Chapter 19. Radio Frequency

19.0 Introduction

It used to be that any electronics book worth its salt would include discrete circuits for AM and FM broadcast receivers and maybe even discuss how an analog TV works. In these digital days, topics like this are only for historical interest.

For this reason, this chapter mostly concentrates on the use of digital communications using radio frequencies. The discussion of the nature of radio is restricted to this introduction. The recipes themselves are fundamentally practical.

We take radio frequency for granted now, but when it first became popular, it must have seemed like magic. After all, it allowed you to talk to someone miles away without any wires connecting them. How could this be possible?

Transmitters and the Law

Recipe 19.1Recipe 19.2, and Recipe 19.4 are radio transmitters. Although most countries allow the use of low-power, short-range FM transmitters and allocate certain bands for use with packet radio, you should check your local regulations and make sure you are not breaking the law by using any of these designs.

Amplitude Modulation (AM)

It all starts with AM transmissions, which use a carrier frequency modulated by the audio signal that is being transmitted. Figure 19-1 shows how this works.

In Figure 19-1 the carrier frequency is only about 4.5 times the audio signal. In a real AM broadcast, the audio signal will generally be a maximum of 16kHz against a carrier frequency of 500kHz, making at least ten times ...

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