[time-nuts] Rubidium Drift

Tom Van Baak tvb at leapsecond.com
Fri Nov 3 04:01:09 UTC 2006


> I've recently acquired a Rubidium Osc ( Efratom FRS-C 10Mhz) as my 
> first venture into this field. It's working as per the spec given 
> ,checked against Trimble GPS with 1pps output.
> Can you point me to somewhere where there is  a description of the 
> physics behind the drift that occurs?Drift is speced as <5x10^-11 per 
> month....but why should there be any "drift" at all?
> "Wandering" about nominal I can expect ,but consistent "drift" ?
> Regards
> Peter
> ZL2AYX

Hi Peter,

You're off to a good start. With enough days of data
you should be able to measure the drift quite nicely.

To answer your good question; we all have come to
know that cesium is accurate and rubidium drifts. But
there's much more to the story...

There are different ways to partition the world of atomic
clocks. One is by atom: we have Cesium, Rubidium,
Hydrogen, Mercury, and several others.

The other is by technique. We have masers, thermal
beams, optical pumping, fountains, and several others.

The existence of drift in an atomic clock is a function
of the technique rather than the atom.

So, yes, all commercial Rb clocks drift, but not really
because they use rubidium. A Rb beam clock (if one
existed) would, like a Cs beam, not drift.

Similarly, a rubidium fountain is just as driftless as a
cesium fountain (in fact, a rubidium fountain may well
outperform a Cs fountain).

One clue is that you hear the phrase "rubidium vapor
frequency standard" (e.g., hp 5065A) as opposed to
the phrase "cesium beam frequency standard" (e.g.,
hp 5061A). The physics is completely different.

To read more about why all commercial, compact,
low-cost, low-power rubidium standards have drift
google for words like rubidium buffer gas.

For a quick overview of Cs and Rb standards see:
http://tf.nist.gov/general/enc-re.htm#rubidiumoscillator
http://tf.nist.gov/general/enc-c.htm#cesiumbeam
http://tf.nist.gov/general/enc-h.htm#hydrogenmaser

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/tvb
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