Hi
Something to think about as you go to tune your Rb: 1) If the tune range is 5 V for 5 x 10^-9 (+/- 2.5x10^-9) then its a ppb / volt or a ppt / mV. (from http://www.ham-radio.com/sbms/LPRO-101.pdf) 2) If you are after a goal of 1x10^-13, then thats 0.1 ppt and 0.1 mV 3) The Rb pulls a half amp to an amp running normally. 4) One foot of number 18 wire is 6.3 mOhms (from http://www.thelearningpit.com/elec/tools/tables/Wire_table.htm) 5) One amp in that wire will give you 6.3 mV 6) Copper wire has a temperature coefficient of 0.00393 / C at room (1% change in 2.5 C) (from http://www.cirris.com/testing/temperature/copper.html) 7) The current in the Rb heater will move around a bit The current and resistance would have to be stable to 1/630 or 0.15% for it to be negligible relative to your goal. At 1.5% it would be the same as the goal. Your Rb may only be ½ as sensitive as the one in the example. It also may pull ½ A compared to the one amp I used. That gets you to a stability that still needs to be better than 1%. I suspect you also will find that the connections to the wire have a *lot* more resistance than the foot of wire back to 0.1% land. Theres also the chance that you needed more than a foot of wire or used something smaller than number 18. For 10 feet you would need number 8 to get the same resistance. There are a couple of solutions: 1) Reference your tuning voltage directly to the Rb ground via sense leads. 2) Float the controller and single point ground it at the Rb 3) Attenuate the control signal at the Rb by 600:1, your tune range is now 8x10^-12 (or +/- 4x10-12). 4) Bolt everything to a 6 x 6 copper buss bar. (or is that to small .) Sense leads probably bring in another amp in the design. Its stability could be an issue. Floating the controller may or may not be possible depending on the design. Attenuating the control voltage at the Rb by 10:1 looks like a real good idea, regardless of what else you do. Of course thats not the whole story. The connections also have thermo electric effects. So many things to worry about. Bob _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
