This book grew out of a series in a British magazine called Electronics World/Wireless World.
In fact, most of the material presented in this book first appeared, at least in general form,
in that publication. Other material was written new for this book.
What are the ‘radio frequencies’?
The radio frequencies (RF) are, roughly speaking, those which are above human hearing,
and extend into the microwave spectrum to the edge of the infrared region. That means
the RF frequencies are roughly 20 000 Hz to many, many gigahertz. In this book, we will
assume the radio frequencies are up to about 30 GHz for practical purposes.
There are radio frequencies below 20 kHz, however. In fact, there are radio navigation
transmitters operating in the 10 to 14 kHz region. The difference is that the waves
generated by those stations are electromagnetic waves, not acoustical waves, so humans
cannot hear them.
Why are radio frequencies different?
Why are radio frequencies different from lower frequencies? The difference is largely due
to the fact that capacitive and inductive stray reactances tend to be more significant at
those frequencies than they are at lower frequencies. At the lower frequencies, those stray
or distributed reactances exist, but they can usually be ignored. Their values do not
approach the amount required to establish resonance, or frequency responses such as high
pass, low pass or bandpass. At RF frequencies, the stray or distributed reactances tend to
be important. As the frequency drops into the audio range (1–20 kHz), and the ultrasonic
range (20–100 kHz) the importance of stray reactances tends to diminish slightly.
What this book covers
We will look at a number of different things regarding RF circuits. But first, we will take
a look at signals and noise. This sets the scene for a general look at radio receivers. Most
radio frequency systems have one or more receivers, so they are an important type of
