0195136047.pdf

700 COMMUNICATION SYSTEMS

concern, PM and particularly FM systems are least favorable. Only in terms of their high level of

noise immunity, their usage may sometimes be justified.

FM, with its high level of noise immunity, and hence power efficiency, is widely used

on high-fidelity radio broadcasting and power-critical communication links such as point-to-

point communication systems. It is also employed for satellite links and voice transmission on

microwave LOS systems. When the transmitted power is a major concern, conventional AM

and VSB (being the least power-efficient systems) are not used, unless their development can be

justified by the simplicity of the receiver structure.

From the viewpoint of ease of implementation, the simplest receiver structure is that of

conventional AM. Standard AM, VSB, and FM are widely used for AM, television, and high-

fidelity FM broadcasting. The relative power inefficiency of the AM transmitter is compensated

for by the extremely simple structure of several billions of receivers. The receiver structure

is much more complicated for DSB SC and SSB SC systems, since they require synchronous

demodulation. These systems, therefore, are never used for broadcasting purposes. Note that

DSB SC also suffers from its relative bandwidth inefficiency.

Radio and Television Broadcasting

Radio (AM and FM) and television broadcasting are the most familiar forms of communication

via analog transmission systems. The receiver most commonly used in AM radio broadcasting is

thesuperheterodyne receiver,shown in Figure 15.2.20, which consists of a radio-frequency (RF)

tuned amplifier, mixer, local oscillator, intermediate frequency (IF) amplifier, envelope detector,

audio-frequency amplifier, and a loudspeaker. Tuning at the desired radio frequencyfcis achieved

by a variable capacitor, which simultaneously tunes the RF amplifier and the frequencyfLOof the

local oscillator. Every AM radio signal, in a superheterodyne receiver, is converted to a common IF

frequency offIF=|fc−fLO|=455 kHz, which allows the use of a single tuned IF amplifier for

signals from any radio station in the frequency band. Matching the bandwidth of the transmitted

signal, the IF amplifier is set to have a bandwidth of 10 kHz.

The frequency conversion to IF is done by the combination of the RF amplifier and the mixer.

The tuning range of the local oscillator is 955–2,055 kHz.fLOcould be either higher or lower than

fc.IffLOis higher thanfc, thenfLO=fc+fIF. By tuning the RF amplifier to the frequencyfcand

mixing its output with the local oscillator frequencyfLO, we obtain two signal components: one

centered at the difference frequencyfIF; and the other centered at the sum frequency 2fc+fIF,

known as theimage frequency. Only the first component is passed on by the IF amplifier. The

RF

amplifier Mixer

IF

amplifier

Local

oscillator

Common tuning

Automatic

volume control

Detector

Loud speaker

Audio

frequency

amplifier

Figure 15.2.20Superheterodyne receiver.