RF and Microwave Circuit Design

14Receivers and Sub-Systems

14.1 Introduction

The superheterodyne (superhet) receiver is the most widely used receiver configuration for RF and microwave applications, and it is the main focus of this chapter. This book is concerned primarily with RF and microwave design, and consequently only the front-end aspects of receiver design will be considered. Noise is an important issue in the design of receiver circuits, and the chapter commences with a review of noise sources and their circuit specifications, leading to an analysis of the effects of noise in superhet receivers, and the use of noise budget graphs. Frequency mixers, which provide the frequency down-conversions in superhet designs, are often regarded as the critical components in the front-end of a superhet receiver, and the chapter concludes with a review of the various types of mixer used at RF and microwave frequencies.

14.2 Receiver Noise Sources

Noise refers to random, unwanted signals that occur naturally in an electronic system. In this section, we describe two of the most common sources of noise in electronic circuits, namely thermal noise and shot noise, and provide mathematical expressions that enable the noise to be quantified.

14.2.1 Thermal Noise

Thermal noise is due to the random motion of charge carriers in a conductor caused by thermal agitation. The random motion of the carriers produces small random currents, and consequently small random noise voltages. Thermal noise is sometimes referred ...