[time-nuts] So what’s inside that Cs Beam Tube anyway?

Tom Van Baak tvb at LeapSecond.com
Thu Nov 3 03:34:06 EDT 2016

Hi Magnus,

About the high-perf CBT, I wrote: "The difference would be in things like the oven temp and detector gain, both of which are controlled outside the tube. My first hint of this was in a paper by Carroll Alley."

Since you're curious, let me explain the hint in the paper by Alley [1][2]. He says:

"The clocks were modified in order to give the performance needed. Following suggestions by Len Cutler and others at Hewlett Packard, we increased the beam current by a factor of 2, we added an integrating loop in the crystal control, and there was a proprietary modification of the beam tube (now standard on all high performance tubes)."

"All in all, we could achieve stabilities over the 15 hours at a couple of parts in 10^14 with standard commercial clocks, as shown in Figure 29. We paid much attention to providing a stable environment for the clocks. Let us look at some pictures to show you the equipment and give you some feeling for the experiment."

I hope that answers your question.


[1] C.O.Alley, "Proper Time Experiments in Gravitational Fields with Atomic Clocks, Aircraft, and Laser Light Pulses" in "Quantum Optics, Experimental Gravity, and Measurement Theory".

This article / book is behind paywalls. I can send you a PDF off-list. But...

[2] There's papers with about the same content. C.O.Alley, "Introduction to some fundamental concepts of general relativity and to their required use in some modern timekeeping systems", PTTI, 1981.

And this PDF for this is freely available, from all the usual suspects:


The quote above is from page 710.

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