The phenomenon of Doppler invariance in an FM signal is discussed first. This is followed by a description of how this signal may be detected by passive sampling, a variable sampling rate being chosen to make each sample correspond to a signal peak. This results in signal folding, which reduces the total number of samples required to detect the signal. Certain characteristics of the signal are described and a formula is derived for the output of the detector when a weighting function designed to reduce the sidelobes is applied to the taps. The output is a discontinuous function because the signal is not always present at all of the taps all of the time, and the nature of this function is illustrated by means of graphs drawn for a number of typical weighting functions.