The performance of any frequency standard, whether atomic or crystal, is described by three characteristics:
Accuracy is a measure of how closely the frequency generated by the standard corresponds to its assigned value (e.g., the atomic transition frequency for an atomic standard). Accuracy can also be applied to a frequency measuring instrument, where it tells how close the instrument’s reading is to the actual frequency being measured. A measurement of a 100-Hz frequency that is accurate to the sixth decimal place is said to be accurate to 1 part in 108, or to have 10−8 accuracy.
Precision is a measure of the repeatability of a frequency measurement. It is generally expressed in terms of a standard deviation of the measurement.
Stability is a measure of the maximum deviation of the standard’s frequency when operating over a specified parameter range. Frequency aging, or long-term stability, is the slow change of the standard’s frequency with time if all other parameters are fixed. Short-term stability is the deviation of the standard’s frequency with time as the result of noise or other internal effects if other parameters are fixed. It is often expressed in terms of the Allan deviation (see Appendix D, Glossary).