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Demodulator

A Demodulator is an electronic circuit used to recover the information content from the carrier wave of a signal. The term is usually used in connection with radio receivers, but there are many kinds of demodulators used in many other systems. Another common one is in a modem, which is a contraction of the terms modulator/demodulator.

AM and FM demodulators

An AM signal encodes the information onto the carrier wave by varying its amplitude in direct sympathy with the analogue signal to be sent. A demodulator for this technique is extremely simple, for it consists of anything that will pass current in one direction only, that is, a rectifier. This may be in the form of a single diode, or may be more complex. Many natural substances exhibit this rectification behaviour, which is why it was the earliest modulation and demodulation technique used in radio. The crystal set exploits the simplicity of the modulation to produce an AM receiver with very few parts.

Frequency modulation or FM is more complex. It has numerous advantages over AM, such as better fidelity and noise immunity. However, it is much more complex to both modulate and demodulate a carrier wave with FM, and AM predates it by several decades.

There are several common types of FM demodulator:

The Quadrature detector, which phase shifts the signal by 90 degrees and multiplies it with the unshifted version. One of the terms that drops out from this operation is the original information signal, which is selected and amplified.

The Foster-Seeley discriminator[?].

The Phase-locked loop

In addition, FM can be demodulated using a digital signal processor, a technique used in software defined radio.



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