Photograph 3 shows the amount of sync clipping for a 560 Ω
source resistor. A graph of VCLIP versus RS 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.
Anotherwaytominimizetheamountofattenuationistocontrol
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.
In applications where signal levels are small the amount of
attenuationshouldbeminimized. ItfollowsfromFigure23and
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.
CH1
CH2
CH1
CH2
VIDEO
8
6
2
560Ω
75Ω
0.1µF
4
680k
0.1µ
Test Circuit 3
Photograph 3
100
90
80
70
60
50
40
30
20
10
0
10
9
8
7
6
5
4
3
2
1
0
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
SERIES RESISTOR (Ω)
SERIES RESISTOR (Ω)
Fig. 24 Cƒ vs Series Resistor
Fig. 23 V
vs Series Resistor
CLIP
VCC
VCC
5.6k
5.6k
VIDEO
INPUT
8
8
VIDEO
INPUT
2
2
5.6k
CC
CC
FILTER
6
4
6
4
680k
0.1µ
75Ω
56p
75Ω
680k
0.1µ
-5V
-5V
Fig. 25 PNP Emitter Follower Buffer
Fig. 26 Typical NTSC Application Circuit
520 - 23 - 03
13
GENNUM [ GENNUM CORPORATION ]
