[time-nuts] Rb Oscillator - rather fundamental question

[John Ackermann N8UR]

I don't think I agree with that, Warren.  I'd view a primary standard as 
an intrinsic one -- that is accurate by definition and doesn't need 
calibration against another, higher level, source.

A cesium beam standard is based on the same physical phenomenon that 
defines the second, so if it's working, it's "right" within some degree 
of tolerance.

As others have pointed out, a gas cell standard is subject to pulling 
and needs to be set to a correct value (and also has drift over time). 
And standards based on other ions may actually be more accurate and 
stable than cesium, but they still need to be related to the official 
definition of the second through a measurement.

In either case, they need to be referred to the cesium transition, so 
they are not primary standards.

Make sense?

John
----

WarrenS wrote:
> All very informative and useful information for sure and good to know,
> But I'm thinking the real difference between a primary and secondary 
> standard,
> Has More to do with if there is anything else more accurate and 
> repeatable available.
> 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.)
> 
> ws
> ************
> [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")?
> 
> Cheers
> Dave
> 
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at 
> febo.com] On
> 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
> 
> 
> <snip>
>> Well, what you said is true as far as it goes, but not the whole story.
> <snip>
> 
> 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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