift
components in them. There are a number of systematic issues to handle.
The magnitude of these depends on how they are handled in the design as
well as how they are maintained.
Also, general relativity comes in so the systematic error of running the
clocks above the gravity of standard sea level acceleration comes into
mind. For a relative comparision at that site it does not care that much
There are many features of a Cesium beam which affects the precision,
but none of them actually runs at the magical number, they are all
skewed by the C-field and compensation for this needs to be done.
However, while the C-field skew can be calculated, it is hard to know
the actual C-field. Modern "digital" Cesiums use a mechanism to measure
and stabilize the C-field by measuring the position of the nearby Rabi
responses in addition to the central Rabi and Ramsey features. Such an
approach can remove the major part of the frequency error traceable to
that systematic issue. For older cesium beams you adjusted the current
in the C-field to match up, so calibration to match frequency is needed.
Older Cesiums does not have this "digital" control loop, so it would be
inaccurate to say that they by definition is sharp on. They are not and
never really be sharp on.
There is a number of other stability issues of various forms to add onto
this, but I think it is out of scope here.
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