[time-nuts] OCXO pressure sensitivity before TCXO

[jimlux]

On 8/3/19 7:22 AM, ew via time-nuts wrote:
> Are wee missing something. Reading Bill Slade post and old NASA attachments the attached plot to me was an eye opener. Our work has always been focused on Rb pressure compensation. Are we overlooking something. Has any one done some work on this subject
> Thanks    Bert Kehren
> 

Those papers are reporting on designs from the 80s - things have 
improved a bit since then.

The thing that potentially causes problems in all "sealed" oscillators 
are mechanical stresses - whether from a hermetic enclosure flexing due 
to pressure changes or from thermal expansion.

The latest USOs from APL take pains to minimize all of those - One thing 
that is interesting is that the reason for a dewar is partially for 
thermal isolation - in space, you're in a very hard vacuum, which is a 
pretty decent insulator so the dewar isn't improving things - and also 
for mechanical consistency - you can mount everything on a subchassis 
that sits inside the dewar and is mechanically and thermally isolated 
from the surrounding dewar.  It also lets you test in air on Earth.

The vacuum in the dewar is probably worse than the vacuum in space 
(depending on where you are "in space"), but nobody has figured out a 
good way to reliably "vent" things - and then you still have the "test 
on the ground" which is in a not very good vacuum: 1e-5 torr (1e-3 Pa) 
is typical for thermal vacuum testing.

One other thing to know about those USOs - they start about 1000 quartz 
blanks that are mounted in holders, and wind up with a couple dozen 
oscillators in dewars, and then run them all for a long time (many 
months) and watch them.  2 or 3 out of a batch will be the ones selected 
for use - based on their observed stability -


This is a very, very different manufacturing model from something where 
you make 100s a month - and that's why they cost $1M each and take 2-3 
years to make. (well, there's a heapin' helpin' of paperwork that goes 
with the USO, too, and that isn't inexpensive to produce)