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, for a signal modulated with a lineal modulation, like AM (Amplitude Modulated), we can use a synchronous detector. On the other hand, for a signal modulated with an angular modulation, we must use an FM (Frequency Modulated) demodulator or a PM (Phase Modulated) demodulator. Different kinds of circuits perform these functions.
A demodulator is an electronic circuit used to recover the information content from the carrier wave of a signal. The term is traditionally used in connection with radio receivers, but many other systems use many kinds of demodulators. Another common one is in a modem, which is a contraction of the terms modulator/demodulator.
Many techniques -- such as carrier recovery, clock recovery, bit slip, frame synchronization, rake receiver, pulse compression, Received Signal Strength Indication, error detection and correction, etc. -- are only performed by demodulators, although any specific demodulator may perform only some or none of these techniques.
The envelope detector is a very simple method of demodulation. 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.
The product detector multiplies the incoming signal by the signal of a local oscillator with the same frequency and phase as the carrier of the incoming signal. After filtering the original audio signal will result. This method will decode both AM and SSB, although if the phase cannot be determined a more complex setup is required.
An AM signal can be rectified without requiring a coherent demodulator. For example, the signal can be passed through an envelope detector (a diode rectifier). The output will follow the same curve as the input baseband signal. There are forms of AM in which the carrier is reduced or suppressed entirely, which require coherent demodulation. For further reading, see sideband.
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:
QAM demodulation requires a coherent receiver.
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