[time-nuts] Another GPSDO project

[Don Johnson]

Hi group,

I an still somewhat in the middle of this project but what's left is mostly fine tuning and the cosmetics of getting it enclosed. I have been using a Trimble Thunderbolt for over a year now and, for the most part, find it a very nice long term comparative reference for my Efratom Rb. I purchased this Thunderbolt as a Starter kit that included the antenna and downlead on eBay and like the overall performance. The weakest part however, of the TB is the oscillator. With room temperature cycling with heating and AC, I would typically see a +-20 nS sine wave looking steering pattern with a period of about 10-20 minutes. I always wanted to modify my unit to use a better doubled ovenized oscillator. 

Recently, George Dubovsky offered some Trimble Thunderbolts (boards only) for sale and I jumped at the chance to use one of his boards as an experimental set up and not mess up my only GPS reference. Some of you may know, the TB board is made to solder SMB connectors for an external oscillator. These connector pads are useable only after you remove the onboard oscillator. There is one for the 10Mhz in and another for the VFC out. The electrical characteristics nicely match an HP 10811 and that's what I used. Using Thunderbolt Monitor software, the VFC slope was modified from -5.00 to -0.45 Hz/V and the Filter TC and Damping were changed to 400 and 1.1 respectively to more gently steer the 10811. The results seem to be working great. I now have a  very gentle 40-minute, +- 5 nS pattern as referenced to my local Rb. I am using the more stable and low phase noise 10811-80168 so I didn't compromise on the low phase noise of the original onboard oscillator but greatly improved stability. I am still working with filter TC and damping settings to see if further improvement can be had. The trick seems to be very gentle nudges without over steering on one side or "loosing it" on the other.

I would also like to experiment with the Fury board with an external oscillator to compare results. Are you listening Said?

Regards...
Don