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The terms accuracy, stability, and
precision are often used in describing an oscillator's quality. Above is an illustration of the meanings of
these terms for a marksman and for a frequency source. (For the marksman, each bullet hole's
distance to the center of the target is the "measurement.") Accuracy is the extent to which a
given measurement, or the average of a set of measurements for one sample,
agrees with the definition of the quantity being measured. It is the degree of "correctness"
of a quantity. Reproducibility is the ability of a single frequency
standard to produce the same frequency, without adjustment, each time it is
put into operation. From the user's
point of view, once a frequency standard is calibrated, reproducibility
confers the same advantages as accuracy.
Stability describes the amount something changes as a function
of parameters such as time, temperature, shock, and the like. Precision is the extent to which a
given set of measurements of one sample agrees with the mean of the set. (A related meaning of the term is used as a
descriptor of the quality of an instrument, as in a "precision instrument." In that context, the meaning is usually
defined as accurate and precise, although a precision instrument can also be
inaccurate and precise, in which case the instrument needs to be calibrated.)
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The military specification for crystal
oscillators, MIL-PRF-55310D*, defines “Overall Frequency Accuracy” as “6.4.33
Overall frequency accuracy. The maximum permissible frequency deviation of
the oscillator frequency from the assigned nominal value due to all
combinations of specified operating and nonoperating parameters within a
specified period of time. In the general case, overall accuracy of an
oscillator is the sum of the absolute values assigned to the following:
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a. The initial
frequency-temperature accuracy (see 6.4.24).
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b.
Frequency-tolerances due to supply voltage changes (see 6.4.17) and other
environmental effects
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(see 6.4.12).
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Total frequency
change from an initial value due to frequency aging (see 6.4.11) at a
specified temperature.”
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The International System (SI) of units for
time and frequency (the second and Hz, respectively) are obtained in
laboratories using very accurate frequency standards called primary
standards. A primary standard operates at a frequency calculable in terms of
the SI definition of the second**:
"the duration of 9,192,631,770
periods of the radiation corresponding to the transition between the
two hyperfine levels of the ground state of the cesium atom 133”.
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* MIL-PRF-55310,
http://www.dscc.dla.mil/Programs/MilSpec/ListDocs.asp?BasicDoc=MIL-PRF-55310
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** XIIIth General
Conference of Weights and Measures, Geneva, Switzerland, October 1967
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