Demodulation

Demodulation is the process by which the original information-bearing signal is extracted from a modulated carrier wave. This function is performed by a demodulator, implemented either as an electronic circuit or as a software routine in software-defined radio systems. Because modulation techniques vary widely, a corresponding range of demodulation methods exists. The recovered output may take the form of analogue audio, analogue video or digital data. Although the terminology originated in the context of radio receivers, demodulators are used in numerous modern communication systems, including modems that transfer digital data via telephone lines, coaxial cables or optical fibres.
Demodulation enables the separation of meaningful information from high-frequency carriers used for reliable transmission. It is a central concept in telecommunications, radio engineering and digital communications.

Historical Development

Demodulation first appeared in early radio receivers during the late nineteenth and early twentieth centuries. In the wireless telegraphy systems of the period 1884–1914, transmitters sent information not through sound but through pulses of radio waves representing Morse code characters. Receivers therefore needed only to detect the presence or absence of a radio signal, converting it into audible clicks. The device responsible for this operation was known as a detector, the earliest form of demodulator.
The first such detectors were coherers, which operated as simple switches triggered by radio waves. The term “detector” persisted and continues to refer to demodulators today. As radio technology advanced to include amplitude-modulated (AM) sound transmissions, more sophisticated demodulators emerged. Reginald Fessenden developed the first AM demodulator in 1904, the electrolytic detector, which used a fine wire immersed in dilute acid. In the same year, John Ambrose Fleming introduced the thermionic diode (Fleming valve), an effective rectifier capable of demodulating AM signals.

Demodulation Techniques

Different modulation schemes require corresponding demodulation strategies based on how the information is encoded in the carrier. Linear modulation, such as AM, typically relies on synchronous or envelope detection, whereas angular modulation, such as frequency modulation (FM) or phase modulation (PM), requires frequency- or phase-sensitive demodulators.
Many demodulators incorporate auxiliary functions, including carrier recovery, clock recovery, frame synchronisation, rake reception, pulse compression and error correction. Although these operations vary by system, they are often essential for accurate extraction or reconstruction of the information signal.

Amplitude Modulation (AM) Demodulation

AM signals represent information through variations in the amplitude of a carrier wave. Two principal demodulation methods are used:

• Envelope detection, a straightforward technique requiring only a rectifying component, such as a diode or other nonlinear device, and a low-pass filter. The rectifier emphasises one half of the waveform, and the filter removes high-frequency components, leaving the audio or data signal. Early crystal radio receivers used mineral crystals as rectifiers and relied on the limited frequency response of headphones as the filter.
• Product detection, which multiplies the incoming signal by a locally generated carrier that matches the frequency and phase of the original transmitter. After filtering, the baseband audio signal appears. This method is crucial for single-sideband (SSB) signals where the carrier is suppressed.

Frequency Modulation (FM) Demodulation

FM demodulation is more complex because the signal encodes information through frequency variations rather than amplitude changes. Several established techniques are employed:

• Quadrature detection, which shifts the phase of the incoming signal by 90 degrees and multiplies it with the original. This operation produces terms that include the modulating signal, which is then filtered and amplified.
• Phase-locked loop (PLL) demodulation, in which the received signal drives a PLL. The error voltage used to correct the loop forms the demodulated output.
• Foster–Seeley discriminator, a combination of an electronic filter with an AM demodulator. If the filter’s amplitude response varies linearly with frequency, the output corresponds to the frequency deviations of the FM signal.
• Ratio detector, a variation of the Foster–Seeley approach, offering improved noise performance.
• Dual-detector method, using two AM demodulators tuned to opposite ends of the FM signal’s deviation range. Their outputs are combined to produce a signal proportional to frequency.

Phase Modulation (PM) and Quadrature Amplitude Modulation (QAM)

PM demodulators track phase variations in the carrier and often share circuitry with FM demodulators because PM and FM are forms of angular modulation. QAM, a widely used digital modulation format, requires coherent demodulation. It uses two product detectors fed by local oscillator signals 90 degrees out of phase to obtain the in-phase (I) and quadrature (Q) components. The demodulator maintains alignment using pilot signals or carrier-recovery algorithms.