[time-nuts] Cesium DOES Drift, not

Tom Van Baak tvb at leapsecond.com
Tue Apr 19 04:10:41 EDT 2005

I wanted to follow up on this thread even though
Brooke and I have talked offline.

To the question -- could I measured a 5071A for a
month and what is its stability? -- attached are
200-day 5071A frequency plots; one is hourly
averages (2freq2.gif) and the other daily averages
(2freq4.gif). The plots are flat (no frequency drift).

Also attached (1sigma1.gif) is a stability plot going
out to tau 4 million seconds (45 days). You can see
these high-perf 5071A are in the 15's and still have
not quite hit their noise floor.

As for accurate definitions of stability, aging, and drift
have a look at pages like:


It is true that no frequency standard is perfect and all
have finite stability. Because of instability, frequency
will in fact wander around. But drift is something else;
it's the systematic (think gradual, continuous, unbounded
monotonic, sometimes linear) change in frequency over
time -- and this is typically not observed in a Cesium

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