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[time-freq] Rubidium standards

Eric Lemmon wb6fly at verizon.net
Fri Feb 8 19:04:41 UTC 2008


An excellent question!  Basically, there are two types of "atomic"
oscillators that are compact enough to find application in a small
laboratory:  Cesium and rubidium.  A cesium oscillator is absolutely
correct, but it has some jitter.  A rubidium oscillator is very stable, but
it must be calibrated periodically with a cesium or hydrogen maser

I have some experience using Z3801 GPS-disciplined oscillators, and I can
state that they are quite stable and accurate.  However, they use an
oven-stabilized crystal oscillator which is nowhere near as stable as a
rubidium oscillator.  The GPS constellation relies on periodic corrections
from a ground-based control station, so there are some very small errors
that can be corrected over the long term, along with some quantization
jitter, as well as atmospheric effects to contend with.

The primary NIST standard is a cesium fountain, described here:


There are several very interesting papers available on the NIST site
regarding cesium and rubidium clocks.

73, Eric Lemmon WB6FLY

-----Original Message-----
From: time-freq-bounces at lists.tapr.org
[mailto:time-freq-bounces at lists.tapr.org] On Behalf Of Randy
Sent: Friday, February 08, 2008 9:15 AM
To: 'TAPR time and frequency projects'
Subject: [time-freq] Rubidium standards

I see advertised once in a while Rubidium frequency standards that include
the following statement:

"The Rubidium standard is carefully calibrated to a GPS Disciplined

I thought those standards were based on the atomic physics of Rubidium and
that was what determined the frequency.  Why do they need to be "calibrated"
at all?  This also suggests that a GPS-disciplined oscillator is better than
the Rubidium standard.  It that true?  Are the HP Z3801 oscillators, for
example, ordinarily more accurate than a good Rubidium standard?


Randy Stegemeyer, W7HR
Port Orchard, WA

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