0195136047.pdf

692 COMMUNICATION SYSTEMS

1

fc

<< RC <<

1

W

(15.2.8)

The simplicity of the demodulator has made conventional DSB AM a practical choice for AM

radio broadcasting. A relatively inexpensive demodulator is very important in view of the several

billions of radio receivers. With only a few broadcast transmitters and numerous receivers, the

power inefficiency of conventional AM is justified. Thus, it is cost-effective to construct powerful

transmitters and sacrifice power efficiency in order to have simpler signal demodulation at the

receivers.

Demodulation of DSB SC AM signalsrequires asynchronous demodulator,which is also

known ascoherentorsynchronous detector.That is, the demodulator must use a coherent phase

reference. This is usually generated by means of aphase-locked loop(PLL), which forces the

phase of the voltage-controlled oscillator to follow the phase of the reference signal, to demodulate

the received signal. The need for such a device is a chief disadvantage of the DSB SC scheme.

Figure 15.2.9 shows the general configuration. A PLL is utilized to generate a phase-coherent

carrier signal that is mixed with the received signal in a balanced modulator. The output of the

balanced modulator is fed into a low-pass filter of bandwidthW, which passes the desired signal

and rejects all signal (and noise) components aboveWHz.

Demodulation of SSB signalsalso requires the use of a phase-coherent reference. Figure

15.2.10 shows the general configuration to demodulate the SSB signal. A small carrier component,

which is transmitted along with the message, is inserted. A balanced modulator is used for

frequency conversion of the bandpass signal to low pass or baseband.

Demodulation of VSB signalsgenerally uses a synchronous detector in the configuration of

Figure 15.2.10. In VSB, a carrier component is generally transmitted along with the message

sidebands. The existence of the carrier component makes it possible to extract a phase-coherent

reference for demodulation in a balanced modulator. In some applications, such as television

broadcasting, however, a large carrier component is transmitted along with the message in the

VSB signal, and in such a case it is possible to recover the message by passing the received signal

through an envelope detector.

Balanced

modulator

Low-pass

filter

Phase-locked

loop

sDSB(t)

Received

signal

x(t)

Message

signal

Figure 15.2.9Demodulator for

the DSB SC AM signal.

Balanced

modulator

Low-pass

filter

Estimate

carrier

component

sSSB(t)

Received

signal

x(t)

Message

signal

Figure 15.2.10Demodulation of

SSB AM signal with a carrier

component.