[time-nuts] Re: Primary frequency standard

[Attila Kinali]

On Fri, 26 Mar 2021 14:42:25 -0700
<cdelect at juno.com> wrote:

> "It would be cool to say one has a "primary standard", .... So, I think
> we should be careful with the term, it's get thrown around too lightly."
> 
> Any of the hobby grade used Cesium standards is actually a Primary
> frequency standard as it does meet the definition of:
> 
> A frequency source that meets national standards for accuracy and
> operates without the need for calibration against an external standard.


It is a bit more complicated than this. What the "national standard
for accuracy" is depends on the application and the needs thereof.
Besides, for most people "primary standard" means more something 
along the lines of "does not need calibration to be accurate."

If you only need an accuracy of 1e-6, then a (good) OCXO will
be your primary standard.

If you need an accuracy of 1e-9, then a Rb vapor cell will
be your primary standard.

If you need an accuracy of 1e-12, then a Cs beam will be
your primary standard.

If you need an accuracy of 1e-15, then a Cs fountain will be
your primary standard.

For the better equiped NMIs, the Cs beam is only a secondary
standard that provides the flywheel between the times when
the Cs fountain is running.

E.g. METAS uses an active hydrogen maser as their main source
for the local realization of UTC. The drift of this maser
is corrected by two HP 5071 against which it is measured.
And the frequency drift of the HP 5071s is corrected by
measuring them against their Cs fountain once a month (IIRC).
Though, I think METAS is more of an outlier when it comes
to NMIs than the norm. There are plenty of NMIs that only
have only some Cs beam standards. And then there are some
that don't even have a local realization of UTC.

			Attila Kinali

-- 
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always 
wonder about. And that's why we do research.
		-- Kobayashi Makoto