Amplitude modulation( AM) is a modulation method in which the information signal is modulated to the amplitude of a carrier frequency. The amplitude of the carrier frequency thus changes depending on the level and frequency of the modulation signal. Modulation only occurs if the frequencies of the carrier signal and the modulation signal are sufficiently different. If they are close to each other, beating occurs.
In amplitude modulation, a high-frequency carrier frequency is modulated in amplitude by a low- frequency modulation frequency. If the modulation signal has a high amplitude, then the positive and negative amplitudes of the modulated carrier signal increase.
In terms of frequency, amplitude modulation forms upper and lower sidebands in addition to the carrier oscillation. If only a fixed modulation frequency is modulated, the frequency spectrum to be transmitted is composed of the carrier frequency, the carrier frequency minus the modulation frequency, and the carrier frequency plus the modulation frequency. The bandwidth required by the carrier signal is twice the bandwidth of the modulation signal. For example, if the modulation signal has a bandwidth of 1 MHz, the carrier signal requires a bandwidth of 2 MHz. The signal-to- noise ratio( SNR) for amplitude modulation depends on the amplitude of the modulated carrier signal.
An important characteristic value of amplitude modulation is the modulation depth. It is given as a percentage and is calculated from the ratio of the amplitude of the modulation frequency to the amplitude of the unmodulated carrier frequency.
In addition to amplitude modulation with double sideband modulation(DSB), there are various variants such as amplitude modulation with suppressed carrier, single sideband modulation( SSB), residual sideband modulation( VSB), quadrature amplitude modulation ( QAM), pulse amplitude modulation( PAM), amplitude shift keying( ASK) and some more.