[time-nuts] Rb Oscillator - rather fundamental question

Magnus Danielson magnus at rubidium.dyndns.org
Wed Feb 24 02:27:21 UTC 2010


Richard (Rick) Karlquist wrote:
> 
> 
> 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.

Indeed.

> 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.

It should be pointed out that just because you have a caesium beam 
clock, or lately caesium fointain clock, means that you achieve the full 
definition of "primary standard" as give above.

A beam standard has many different flaws. Older beam standards will age 
since the C-field is not being maintained. Modern "digital" clocks has a 
servo-loop to ensure that.

RF-amplitude, phase-difference betweeen the interaction fields, 
temperature/average speed of beam provides doppler shifts etc.

The repeatability brings many issues in. Caesium beams have excellent 
repeatability compared to rubidium gas-cells.

Any gas-cell standard has wall-shifts, buffer-gas shifts, temperature 
shift, excitation signal strength and polarisation etc. etc. etc.

Rubidium cells forms nice gas-cell standards even if the gas cell 
technology is limited. Price/performance is usually very good.

So... beyond the atom being used, the clock type and the details of its 
operation needs to be overviewed before a well-founded judgement of it's 
  stability and repeatability... and thus primary standard ability, can 
be given.

Cheers,
Magnus



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