Demodulation

Demodulation recovers an information-bearing signal from a modulated carrier wave. Its method depends on how the carrier encodes information in amplitude, frequency, phase, or another property.

Envelope detector: A circuit that extracts a signal from the changing amplitude of an amplitude-modulated wave. It recovers ordinary AM audio by following the carrier’s amplitude envelope.
Modulation: The encoding of information by varying a property of a carrier signal. Demodulation reverses the encoding operation performed by modulation.
Software-defined radio: A radio system that implements signal processing functions largely in software on digitized signals. Software can implement and change demodulation methods without replacing dedicated hardware.
Envelope detection: A method for recovering amplitude-modulated information by tracking the signal’s envelope. It is simpler than coherent detection but cannot recover suppressed-carrier AM.
Product detector: A circuit that multiplies a received signal by a locally generated oscillator signal to recover modulation. It demodulates suppressed-carrier signals by restoring a phase-coherent reference.
Carrier wave: A periodic signal whose properties are varied to carry information. Its amplitude, frequency, or phase changes provide the information that demodulation recovers.
Digital communications: The transmission and reception of information represented by discrete symbols or bits. Digital receivers demodulate waveforms to estimate which symbols were transmitted.
Noncoherent detection: Signal detection that does not require a phase-synchronized carrier reference. It avoids carrier synchronization, unlike many phase-sensitive demodulation methods.
Frequency discriminator: A circuit that converts changes in input frequency into corresponding changes in output voltage. It turns frequency deviations in an FM signal into the original modulation waveform.
Amplitude modulation: A modulation method in which the amplitude of a carrier varies with the information signal. Its envelope contains the information that simple detectors can recover.

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