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A PSK31 Audio Beacon Project Provides a Laboratory Capstone Design Experience In Digital Communications

Abstract

This paper focuses on combining a 433.92 MHz wireless temperature sensor with a PSK31 Audio Beacon transmitter to provide a laboratory capstone experience for junior students in Electrical and Computer Engineering Technology. The laboratory capstone PSK31 Audio Beacon Project is structured to support course goals and objectives of "Digital Communications" offered at the beginning of the junior year. The paper presents the "project concept" together with its theory of operation and project schematic. Also, students are required to build a prototype audio beacon transmitter using surface mount components, implement a packaging concept, and write a complete project specification. The secondary goals of the project are to introduce students to radio frequency (RF) amplifier concepts, and to prepare students for the upcoming Senior Design Capstone Experience required for a baccalaureate degree in engineering technology. To this end, students are required to maintain a project journal to record ideas, data, test results, and experiences throughout the project. The authors have found that student attitude towards the course and subsequent feedback to be most rewarding, and were very encouraged by the level of student involvement in the project.

Introduction

Over the past four years a phase shift keying (PSK31) Audio Beacon project has been introduced to a group of students in a Digital Communications at the University of Cincinnati. After a couple of iterations of the project, the authors noticed that student comments on the “project evaluation” form tended to center around adding additional capabilities to the project in order to make it more interesting. This paper is a direct result of those student evaluations together with a conscientious attempt on behalf of the authors to utilize projects in the laboratory portion of the course to stimulate student interest in RF communication concepts.

This laboratory capstone project illustrates the integration of a 433.92 MHz wireless temperature sensor into a PSK31 modulated audio beacon transmitter. The temperature sensor is a Dallas Semiconductor DS18S20 integrated with a standard "off-the-shelf" 433.92 MHz RF transmitter module that uses amplitude modulation. The transmission method known as "On-Off Keying (OOK)" transmits the temperature data by simply switching the carrier signal on and off. The RF data-link serial receiver can be located up to 500 feet from the transmitter and interfaces to the PSK31 Audio Beacon via an RS232 interface. The Audio Beacon features PSK31 encoding and audio waveform generation using a single-chip SX28 RISC microcontroller operating at 50 MHz. The beacon has a choice of three base carrier audio frequencies, namely: 500 Hz, 1 kHz, and 2 kHz. As an added bonus, students are introduced to RF amplifier concepts by taking the output from the audio beacon and injecting the audio signal into the microphone input of a five watt single-sideband transmitter. To form a conventional RF Beacon operating on 3.579 MHz (Amateur Radio 80 meter band), the output of the Class AB push-pull RF power amplifier is sequentially reduced by switching in-line a series of 3 dB attenuators until the output power reaches 0.63 watts. The transmit Beacon data string consists of station identification (Amateur

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Everly, J., & Resnick, B. (2008, June), A Psk31 Audio Beacon Project Provides A Laboratory Capstone Design Experience In Digital Communications Paper presented at 2008 Annual Conference & Exposition, Pittsburgh, Pennsylvania. 10.18260/1-2--3752