0195136047.pdf

702 COMMUNICATION SYSTEMS

f

f

f

0

(b)

(c)

(a)

fIF

Bc

0 fc = fLO − fIF

BRF < 2 fIF

0 fLO

2 fIF

fc f'c = fLO + fIF

Bc

BRF

Figure 15.2.22Frequency response

characteristics of IF and RF ampli-

fiers.(a)IF amplifier.(b)RF am-

plifier.(c)Desired signal and image

signal to be rejected.

reduce the gains of the RF and IF amplifier stages. A heavily filtered output from the demodulator

is often used to provide an automatic frequency control (AFC) loop through the local oscillator

that can be implemented to have electronic tuning by using a varactor. After manual tuning, the

AFC loop locks the receiver to the selected station. Finally, the response of the demodulator is

fed into the stereo demodulator, which is implemented as shown in Figure 15.2.19(b).

Television signalsin television signal transmission are the electric signals generated by

converting visual images throughraster (TV image area) scanning. The two-dimensional image

or picture is converted into a one-dimensional electric signal by sequentially scanning the image

and producing an electrical signal that is proportional to the brightness level of the image. A

television camera, which optically focuses the image on a photo cathode tube that consists of

a photosensitive surface, is used for scanning. An electron beam produces an output current or

voltage that is proportional to the brightness of the image, known as avideo signal. The scanning

of the electron beam is controlled by two voltages, as shown in Figure 15.2.24, applied across

the horizontal and vertical deflection plates. In the raster scanning in an NTSC TV system, the

image is divided into 525 lines which define aframe,as illustrated in Figure 15.2.25. The resulting

signal is transmitted in^1 / 30 second. The number of lines determines the picture resolution and,

along with the rate of transmission, sets the channel bandwidth needed for image transmission.

However, the time interval of^1 / 30 second to transmit a complete image is not generally fast

enough to avoid flickering, which is annoying to the average viewer. Therefore, to overcome the

flickering, the scanning of the image is performed in aninterlaced pattern,as shown in Figure