[time-nuts] Cesium DOES Drift
brooke at pacific.net
Mon Apr 18 18:45:15 EDT 2005
I've been reading up on real world Cesium standards and they do drift.
For example see the first paragraph on page 4 of:
Introduction to time and frequency metrology by Judah Levine
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 70, NUMBER 6 JUNE 1999
Cesium is just like a crystal oscillator in that it has an offset and a
drift. The above article says:
"A good commercial cesium frequency standard, for example, might
exhibit fractional frequency fluctuations of 2E-14 for averaging
times of about one day. The frequency of the same device might
differ from the SI definition by 1E-13 or more (sometimes much
more), and this frequency offset may change slowly with time as the
device ages. (This frequency offset is what remains after
corrections for the perturbations mentioned above have been applied.
If no corrections are applied, the fractional frequency offset is
usually dominated by Zeeman effects, which can be as large as 1E-10.)"
The FTS4060 time interval is following the following equation (it takes
about a month to get this equation):
y = -1.2594x2 + 236.37x - 10318
where Y is in ns and X is the Day Of the Year. The first term is the
fractional frequency stability, i.e. drift rate and is
1.14E-14 per day which is pretty good. The HP-Agilent 5071A is
specified at <1E-14 per day.
Note that the fractional frequency stability of a good lab grade crystal
standard is about 1E-10 per day, so Cesium is 10,000 times better, but
still has drift.
This explains a lot about why setting the C field near 1E-14 is
difficult, the frequency is changing all the time.
Now Having Fun,
Brooke Clarke, N6GCE
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
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