Time-Varying Spectral Approximation of Filtered Signals

An approximation of the time-varying (Wigner) spectrum of a filtered signal is presented. The approximation is simple to apply, yet insightful, in that it shows the effects of the magnitude and phase of the frequency response of the filter on the time-frequency properties of the signal. In addition, the approximation yields the correct Fourier spectrum of the filtered signal, to within a complex constant