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Session 3548

QPSK Modulator: A Design Example for EET Kenneth Burbank Western Carolina University

Abstract Quaternary Phase Shift Keying (QPSK) is a fundamental method of encoding digital data in the phase of an analog carrier. This technique was used by early modems using the public telephone network. While the technology is no longer economically valid, the components of the modem span the coursework of most EET curricula; i.e. both analog and digital circuits, within a communications framework.

A directed design project approach was used. The instructor defined the objectives of the project, and directed the sequence and tools used in the design. The class was given a QPSK modulator constructed from TTL 7400 series gates and 741 op amps. From this circuit, the students had to identify parts and create a schematic. The linear circuits included an oscillator and a band pass filter, and both were analyzed using PSpice. The digital circuits were split into combinational and sequential sections, and timing diagrams were developed. The digital circuits were then divided into sections suitable for implementation using programmable logic devices. The signal waveforms were observed in both the time- and frequency-domains, and the concept of bit rate vs. baud rate was introduced.

This design project was first implemented in a sequential digital circuits course, after the students had taken a linear circuits course, both at the junior level. For most students, this was their first exposure to mixed digital-linear circuits, and the first circuit that they had to reverse engineer. The directed design approach is used as a prelude to the senior design projects. While the circuit is straightforward, it serves to reinforce the students’ knowledge of linear and digital circuits, their ability to analyze and design simple state machines, and their schematic capture and laboratory skills.

Introduction A key tenet for engineering technology is hands-on experiential learning. For Electrical Engineering Technology (EET), this most often translates into building, testing, and troubleshooting electronic circuits. Most commercial designs are mixed analog and digital circuits. For our curriculum linear and digital circuits are covered in separate courses. This means that portions of the digital circuits class must be combined with previously studied analog op amp circuitry. The design example described here is a modem, which is then covered in a later class in digital communications.

The goal of this project is to incorporate analog and digital circuit elements in a practical design that mimics a commercial device. Most students at least know that a modem is used between a computer and the Internet. From this fundamental knowledge, a modulator can be built using circuits already covered in the junior year. This design can be simulated using PSpice and Altera Proceedings of the 2003 American Society for Engineering Education Annual Conference & Exposition Copyright © 2003, American Society for Engineering Education

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Burbank, K. (2003, June), Qpsk Modulator: A Design Example For Eet Paper presented at 2003 Annual Conference, Nashville, Tennessee. 10.18260/1-2--11628