[time-nuts] Rb Tuning

Didier Juges didier at cox.net
Thu Jul 29 17:10:29 EDT 2010


Attenuating the control voltage near the point of use is always a good idea when you are concerned about common mode or ground noise and particularly when you have excess dynamic range, but you have to consider the noise added by the divider. Some filtering may be required, and use as low a set of resistor values as practical.

Didier

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-----Original Message-----
From: "Bob Camp" <lists at rtty.us>
Sender: time-nuts-bounces at febo.com
Date: Thu, 29 Jul 2010 12:41:47 
To: 'Discussion of precise time and frequency measurement'<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: [time-nuts] Rb Tuning

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 it’s 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 that’s 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. There’s 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. It’s 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 that’s not the whole story. The connections also have thermo
electric effects.

 

So many things to worry about.  

 

Bob

 

 

 

 

 

 

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