Demodulation

From Wikipedia, the free encyclopedia

Demodulation is the act of removing the modulation from an analog signal. Demodulating is necessary because the receiver system receives a modulated signal with specific characteristics and it needs to turn it to base-band.

There are several ways of demodulation depending on what parameters of the base-band signal are transmitted in the carrier signal, such as amplitude, frequency or phase. For example, if we have a signal modulated with a lineal modulation, like AM (Amplitude Modulated), we can use a synchronous detector. On the other hand, if we have a signal modulated with an angular modulation, we must use a FM (Frequency Modulated) demodulator or a PM (Phase Modulated) demodulator respectively. There are different kinds of circuits that make these functions.

To demodulate an AM signal, pass it through a diode rectifier. The amplitude variation will integrate into the original modulating signal.

There are several ways to demodulate an FM signal. The most common is to use a discriminator. This is composed of an electronic filter which decreases the amplitude of some frequencies relative to others, followed by an AM demodulator. If the filter response changes linearly with frequency, the final analog output will be proportional to the input frequency, as desired. Another one is to use two AM demodulators, one tuned to the high end of the band and the other to the low end, and feed the outputs into a difference amp. Another is to feed the signal into a phase-locked loop and use the error signal as the demodulated signal.

An example of a demodulation system is a modem, which receives a telephone signal (electrical signal) and turns this signal from the wire net into a binary signal for the computer.