Spectral Analysis of Pulse-Modulated rf Signals

The parameters that characterize a rectangular-shaped pulse-modulated sinusoidal signal are the carrier frequency, the pulsewidth, the repetition frequency, and the number of pulses in or the duration of the signal. We use a Fourier series representation to show the influence of these parameters on the spectrum of a pulse-modulated signal at a microwave carrier frequency. When an additional amplitude modulation is applied at audio frequencies, the resulting transient cannot be efficiently analyzed with numerical transform techniques. We present approximate numerical and analytical techniques to obtain the frequency spectrum of such signals. This approach allows the near-real-time spectral analysis of modulated signals. Thus, the resulting spectrum can be easily calculated for idealized modulation waveforms. A typical example is presented and the effect of pulse modulation on the spectral content of an rf signal burst is discussed.