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  • QPSK Demodulator

  • Created: Feb 25, 2014

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Summary

This circuit is a demodulator for Quadrature Phase-Shift Keying (QPSK). QPSK is under the Phase-shift keying (PSK) type of modulation that is a digital modulation scheme that conveys data by changing the phase of a reference signal (the carrier wave). QPSK, also known as quadriphase PSK, 4-PSK, or 4-QAM, uses four points on the constellation diagram, equispaced around a circle. With four phases, QPSK can encode two bits per symbol. With this technique, the bit error rate of a modulation system can be minimized.

The typical approach to QPSK demodulation is the decomposition of the modulated input signal via two multipliers driven by a fixed-frequency oscillator whose two outputs are 90° apart. To perform the demodulation, the circuit uses amplifiers, mixers, and logic gates to recover the data signal. The LM318, a high-speed operational amplifier, is used to amplify the input signal to prepare the signal for the next steps in the demodulation process. The HEF4013B, a D-type flip-flop, is used to toggle the states of the input signal. The circuit also uses register such as the CD4076BMS, a CMOS 4-bit D-type register, and the HEF4015B Dual 4-bit static shift register. The registers are used as the memory of the system as it stores the previous state of the digital signal. The HEF4030BP 2-input Exclusive-OR gate is also used in the circuit which detects the value of the received phases and produces a corresponding logic output to each phase: for 0° input phase, it produces logic “0” in both branches, for a phase of 90°, it produces logic “1” in the upper branch, for a phase of 270°, it produces logic “1” at the lower branch, and for a phase of 180°, it produces logic “1” in both branches.

Wireless Local Area Network is one application where Phase-shift keying is used. The variety of PSK that is utilized depends on the data rate requirement. At the basic-rate of 1Mbit/s, it uses DBPSK (differential BPSK). For the extended rate of 2Mbit/s, DQPSK is used. For 5.5Mbit/s and the full-rate of 11Mbit/s, QPSK is employed, but has to be coupled with complementary code keying. The IEEE802.15.4 also used PSK which allows the use of two frequency bands: 868–915MHz using BPSK and at 2.4GHz using OQPSK.

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