Abstract: A frequency modulated signal is converted into a pair of baseband signals by mixing the signal with a pair of phase-quadrature reference signals of substantially carrier center frequency. The double reference frequency component of each of the baseband signals is filtered to obtain I and Q signals. The I and Q signals constitute in rectangular coordinates the components of a vector represented in polar coordinates by R and .theta. where R is the maximum amplitude of the I and Q signals and .theta. is the instantaneous angle represented by the arctangent of the I signal divided by the Q signal. The I and Q signals are inverted to provide respective -I and -Q signals. The I, -I, Q, -Q signals are sampled to provide a sequence of sets of signal samples. For each set of signal samples the angle .theta. is obtained. A sequence of changes in the values of the angle .theta.