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

[Richard (Rick) Karlquist]

On 3/13/2020 11:00 PM, 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.
> 

When I worked for HP, there was a saying that a primary frequency
standard was something you could build on desert island with no
access to another frequency standard to calibrate it against.
A synthesizer for C field adjustment doesn't count because it
could use the Cs under construction for a reference.  This saying
predates GPS, but you also don't get to have a GPS reference.
It sounds like something Chuck Little would have said.

Plug and play is a different criterion than "primary".  As
a member of the 5071 design team, I am proud to say that the
5071 is truly plug and play.  We took one of the prototypes
to NIST and set it up.  Plug it in, push the on button, wait
10 minutes, and we had a few parts in 10^13 accuracy as measured
by NIST.  After gravity normalization of course.  And remember,
the 5071 can run on its built in battery.  We had to go to
a lot of trouble to keep power consumption down to a few
dozen watts.

And of course there are TVB's adventures with 5071's.

The other end of the spectrum are laboratory standards that are
many orders of magnitude more accurate, but require a staff
of post-docs to maintain them.

At one time, only cesium standards were considered truly primary
because of the definition of the second.  However, the
quantum mechanical constants of other atoms such as Rb
have been measured to much more accuracy than the 5071
so that Rb standards can be considered traceable to Cs
if they are otherwise of primary architecture.  The key
idea is that all Rb atoms are absolutely identical.  Rb gas
cells are of course never primary.

If you want to build the worlds most accurate clock,
however, it has to be cesium, because the quantum mechanics
of other atoms can never be known to more accuracy than
the best cesium standard, AFAIK.  At least until the
definition of the second is changed.  It's under discussion,
but no decision yet.

I hope that makes sense.  It took me a while to grasp these
concepts.

Rick N6RK