[time-nuts] fleabay cesium

WB6BNQ wb6bnq at cox.net
Sat May 9 09:23:40 UTC 2009


Magnus,

That is why I said ". . . setup and properly adjusted . . "

Bill....WB6BNQ


Magnus Danielson wrote:

> WB6BNQ skrev:
> >    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
> usually.
>
> 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.
>
> Cheers,
> Magnus
>
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