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@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.