What is Amplitude Modulation?

Amplitude modulation is defined as the multiplication of one time-domain signal by another time-domain signal. The signals may or may not be complex in nature, i.e., either or both signals may contain harmonics components. It is impossible to have amplitude modulation unless at least two different signals are involved. The signals may be electrical in nature, or they can be vibration signals. Modulation is inherently a non-linear process, and always gives rise to frequency components that did not exist in either of the two original signals

Amplitude Modulated Wave Form

If the amplitude-modulated signal shown here is passed through a frequency analyzer, the following spectrum is the result. The highest peak is the carrier frequency. The right-hand peak is the “upper sideband”, and has a frequency of the carrier frequency plus the modulating frequency. The left-hand peak or “lower sideband” has a frequency of the carrier minus the modulating frequency. The sidebands are sometimes called “sum and difference” frequencies because of their symmetrical spacing around the carrier.

Amplitude modulation also occurs in sound reproducing equipment, where it is called Intermodulation Distortion. The sum and difference frequencies are not in musical harmony with the tones that cause them, making intermodulation a particularly noticeable form of sound distortion.

Spectrum of Modulated Wave Form

Rectified Wave Form

Recovered Modulating Signal

This process of demodulation is exactly what happens in an AM radio -- the carrier is a very high frequency signal generated by the radio station, and the modulating signal is the voice or music that constitutes the program. The radio receives the modulated carrier, amplifies it, and rectifies (“detects”) it to recover the program.

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