[time-nuts] Ultra Stable Rb

[djl]

Very nice experiment. What do you think might be affected by pressure? 
At least in the physics pkg?
Don

On 2020-04-09 09:36, 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
> contemplated which Rb oscillator to use and finally decided on the
> LPRO-101 since it had no DDS (and thus no discrete steps) and a wide
> supply voltage range (and I had several of them).  The unit was
> mounted to the heavy aluminum lid of the chamber (which had five
> 61-pin electrical feedthru connectors, so no problem there).  Kapton
> heaters were applied to the outside of the chamber and connected to a
> temperature controller, and a low noise power supply that could be
> varied from 18-32 volts was used to power the LPRO.  A modified NTBW50
> is used to monitor the output of the LPRO.  A UPS and line conditioner
> were also added to the system.
> 
> 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.
> 
> 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.
> 
> At this point (Jan 2019) I connected the NTBW EFC drive to the LPRO.
> Now the LPRO could be disciplined, but as we all know GPS degrades the
> short-term performance.  So, I run the unit with discipline disabled
> and just manually change the DAC voltage to keep the LPRO on
> frequency.  The supply voltage, chamber pressure, and chamber
> temperature have not been touched since that point.  By knowing the
> EFC gain I can calculate the aging.
> 
> For the 238 day period ending 3Nov19 the aging was -3.76x10E-14 /day.
> Not bad as far as Rb goes, but I can certainly do better.
> 
> The next step I would like to take is to move the C-field adjustment
> outside the chamber (and increase its resolution) so that I can put
> the oscillator on frequency without any changes in supply voltage.
> And again disconnect the EFC (since there is a temperature dependence
> on the DAC value).  Then I should be able to get back to finding the
> exact pressure the chamber should be set at.
> 
> The whole system is contained in a very short rack with the chamber on
> top of it and an insulation shield over it.  I call it RUFUS (RUbidium
> Frequency Ultra Stable).  It lives underneath the stairs going to the
> basement.
> 
> I have also considered just building a box to drive the EFC and
> increment the voltage at the proper rate for whatever the drift might
> be for the temperature, pressure, and supply voltage that the
> oscillator might be at.  Too much fun!
> 
> Sorry for the long post.  I'm hoping to have a detailed paper with all
> the details at some point, but getting all the data of course goes
> very slow.
> 
> Regards,
> Skip Withrow
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.

-- 
Dr. Don Latham  AJ7LL
PO Box 404, Frenchtown, MT, 59834
VOX: 406-626-4304