[time-nuts] Rb Oscillator - rather fundamental question

Richard (Rick) Karlquist richard at karlquist.com
Tue Feb 23 17:13:23 UTC 2010

David C. Partridge wrote:
> Cough - the rubidium clock or oscillator does have an intrinsic frequency,
> which is the rubidium hyperfine transition of 6 834 682 610.904 324 Hz, it's
> just that the frequency generated by the transition in question isn't used
> to DEFINE the second, so by definition, it must be secondary.  Only a
> Caesium clock is a primary standard, as the second is DEFINED to be the time
> taken for 9,192,631,770 cycles of the radiation corresponding to the
> transition between the two hyperfine levels of the ground state of the
> caesium 133 atom.[1].
> Unless of course they changed the rules recently ...
> [1] <http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html>
> Dave

Well, what you said is true as far as it goes, but not the whole story.
The fact that a clock is based on cesium does not necessarily mean it
is a primary standard.  For example the "chip scale atomic clock" uses
cesium and is a secondary standard.  OTOH, certain experimental clocks
based on atoms such as rubidium, mercury, etc could be considered
primary standards in spite of the definition of the second.

It's not the type of atom, but the type of clock that is crucial.
"Cesium" usually refers to an atomic beam clock and "Rubidium" usually
refer to a gas cell device.  In an atomic beam, the atoms are, on the
average, unperturbed, and will transition at exactly the 9192... 
frequency in the definition of the second.  Except that they are offset
from this frequency by a known amount due to the C-field.  In a gas
cell device, the atoms are perturbed by the buffer gas which results
in a unknown frequency shift from the 6834... frequency.  You have
to remove this offset by comparing to a primary standard.

We used to say that in theory you could build a cesium beam standard
from a kit of parts on a desert island having no other clocks, and when 
you turned it on, it would be on the correct frequency (within a
tolerance) guaranteed by design/physics.  There is no way you
could do this with a rubidium or cesium gas cell standard
to any kind of accuracy associated with atomic clocks (it would only be
in the general neighborhood of 6834...)

That is the difference between primary and secondary standards.
Another difference is that secondary standard have "aging" and
primary standards don't.

Rick Karlquist N6RK

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