Modulation

Modulation is the variation of one parameter of a signal by the action of another signal. A common type of modulation is amplitude modulation, where the amplitude of one signal (called the "carrier") is caused to fluctuate in response to a modulating signal. This is the way AM radio transmission works; a high-frequency wave called the carrier is caused to fluctuate in level in accordance with the voice or music signal being transmitted. The radio receiver picks up the modulated carrier and performs a demodulation to extract the audio signal. Frequency modulation is another type where the frequency of the carrier is varied rather than the amplitude. Modulation of a carrier causes new components to appear in the spectrum and they are called sidebands. The frequencies of the sidebands are equal to the carrier frequency plus and minus the modulating frequency.

In rotating machinery there are many fault mechanisms which can cause amplitude and frequency modulation, and vibration analysis exposes the sidebands. Demodulation can be performed to detect the modulation frequencies directly.