The shape of low excursions of the envelope of a signal plus noise

Like low excursions of the envelope of narrowband noise, those of the sum of a signal plus narrowband noise are hyperbolic in shape if the signal-to-noise ratio r is not high. When r/spl Gt/1, low excursions of the envelope are hyperbolic if the signal is frequency-modulated but have a quartic shape with one or two minima if it is not. This article illuminates various aspects of FM clicks.