[time-nuts] Ultra Stable Rb

[jimlux]

On 4/9/20 8:36 AM, Skip Withrow wrote:
> Hello Time-Nuts,
> This is a subject that I have been interested in for quite some time.
> 
> If you do some searching on Rb oscillator aging, there is a paper from
> FEI that showed GPS units aging in vacuum  (and space) differently (as
> in opposite sign) than at sea level.  My thought was that there should
> be a pressure where the aging should go to zero.
> 
> I have been told by several people (that should know) that this has
> been studied in the industry and has never come to anything.
> 
> But I'm a sucker for punishment.  In April of 2018 I acquired a very
> nice vacuum chamber surplus from the aerospace industry.  I
<snip>

> I did not want to  drive the EFC (to remove a many variables as
> possible).  The C-field was set to get the unit about on frequency at
> around 20Torr, then the supply voltage was tweaked to put it exactly
> on frequency.


If I had to guess (and that's what it is) I would say you're seeing an 
effect of the internal temperature distribution being dominated by gas 
conduction/convection to radiation.  The mean free path of air at 10 hPa 
(10 mBar, 7.6 Torr) is about 6.7 microns.  That's probably pretty small 
compared to the assembly dimensions, but it's starting to get bigger.

At 1 micron (0.001 Torr) MFP is  around 7cm



> 
> I have found that, indeed, the aging direction can be changed with
> pressure.  And there is a pressure that you can get the drift to zero.
> However, another fly in the ointment is that changing the supply
> voltage to put the unit on frequency also changes the aging.