Phase-shift keying is another form of angle-modulated, constant –amplitude digital modulation. It is similar to conventional phase modulation with the difference that the input signal is a binary digital signal instead of an analog baseband signal. Furthermore, phase-shift keying has a limited number of output phases. There are different levels of phase-shift keying that is characterized by increasing number of input levels and proportional increase of possible output phases.
Binary phase-shift keying has only one level and thus two possible output phases. One phase represents a logic 1 and the other represents a logic 0. With binary phase-shift keying, the phase of the output carrier shifts are separated by 180° which is why it is sometimes called phase reversal keying and biphase modulation. BPSK transmitter is essentially composed of a level converter feed and an oscillator input into a balanced modulator which outputs the product of two sinusoidal waveforms. Its receiver just reverses the direction of the feeding of the inputs and also has the inclusion of clocking and carrier recovery components.
Quaternary phase-shift keying is another form of angle-modulated, constant-amplitude modulation with two input levels and four output phases. The inputs are composed of 00, 01, 10 and 11 which are sometimes called dibits. In the modulator, each dibit code generates one of the four possible output phases, each phase separated by 90°. A QPSK transmitter is composed of a bit splitter to separate input dibits. Each are then fed into balanced modulators with reference oscilations 90° out of phase. The outputs of the modulators are fed into a summer to release the QPSK output. A QPSK receiver uses a power splitter that splits the In-Phase channel from the Quadrature-channel. It is then fed to a product detector with reference oscillations 90° out of phase.
Posted 2010-12-14 and updated on Dec 14, 2010 1:41pm by crisd