Encoder for digital communication systems ‘quadrature phase shift keying modulator'

The need for a digital communications system has been increased due to its advantages over the analogue system. Quadrature Phase Shift Keying is a very efficient digital modulation method; widely used as CDMA (Code Division Multiple Access) cellular service, wireless local loop, Iridium (a voice/data satellite system) and DVB-S (Digital Video Broadcasting-Satellite). This report covers certain aspects of the operation and construction of such modulator. It will show how the digital signal can be conveyed over a bandpass channel with the aid of this modulation technique, similar to those for analogue messages. The QPSK was modulator designed and constructed for this project consists of several parts; Data Splitter, I-Q modulator and Summing Circuit. As results, at the part of data splitter, the data taken from random binary counter was split into two channels (I and Q channel) at half the data rate. The output was changed to a normalized value ±5 Vdc by using PNP transistor. At the part of I-Q modulator; multiplication of a carrier wave cos ooct, from quadrature oscillator, by logic '1' symbol state (+5 V) does not change the phase of the unmodulated carrier and if a logic '0' symbol state (-5 V) there has 180° phase change. The final results we look at the part of summing circuit, the QPSK involves transmitting one of four difference phase signals to represent each four digital data state; 50°, 130°, "221.7, and 313.3°.