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10 An unsymmetrical power spectrum can arise when the carrier is frequency modulated with two or more different frequencies (36).

In recognition of the October 1964 declaration of the International Committee of Weights and Measures that the physical measurement of time be based on a particular transition between two hyperfine levels in the ground state of cesium 133, a review of the characteri~tics of cesium beam atomic frequency standards is presented; This article discusses the general requirements for frequency and time standards, advantages offered by the atomic standard as compared to astronomical standards, various other atomic standards in brief, the operating principles of cesium standards, measures of performance, error sources in cesium standards, characteristics of several standards in current operation, comparison of cesium standard!!, and atomic time standards derived from atomic frequency standards.

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