[time-nuts] Rb Oscillator - rather fundamental question

Rick Karlquist richard at karlquist.com
Tue Feb 23 20:00:54 UTC 2010


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.)
>

Sorry but you have completely misunderstood the concept.
It is admittedly a difficult concept to grasp; I know it took
me a long time.

A hydrogen maser with the wall shift servo'ed out will run rings
around a compact Cs beam clock like the HP5062, used on submarines.
(An interesting trivia item is that I don't believe the 5061 can
fit through a submarine hatch).  The 5062 is still a primary
frequency standard and the hydrogen maser is still a secondary
frequency standard.

Regarding "drift" of primary cesium beam standards:  the 5071A has
unmeasurable drift, aging and tempco, down to a measurement limit
of at least 1E-15.  It has a typical *random* error of a few parts
in 1E-13.  The systematic error (average error of all 5071A's built)
has been established to be below 1E-14.  It will always be a
primary standard even in the presence of longer reversible optically pumped
laboratory Cs beam standards of higher accuracy and better short term
stability, or cesium fountains, etc.  Even the 5061A/B is considered
a primary standard, albeit with reduced accuracy, even though it
has a measurable tempco.  We were very proud of the E1938A crystal
oscillator when it was able to meet the 5061 tempco spec.  It
is in no way a primary frequency standard regardless of that
or any other accomplishment.

Primary means that the clock will meet its spec without being
"calibrated" against a better clock.  Secondary means that
calibration against a primary standard is necessary.

Rick Karlquist N6RK




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