demodulator


Also found in: Dictionary, Thesaurus, Acronyms, Wikipedia.

demodulator

[dē′mäj·ə‚lad·ər]
(electronics)

Demodulator

A device used to recover the original modulating signal from a modulated wave. A demodulator is also known as a detector.

In communications systems and in some automatic control systems, the information to be transmitted is first impressed upon a periodic wave called a carrier. The carrier is then said to be modulated. After reception of the modulated carrier, the original modulating signal is recovered by the process of demodulation or detection.

The amplitude, frequency, or phase of a carrier may be changed in the modulation process. Therefore, the process of demodulation and the practical circuits for accomplishing it differ in each case. However, all demodulators require the use of a nonlinear device in order to recover the original modulating frequencies, because these frequencies are not present in the modulated carrier and new frequencies cannot be produced by a linear device.

A semiconductor diode is frequently used to demodulate an amplitude-modulated (AM) carrier. A simple filter consisting of capacitance and resistance is used to eliminate the carrier and other undesired frequencies from the output of the demodulator. Another common AM detector uses a multiplier circuit, available as a semiconductor chip. A square-law detector is often used to demodulate single-sideband (SSB) signals. A multiplier chip with both inputs tied together serves nicely as a squaring circuit and may be used as a low-distortion demodulator for SSB signals. See Amplitude-modulation detector, Rectifier

Frequency-modulated (FM) signals and phase-modulated (PM) signals may generally be demodulated by the same type of circuits, the only difference being the filter circuits in their respective outputs.

There are two basic classes of FM or PM demodulators. The first type, known as discriminators, use tuned circuits to change frequency or phase variations into amplitude variations and then use amplitude-demodulating devices such as diodes or a multiplier to recover the modulating frequencies. The second class or type of FM demodulator is the phase-locked loop, which includes a phase detector that may be a multiplier, a low-pass filter, and a voltage-controlled oscillator that produces a frequency proportional to its control voltage. The output of the phase detector is proportional to the phase difference between the incoming FM or PM signal and the voltage-controlled oscillator output. This phase detector output, after filtering, is the desired original modulating signal and also provides the control voltage needed to keep the voltage-controlled oscillator locked to the incoming signal frequency. These phase-locked loops are available as integrated semiconductor circuits, or chips. See Frequency-modulation detector, Phase-modulation detector

Amplitude modulation and demodulation may be accomplished with the same device. For example, a multiplier performs both of these functions. In addition, phase-locked loops incorporate all the basic circuits needed for the modulation and demodulation of FM, PM, and AM signals. Therefore, circuits have been devised that will either modulate or demodulate FM, PM, and AM signals. These circuits are known as modems and are commonly used in modern communications systems. See Modem

A carrier wave may be modulated in both amplitude and phase simultaneously when a digital signal is being transmitted. The commonly used system employing this technique uses a 90° phase modulation and a two-level amplitude modulation and is called quadrature amplitude modulation (QAM).

The development of optical demodulators came with the advent of optical-frequency communications systems. See Modulator, Optical communications, Optical detectors

References in periodicals archive ?
17, an eye diagram for the 8PSK after transmission through a model of the optical transmission path (on the demodulator input) is shown.
Demodulators supporting DVB-S2X enable developers to future-proof their TV and STB designs with advanced channel-recovery features and technology to support the upcoming roll-out of 4K Ultra HD TV reception based on high-efficiency video coding (HEVC) capabilities.
In the proposed QPSK demodulator the incoming signal is sampled 8 times of the incoming signal frequency.
The EVB for ADL5382 is used as I-Q demodulator for the low-IF receiver as shown in Figure 5.
Minor timing offsets due to unequal trigger sensitivities and latencies are accommodated by the demodulators because they inherently synchronize to the nearest symbol.
As a result, the LLR values generated by the demodulator can pass to the turbo decoders that can deal with them as if they were BPSK signals.
2 where it is graphically shown that in an ideal case, a demodulator establishes a transformation between the received ([[GAMMA].sup.RX]) and demodulated ([[GAMMA].sup.DE]) symbol planes which should ideally consist of a simple scaling and rotation that can be easily corrected.
The HX260 multichannel demodulator enables the unit to operate in full mesh mode with a single satellite hop to connect to another HX260 terminal.
"By marrying best-in-class mobile TV tuner technology from the Athena Semiconductor acquisition and our core demodulator IP together with leading-edge process technology, Broadcom has delivered a compelling solution to our customers," said Dino Bekis, Director of New Business Initiatives for Broadcom's Wireless Connectivity Group.
DRX 3975D is the first COFDM demodulator to substantially exceed all requirements of the NorDig-Unified v1.0.2 Receiver Specification.
If the signal is spread by a factor of 1,000, this requires a 30-dB J/S to create an effective 0-dB J/S after the spread-spectrum demodulator to effectively jam a digital transmission.