Page 52 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
2

Modulation

Different modulation schemes have been adopted for radio services, such as
broadband AM and FM for broadcast, narrowband FM for line-of-site two-way
voice communications, single sideband (SSB) for long-distance voice commu-
nications via the ionosphere, and digital modulation for high-speed point-to-
point and multipoint microwave radio communications links.

2.1 Amplitude Modulation
2.1.1 Introduction
Amplitude modulation (AM) is the earliest modulation method for wireless
voice communications. It is very simple and cheap to work with from a hard-
ware standpoint, and it is still extensively used today for commercial and
shortwave broadcast, as well as in certain citizens band and limited ham radio
systems.

2.1.2 Fundamentals
Modulation is the way we insert baseband information on an RF carrier wave.
The baseband information can be voice, digital data, analog video, etc.
Demodulation is the procedure of extracting this baseband information, which
is then sent to a speaker for voice and music, or on to digital circuits for pro-
cessing or storage.
The most basic way we have of imprinting voice, data, or music on an RF
carrier is by modulating the amplitude of the carrier (Fig. 2.1). The unmod-
ulated carrier, which is produced by an oscillator, functions as the RF that
will transport the baseband modulation through space to a receiver. The
baseband is the intelligence—always at a much lower frequency than the
RF carrier—and is inserted onto the carrier by nonlinear mixing of these
two signals. As seen in the time domain, the amplitude of the RF carrier is

51
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.