[time-nuts] Is 5061A by itself a primary reference? Was: Modern Rb atomic reference vs classic Cs

[Magnus Danielson]

Hi,

On 2020-03-14 07:00, Greg Maxwell wrote:
> On Sat, Mar 14, 2020 at 5:35 AM WB6BNQ <wb6bnq at cox.net> wrote:
>> *By definition a Cesium frequency standard is just that, an absolute
>> primary reference !  The only difference between the 5061A and another
>> Cesium reference is a matter of degree of closeness to the absolute
>> value.  For example the 5061A has a spec of +/- 1 in 10 to the -11th for
>> accuracy, whereas, 30 later, the hp5071 has a spec of +/- 1 in 10 to the
>> -12th.*
> Because the 5061a does not automatically perform c-field adjustment by
> itself and you cannot perform it without additional equipment (a
> synthesizer), I wonder if it's pedantically correct to describe the
> venerable 5061a _by itself_ as a primary reference?
>
> The composite system of 5061+synthesizer+person-to-operate-it is a
> primary reference.
>
> Maybe my perspective on this is somewhat warped because I found
> c-field adjustment on the 5061a to be a bit of a pain. :)
>
> Perhaps this is all minutia,  but I also though it might be worth
> reminding people that 5061a is not as plug-and-play as the 5071.

Well, I agree that the open-loop property of C-field is an issue.

The comes the question of what would manifest a primary reference. That
the HP sales people advocate for it is natural, but if we look at how
the metrology world looks at it.

A definition of what manifests a primary reference is found in the VIM
document, and it states:

5.4 (6.4)
primary measurement standard primary standard

measurement standard established using a primary reference measurement
procedure, or created as an artifact, chosen by convention

5.5 (6.5)
secondary measurement standard secondary standard

measurement standard established through calibration with respect to a
primary measurement standard for a quantity of the same kind

NOTE 1 Calibration may be obtained directly between a primary
measurement standard and a secondary measurement standard, or involve an
intermediate measuring system calibrated by the primary measurement
standard and assigning a measurement result to the secondary measurement
standard.

NOTE 2 A measurement standard having its quantity value assigned by a
ratio primary reference measurement procedure is a secondary measurement
standard.

 0.

    2.8
    primary reference measurement

    procedure

    primary reference procedure

    reference measurement procedure used to obtain a measurement result
    without relation to a measurement standard for a quantity of the
    same kind

So, that is kind of interesting, but not very enlightening. At least
there is a definition. So what ends up being the primary reference
clocks for BIPM in their role to maintain internationally recognized
SI-second?

After a bit of searching, this document illustrates what BIPM considers
active primary standards:

https://www.bipm.org/utils/en/pdf/time_ann_rep/Time_annual_report_2018/11_Table6_TAR18.pdf

This being a sample, but just to get out scales calibrated. Not a
HP5061A/B or HP5071A in sight. Those show up as clocks contributing into
EAL, which is then corrected into TAI using the primary reference clocks
measurement of caesium transition frequency.

The caesium fountains being used has a wide range of work done on them
to establish shifts that perturb their absolute frequency observation. I
actually asked about the C-field on the NPL primary reference clock as I
visited there earlier this year. Turns out they actually run it open
loop, but only after making careful checks to see the effect and
conclude that it is actually very stable with the setting they have, and
they have estblished the corrections for it. They have also compensated
for the shift due to cross-section collisions, microlensing etc. Quite a
bit of things goes into ensuring shifts is either first degree cancelled
itself, or compensated but calibrated on the device itself. That is what
is expected of a candidate to become a primary reference clock, and even
as you do it, that's not on itself sufficient to actually become it. If
you do, and achieve the performance, consistently, then naturally after
running a number of hoops it would be welcome to contribute.

Since I am not made out of money, I do not think I will have a primary
reference clock in my lab. Cool clocks, sure, but not that cool. If I
end up being very lucky, maybe I will end up having one, but it does not
seem realistic.

Cheers,
Magnus