Using statistical language, the instantaneous frequency of a deterministic signal, such as an electromagnetic pulse collected at a point in space, is a location function and its deviation is a shape function. The former is better defined than the latter, which can be computed an infinity of different ways, some of which can exhibit pathologies. Some deviations can become pure imaginary in some regions of the time-frequency phase space. The instantaneous frequency of the analytic signal, too, suffers its own pathologies. It can go negative when two or more distinct signals are analysed as a single signal. This paper briefly discusses a limited treatment of the instantaneous frequency and its deviation.
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