Basic Wireless Communication for Microcontrollers
Chapter 2 - Design Project 1: Crystal Radio Receiver
Project Overview
The venerable crystal receiver may seem like a strange place to begin a tutorial on digital wireless communications. However, as complex as the electronics used in modern communications are, a lot of the misunderstandings and difficulties which arise in trying to work with them can be traced back to an incomplete understanding of the basics of radio engineering. Because a crystal receiver is so simple yet uses most of the same vital concepts as all receivers, it is an ideal platform to review the foundations of RF systems. You may well be surprised at all of the considerations that go into designing a crystal receiving system.
The job of the crystal set is to receive AM radio signals, select out one particular signal, and recover and play the audio contained in it. It is the simplest receiver possible which can fulfill this purpose. In contrast to most receivers, it requires no power source other than the received signal itself.
To accomplish this task, the receiver consists of four parts: an antenna system, a tuner, a detector, and an earphone. The antenna converts EM waves into electrical current, the tuner filters the current to select out only the desired station, the detector extracts the audio modulating waveform from the RF, and the earphone converts the audio electrical signal to sound waves for your ear. Our goal, then, for the first project is to design and build such a receiver, while learning the basics of radio engineering, such as impedance matching and antennas, along the way.
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