13

520 - 23 - 03

VIDEO

0.1µF

CH1

CH2

2

75

Ω

560

Ω

4

8

680k

0.1µ

6

Photograph 3 shows the amount of sync clipping for a 560

Ω

source resistor. A graph of V

CLIP

versus R

S

is shown in

Figure 23, and Figure 24 shows the corresponding capacitor

value for a particular series resistor to provide a corner

frequency of 500 kHz.

In applications where signal levels are small the amount of

attenuation should be minimized. It follows from Figure 23 and

Figure 24 that in order to minimize attenuation a small series

resistor and a larger capacitor to ground should be chosen.

This however, increases the capacitive loading of the signal

source.

Another way to minimize the amount of attenuation is to control

the source resistance seen by the sync separator by using a

PNP emitter follower (Figure 25). A PNP emitter follower works

well to drive the sync separator, and does not require much

DC current because the transistor provides the current when

it is needed during sync. Figure 26 is a typical application

circuit that minimizes sync tip clipping.

Test Circuit 3

Fig. 25 PNP Emitter Follower Buffer

Photograph 3

100

90

80

70

60

50

40

30

20

10

0

0

100

200

300

400

500

600

700

SERIES RESISTOR (

Ω)

SERIES RESISTOR (

Ω)

0

100

200

300

400

500

600

700

10

9

8

7

6

5

4

3

2

1

0

Fig. 24 Cƒ vs Series Resistor

5.6k

2

V

CC

75

Ω

VIDEO

INPUT

FILTER

4

8

-5V

680k

0.1µ

6

V

CC

6

-5V

2

75

Ω

VIDEO

INPUT

4

8

5.6k

56p

5.6k

680k

0.1µ

Fig. 26 Typical NTSC Application Circuit

CH1

CH2