demodulator

A demodulator is a device or algorithm that extracts the original information-bearing signal from a modulated carrier. It is the counterpart to a modulator and is used in receivers to recover baseband signals from radio frequencies, optical carriers, or other transmission media. The demodulator must reverse the modulation process applied at the transmitter and often includes filtering, amplification, and synchronization stages.

For analog modulation, common demodulation methods include envelope detection for Amplitude Modulation (AM), where a diode rectifies the RF signal and a low-pass filter recovers the audio; for frequency modulation (FM), discriminators or phase-locked loops convert instantaneous frequency variations back into amplitude variations corresponding to the original signal; phase modulation (PM) typically requires coherent or differential demodulation using a reference carrier.

In digital communications, demodulation maps received symbols to bit streams. Coherent demodulation uses a synchronized reference carrier and matched filtering, followed by decision devices; noncoherent demodulation does not require phase information. Common schemes include PSK, QAM, and FSK demodulation. Timing recovery and carrier recovery are important subfunctions. Equalization may be used to mitigate channel distortion.

Demodulators can be implemented in hardware, such as in radio front-ends and tuners, or in software in software-defined radios. They are evaluated by error performance (bit or symbol error rate), robustness to noise and distortion, and latency. They may also include additional functions like demultiplexing, error correction interface, and data conversion.