[time-nuts] Re: Project Great

[Magnus Danielson]

Hi,

Den 2021-11-29 kl. 10:57, skrev Hal Murray:
> jim at 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?

"primary standard" is an overloaded term, so depending on context a 
particular product may suffice to be a "primary standard" in some 
context but not in others. In general a "primary standard" does not need 
external corrections, and some clocks will have less than perfect 
mechanisms for their systematic variations or drift, which does not 
covers the ADEV, as ADEV is not covering systematic properties but is 
only intended to cover random noise. For metrology contexts, "primary 
standards" is only a handful of cesium foutains to achieve frequency 
accuracy, where as the bulk of atomics clocks contribute stability (i.e. 
optimal ADEV).

In telecom, a "Primary Reference Clock (PRC)" or "Primary Reference 
Source (PRS)" ensures frequency within +/- 1E-11, which used to be what 
analog cesiums could achieve. Requirements have since progressed, 
especially for the phase as time is now an added.

So, in general, it's about the repetitive independent generation of 
phase, frequency and drift. Stability in terms of ADEV and TDEV then 
comes in as othogonal requirement.

I think you will find that IEEE Std 1139 and 1193 has further 
refinements as they pop out of the approval and publishing. 1139 draft 
is now in balloting process. We still work on 1193. I also recommend 
having a look at VIM and GUM documents as available from BIPM.

Cheers,
Magnus