Chapter Overview

In radio communication systems, the transmitted signal is very weak when it reaches the receiver, particularly when it has traveled over a long distance. The signal, which has shared the free-space transmission media with thousands of other radio signals, has also picked up noise of various kinds. Radio receivers must provide the sensitivity and selectivity that permit full recovery of the original intelligence signal. The radio receiver best suited to this task is known as the superheterodyne receiver. Invented in the early 1900s, the superheterodyne is used today in most electronic communication systems. This chapter reviews the basic principles of signal reception and discusses various superheterodyne circuits including direct conversion.

Chapter Outline

9-1 Basic Principles of Signal Reproduction
9-2 Superheterodyne Receivers
9-3 Frequency Conversion
9-4 Intermediate Frequency and Images
9-5 Noise
9-6 Typical Receiver Circuits
9-7 Receivers and Transceivers

Chapter Objectives

• List the benefits of a superheterodyne over a TRF receiver and identify the function of each component of a superheterodyne, including all selectivity functions.
• Express the relationship between the IF, local oscillator, and signal frequencies mathematically and calculate any one of them, given the other two.
• Explain how the design of dual-conversion receivers allows them to enhance selectivity and eliminate image problems.
• Describe the operation of the most common types of mixer circuits.
• Explain the architecture and operation of direct conversion and software-defined radios.
• List the major types of external and internal noise and explain how each interferes with signals both before and after they reach the receiver.
• Calculate the noise factor, noise figure, and noise temperature of a receiver.
• Describe the operation and purpose of the AGC circuit in a receiver.
• Explain the operation of squelch circuits.