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Modulation is the process whereby some characteristic of one wave is varied in accordance with some characteristic of another wave. | MODULATION Modulation is the process whereby some characteristic of one wave is varied in accordance with some characteristic of another wave. The basic types of modulation are angular modulation including the special cases of phase and frequency modulation and amplitude modulation. In missile radars it is common practice to amplitude modulate the transmitted RF carrier wave of tracking and guidance transmitters by using a pulsed wave for modulating and to frequency modulate the transmitted RF carrier wave of illuminator transmitters by using a sine wave. Frequency Modulation FM - As shown in Figure 1 an unmodulated RF signal in the time domain has only a single spectral line at the carrier frequency fC in the frequency domain. If the signal is frequency modulated as shown in Figure 2 the spectral line will correspondingly shift in the frequency domain. Amplitude Modulation AM - If the signal in Figure 1 is amplitude modulated by a sinewave as shown in Figure 3 sidebands are produced in the frequency domain at Fc Fam- AM other than by a pure sine wave will cause additional sidebands normally at Fc nFAM where n equals 1 2 3 4 etc. FREQUENCY DOMAIN TIME DOMAIN PLOT RF Carrier e.g. 10 GHz Time E -------------c Carrier Frequency at 10 GHz Figure 1. Unmodulated RF Signal TIME DOMAIN PLOT RF Carrier I e.g. 10 GHz e.g. 5 GHz t1 t2 t3 FREQUENCY DOMAIN Occurs from t2 to t3 10 Frequency Occurs GHz from t1 to t2 Figure 2. RF Signal with Frequency Modulation TIME DOMAIN PLOT Detected Signal Fam e.g. 100 Hz FREQUENCY DOMAIN Upper Sideband Lower Sideband 9 999 999 900 Hz 10 000 000 100 Hz Frequency GHz Figure 3. Sinewave Modulated RF Signal Pulse modulation is a special case of AM wherein the carrier frequency is gated at a pulsed rate. When the reciprocal of the duty cycle of the AM is a whole number harmonics corresponding to multiples of that whole number will be missing e.g. in a 33.33 duty cycle AM wave will miss the 3rd 6th 9th etc. harmonics while a square wave or 50 duty
