On 11/29/21 1:57 AM, Hal Murray wrote:
j...@luxfamily.com said:
And a lot ofsources may have a low flat spot in the curve, but it
eventually trends up. Except for primary standards like Cs beam.
What's magic about "primary standard" or "Cs beam" that keeps the ADEV from
trending up?
Their ultimate accuracy is dependent only on a invariant physical
property that is independent of time.
A quartz crystal ages. No matter how good your oven is, the frequency
will change over a long time.
Mercury ions, Cs ions (perhaps all ions?) have frequencies that are a
fundamental property of the ion. There might be practical
implementation limits or side effects that limit the lowest achievable
uncertainty - hot atoms will have higher variance than cold atoms, for
instance. So not all Cs primary standards have the same ultimate
performance.
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