[time-nuts] Rb Oscillator - rather fundamental question

WarrenS warrensjmail-one at yahoo.com
Tue Feb 23 19:36:00 UTC 2010

All very informative and useful information for sure and good to know,
But I'm thinking the real difference between a primary and secondary 
Has More to do with if there is anything else more accurate and repeatable 
I'd guess a Rb would of made a great cave man Primary standard.
And sounds like it will NOT be long before the Freq and drift of a CS 
Primary will be consider just another secondary standard that will have to 
be calibrated.
(to get the 1e-16 + or whatever accuracy/repeatability  it is they are now 
working on.)

[time-nuts] Rb Oscillator - rather fundamental question

Sorry - I should have written a longer response - but you've put it all
straight anyway.

I wonder how long it will be before the definition of the second is changed
to use the newer types of clocks using strontium, ytterbium, mercury, or
aluminium (which I believe is the current "front runner")?


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 
Behalf Of Richard (Rick) Karlquist
Sent: 23 February 2010 17:13
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Rb Oscillator - rather fundamental question

> Well, what you said is true as far as it goes, but not the whole story.

Rick Karlquist N6RK

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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