0195136047.pdf

15.2 ANALOG COMMUNICATION SYSTEMS 691

television broadcast system. The generation of VSB is similar to the generation of SSB, except
that the sideband-removal filter has a slightly different transfer function.

Message Demodulation

The major advantage of the conventional (standard) DSB AM signal transmission is the ease with
which the signal can be demodulated. The message signal is received by passing the rectified signal
through a low-pass filter whose bandwidth matches that of the message signal. The combination of
the rectifier and the low-pass filter is known as anenvelope detector, which is almost universally
used. A standard AM envelope detector consisting of a diode and anRCcircuit (which is basically
a simple low-pass filter) and its response are shown in Figure 15.2.8.
During the positive half-cycle of the input signal, the diode is conducting and the capacitor
charges up to the peak value of the input signal. When the input falls below the voltage on the
capacitor, the diode becomes reverse-biased and the input becomes disconnected from the output.
During this period, the capacitor discharges slowly through the load resistorR. On the next cycle
of the carrier, the diode conducts again when the input signal exceeds the voltage across the
capacitor. The capacitor charges up again to the peak value of the input signal, and the process is
continued. The time constantRCmust be selected to follow the variations in the envelope of the
carrier-modulated signal. Generally, it is so chosen that

Ac cos 2πfct

Sideband

filter

H(ω)

x(t)

Message

signal

Balanced

modulator

× VSB AM signal

Figure 15.2.7Generation of a VSB AM

signal.

Voltage proportional to sAM(t)

Proportional to

Ac + f(t)

Ac

t

Envelope

Carrier

R sd(t)

sd(t)

(a)

(b)

C

Figure 15.2.8Envelope detection of conventional (standard) AM signal.
(a)Standard AM envelope detector.(b)Its response.