GS1881 Datasheet, PDF (13/14 Pages) Gennum Corporation – Monolithic Video Sync Separators

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.
CH1
CH2
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.
CH1
VIDEO
2
560Ω 0.1µF
75Ω
4
8
6
680k _0.1µ
CH2
Test Circuit 3
Photograph 3
100
90
80
70
60
50
40
30
20
10
0
0
100 200
300 400
500 600 700
SERIES RESISTOR (Ω)
Fig. 23 VCLIP vs Series Resistor
10
9
8
7
6
5
4
3
2
1
0
0
100 200
300 400
500 600 700
SERIES RESISTOR (Ω)
Fig. 24 Cƒ vs Series Resistor
VIDEO
INPUT
FILTER
75Ω
V_CC
5.6k
C_C
-5V
8
2
6
4
680k _0.1µ
VIDEO
INPUT
5.6k
75Ω
V_CC
5.6k
C_C
56p
-5V
8
2
4
6
680k _0.1µ
Fig. 25 PNP Emitter Follower Buffer
Fig. 26 Typical NTSC Application Circuit
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