[time-nuts] Pressure related rubidium oscillator aging

[Skip Withrow]

Hello Time-Nuts,

For the past several months I have been investigating the change in
aging rate of a rubidium oscillator with change in pressure.  This has
been done with an operating oscillator in a temperature controlled
vacuum chamber.

Obviously, the frequency changes (a lot) with pressure changes due to
'oil canning' of the rubidium cell.  What I have been interested in is
the behavior of the change in frequency (aging) with pressure. If an
inflection point can be found or the aging minimized there is
potential to have an oscillator that performs very well.

The only 'change in aging' mechanism that I have run across is helium
permeation of the rubidium cell.  FEI has a paper published when the
oscillators for the first GPS satellites were being developed.
Temex/Spectratime is the only company that I have seen that addresses
this in their specifications.

So far, I have been operating in the 10-50 Torr range and have seen a
very definite trend on aging with pressure.  But there is much more
research to do.  My big question is - what pressure related mechanism
might affect aging?  I would think that helium permeation would have a
somewhat longish time constant.

I'm wondering if anyone from the group has some experience in this area?

Thanks in advance for any help.

Regards,
Skip Withrow