[time-nuts] Lifetime of Cesium tube

[Chuck Harris]

glenn.tracy wrote:
> What kind of lifetime can one expect to see from an HP 5061-6101 cesium 
> tube?

It is my understanding that you can expect about 5 years in continuous use,
or 2 years on the shelf.

> What typical symptoms do these exhibit near the end of their useable 
> lifetime?

The end of life can be due to several things, but most notable is a shortage
of cesium ions to send hurling down the length of the tube, and an excess
of other molecules for the cesium ions to run into.  Either condition results
in a shortage of cesium molecules for the detector to detect.

A c-beam is essentially a notch filter.  The beam is modulated (mixed?) with
a 9Ghz microwave signal that is derived from a very stable and low noise crystal
reference.  When the modulation signal is at the right frequency, the beam
current will peak.  There is usually a servo mechanism that keeps the crystal
reference tuned so that the beam current stays on top of this peak.

At end of life, the beam current peak becomes indistinct.  There isn't much
distance between the peaks and the valleys, and the servo has trouble figuring
out where the peak is, so it ends up bouncing all over the place.  This shows
up as increased phase variations.  Ultimately, no peaks can be found and the
crystal oscillator ends up slammed against its maximum, or minimum frequency.

-Chuck