[time-nuts] Cesium DOES Drift

[Brooke Clarke]

Hi:

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
http://www.boulder.nist.gov/timefreq/general/pdf/1288.pdf

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

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