19-11-2012, 04:48 PM
Amplitude Modulation Fundamentals
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In the modulation process, the baseband voice, video, or digital signal modifies
another, higher-frequency signal called the carrier, which is usually a sine wave.
A sine wave carrier can be modified by the intelligence signal through amplitude
modulation, frequency modulation, or phase modulation. The focus of this
chapter is amplitude modulation (AM).
Objectives
After completing this chapter, you will be able to:
■ Calculate the modulation index and percentage of modulation of an AM
signal, given the amplitudes of the carrier and modulating signals.
■ Define overmodulation and explain how to alleviate its effects.
■ Explain how the power in an AM signal is distributed between the carrier
and the sideband, and then compute the carrier and sideband powers,
given the percentage of modulation.
■ Compute sideband frequencies, given carrier and modulating signal
frequencies.
■ Compare time-domain, frequency-domain, and phasor representations of
an AM signal.
■ Explain what is meant by the terms DSB and SSB and state the main
advantages of an SSB signal over a conventional AM signal.
■ Calculate peak envelope power (PEP), given signal voltages and load
impedances.
AM Concepts
As the name suggests, in AM, the information signal varies the amplitude of the carrier
sine wave. The instantaneous value of the carrier amplitude changes in accordance with
the amplitude and frequency variations of the modulating signal. Figure 3-1 shows a singlefrequency
sine wave intelligence signal modulating a higher-frequency carrier. The carrier
frequency remains constant during the modulation process, but its amplitude varies in
accordance with the modulating signal. An increase in the amplitude of the modulating
signal causes the amplitude of the carrier to increase. Both the positive and the negative
peaks of the carrier wave vary with the modulating signal. An increase or a decrease in
the amplitude of the modulating signal causes a corresponding increase or decrease in both
the positive and the negative peaks of the carrier amplitude.
An imaginary line connecting the positive peaks and negative peaks of the carrier
waveform (the dashed line in Fig. 3-1) gives the exact shape of the modulating
information signal. This imaginary line on the carrier waveform is known as the
envelope.
Because complex waveforms such as that shown in Fig. 3-1 are difficult to draw,
they are often simplified by representing the high-frequency carrier wave as many equally
spaced vertical lines whose amplitudes vary in accordance with a modulating signal, as
in Fig. 3-2. This method of representation is used throughout this book.
The signals illustrated in Figs. 3-1 and 3-2 show the variation of the carrier amplitude
with respect to time and are said to be in the time domain. Time-domain signals—
voltage or cur
Sidebands and the
Frequency Domain
Whenever a carrier is modulated by an information signal, new signals at different
frequencies are generated as part of the process. These new frequencies, which are called
side frequencies, or sidebands, occur in the frequency spectrum directly above and
directly below the carrier frequency. More specifically, the sidebands occur at frequencies
that are the sum and difference of the carrier and modulating frequencies. When signals
of more than one frequency make up a waveform, it is often better to show the AM
signal in the frequency domain rather than in the time domain.
Pulse Modulation
When complex signals such as pulses or rectangular waves modulate a carrier, a broad
spectrum of sidebands are produced. According to Fourier theory, complex signals such
as square waves, triangular waves, sawtooth waves, and distorted sine waves are simply
made up of a fundamental sine wave and numerous harmonic signals at different amplitudes.
Assume that a carrier is amplitude-modulated by a square wave which is made up
of a fundamental sine wave and all odd harmonics. A modulating square wave will produce
sidebands at frequencies based upon the fundamental sine wave as well as at the
third, fifth, seventh, etc., harmonics, resulting in a frequency-domain plot like that shown
in Fig. 3-11. As can be seen, pulses generate extremely wide-bandwidth signals. In order
for a square wave to be transmitted and faithfully received without distortion or degradation,
all the most significant sidebands must be passed by the antennas and the transmitting
and receiving circuits.
Figure 3-12 shows the AM wave resulting when a square wave modulates a sine
wave carrier. In Fig. 3-12(a), the percentage of modulation is 50; in Fig. 3-12(b), it is
100. In this case, when the square wave goes negative, it drives the carrier amplitude to
zero. Amplitude modulation by square waves or rectangular binary pulses is referred to
as amplitude-shift keying (ASK). ASK is used in some types of data communication when
binary information is to be transmitted.