[time-nuts] Cesium DOES Drift, not
Brooke Clarke
brooke at pacific.net
Mon Apr 18 20:32:23 EDT 2005
Hi Tom:
Think of it this way. Offset is a measure of how well someone has set
an oscillator. Stability is a measure of how well the oscillator
maintains it's frequency. Stability is the money spec. A lab grade
crystal oscillator may have a stability of parts in E-10 per day. That
means that if the offset is set to zero a day later the frequency may by
off by parts in E-10.
The world in not a perfect place and any Cesium oscillator also has
aging, it's not perfect, that's why there's a spec on the 5071A saying
less than 1E-14 per day. NIST has a fountain Cesium where they say less
than 1E-15 per day. That number is the stability NOT the offset. To
see the stability you need to accumulate enough data to see the
parabolic shape. It took me over a month after I got the wrinkles out
of my test system.
Can you hookup a Cesium source and make a time interval measurement for
a period of a month? Do you have software that will tell you the
stability (not offset) of the oscillator being tested?
Having Fun,
Brooke
Tom Van Baak wrote:
>>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.
>>
>>
>
>Do be careful here. Excel will blindly report
>equations with 5 significant digits no matter
>what the data looks like.
>
>Here's something to try: break up your data
>into three 10-day segments and see how well
>the x2 term of the equations agree.
>
>Or convert phase to frequency and then plot
>30 days of frequency. If you have real drift it
>should be clear from this plot.
>
>DougH, JohnA, and I have Stable32 which makes
>this a snap if you want to send any of us the raw
>phase data.
>
>
>
>>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.
>>
>>
>
>All frequency standards have frequency instabilities.
>Hydrogen masers, Quartz, and Rubidium have drift,
>but Cesium standards are generally considered to
>have zero drift. That's one reason UTC is based on
>that technology.
>
>And note that fractional frequency [in]stability is not
>the same thing as frequency drift. I can go into this
>in more detail if you wish.
>
>
>
>>This explains a lot about why setting the C field near 1E-14 is
>>difficult, the frequency is changing all the time.
>>
>>
>
>If you make frequency plots in addition to phase plots
>you will see dramatically why setting the C-field of
>your 4060 to 1e-14 is hopeless. Frequency plots will
>graphically show frequency instability (the width of
>the line) and frequency drift (the slope of the line).
>
>/tvb
>
>
>
>>Now Having Fun,
>>
>>Brooke Clarke, N6GCE
>>
>>
>
>
>
>
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