[time-nuts] CBT EOL

[wa1zms at att.net]

Thanks Dave. Good explination. The unit in question is a standard
HP tube so it could be an issue with the EM.  I'll keep tracking the
beam current and if it continues to fall at the present rate I'll
try boosting the EM voltage. Although that will require a homebrew
modifictaion since the supply is already at the -2500v limit to
match the spec on the tube.

-Brian, WA1ZMS/4


-------------- Original message ----------------------
From: "Dave Carlson" <dgcarlson at sbcglobal.net>
>
> Hello,
> 
> Speaking not from a physicist perspective, but from being in the business 
> for over 20 years...
> 
> The mechanism depends somewhat on the amount of flux that has been traveling 
> through the CBT, which determines if the cesium becomes exhausted before the 
> electron multiplier first dynoe "wears out".
> 
> In general with the HP/Agilent/Symmetricom CBT there is enough getter 
> material inside to trap contaminants well beyond the time that the cesium 
> should run out in the oven. As long as the Ion Pump is working, internal 
> contamination should be the minimal contributor to EOL.
> 
> Whether the cesium runs out or the Electron Multiplier first dynode loses 
> its coating is largely dependent on the amount of flux. The High-Performance 
> CBT run at a hotter oven temperature to gain better S/N and thereby exhaust 
> the available cesium at a faster rate. In these CBT the EOL mechanism is 
> more likely due to that. As the cesium runs out the S/N degrades. Increasing 
> the EM voltage can help, but eventually you're amplifying more and more 
> noise.
> 
> In the standard-performance or long-life CBT the EOL mechanism is more 
> likely due to the first dynode of the EM becoming depleted of its coating, 
> than buildup of internal contaminants.
> 
> Dave
> 
> ----- Original Message ----- 
> From: <wa1zms at att.net>
> To: <time-nuts at febo.com>
> Sent: Monday, February 04, 2008 6:39 AM
> Subject: [time-nuts] CBT EOL
> 
> 
> When a CBT reaches end-of-life, is it because the source of Cs is used
> up? Or is it due to contamination of the electron multiplier?
> 
> I've heard both as causes but wanted more info.
> 
> I have a CBT with very low beam current and it keeps falling. I'm wondering
> if running a higher than normal EM voltage might help keep it on 
> life-support.
> 
> -Brain, WA1ZMS/4
> 
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