Amplitude modulation is the process of using a modulating signal to control the amplitude of a high-frequency sinusoidal carrier so that it varies according to the pattern of the modulating signal. The general model of an amplitude modulator is shown in Figure 1.
1 In the figure, m(t) is the modulating signal, is the modulated signal, h(t) is the impulse response of the filter, then the general expression for the time and frequency domain of the modulated signal are
In the formula, M(w) is the spectrum of the modulating signal m(t),, w is the carrier angular frequency. From the above expression, it can be seen that for the amplitude modulation signal, in the waveform, its amplitude varies with the baseband signal law; in the spectral structure, its spectrum is entirely a simple shift of the baseband signal spectrum in the frequency domain. Since this shift is linear, amplitude modulation is often also referred to as linear modulation, and accordingly, amplitude modulation systems are called linear modulation systems. In the general model of Fig. 1, a variety of amplitude modulated signals can be obtained by appropriately choosing the characteristics of the filter H(w), such as: conventional double-sideband amplitude modulation (AM), suppressed carrier double-sideband amplitude modulation (DSB-SC), single-sideband modulation (SSB), and residual sideband modulation (VSB) signals.