Page 17 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Chapter 1

Introduction

This book will be a journey for both the hobbyist and the engineer on how radios
are designed. The book starts off with simple designs such as an offshoot of crystal
radios, tuned radio-frequency radios, to more complicated designs leading up to
superheterodyne tuners and radios. Each chapter presents not only the circuits but
also how each circuit was designed considering the tradeoffs in terms of

performance, power consumption, availability of parts, and the number of parts.
In the engineering field, often there is no one best design to solve a problem. In
some chapters, therefore, alternate designs will be presented.
Chapters 4 through 12 will walk the hobbyist through various radio projects. For
those with an engineering background by practice and/or by academia, Chapters 13
through 23 will provide insights into the theory of the various circuits used in the

projects, such as filter circuits, amplifiers, oscillators, and mixers.
For now, an overview of the various radios is given below.
Tuned Radio-Frequency (TRF) Radios

The simplest radio is the tuned radio-frequency radio, better known as the TRF
radio. It consists mainly of a tunable filter, an amplifier, and a detector.

A tunable filter just means that the frequency of the filter can be varied. Very much
like a violin string can be tuned to a specific frequency by varying the length of the
string by using one's finger, a tunable filter can be varied by changing the values of
the filter components.
Generally, a tuned filter consists of two components, a capacitor and an inductor.

In a violin, the longer the string, the lower is the frequency that results. Similarly, in
a tuned filter, the longer the wire used for making the inductor, the lower is the
tuned frequency with the capacitor.
In TRF radios, there are usually two ways to vary the frequency of the tuned filter..
One is to vary the capacitance by using a variable capacitor. This way is the most
common method. Virtually all consumer amplitude-modulation (AM) radios use a

variable capacitor, which may be a mechanical type such as air- or poly-insulated
variable-capacitor type or an electronic variable capacitor. In the mechanical type of
variable capacitor, turning a shaft varies the capacitance. In an electronic variable
capacitor, known as a varactor diode, varying a voltage across the varactor diode
varies its capacitance. This book will deal with the mechanical types of variable

capaCitors.
The second way to vary the frequency of a tunable filter is to vary the inductance
of an inductor or coil via a tuning slug. This method is not used often in consumer
radios because of cost. However, for very high-performance radios, variable