FMS6243 Datasheet, PDF (7/10 Pages) Fairchild Semiconductor – Low-Cost, 3-Channel, SD Video Filter Drivers with External Delay Control

Signal Peaking Adjustment
The peaking of a video input signal can be adjusted by
placing a peaking capacitor across the series-75ohm
resistor on the output of the FMS6243. Where the input
video signal to the FMS6243 has a soft roll-off, meaning
the input video signal is attenuated at 4Mhz by -0.5dB,
the Chroma/Luma gain is approximately -5% and fails a
system specification of +- 2.5%. This attenuation can be
adjusted by adding a 150pF capacitor across the series-
75ohm resistor on the output (see Figure 15). This brings
the attenuation at 4.0Mhz to approximately 0dB, giving a
Chroma/Luma gain of 0%. Figure 13 shows the peaking
effect of adding a peaking capacitor across the series-
75ohm resistor. The graph shows a 10pF delay capacitor
with a 10pF, 50pF, and 100pF peaking capacitor.
6.40
6.35
6.30
6.25
6.20
6.15
6.10
6.05
6.00
5.95
5.90
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Frequency (MHz)
10pF10pF
10pF50pF
10pF100pF
Figure 13. Frequency Response Delay Capacitor vs.
Peaking Capacitor
Group Delay and Peaking Adjustment
Simultaneously
If both a group delay adjustment and a peaking adjust-
ment need to be incorporated into the system design, the
following methodology should be followed. Address the
group delay adjustment first, then the peaking adjus-
ment, because the group delay adjustment causes a
video signal attenuation at 4Mhz.
6.35
400KHz
6.30
3.58MHz 4.43MHz
6.25
6.20
10pF
40pF
6.15
80pF
6.10
6.05
6.00
5.95
5.90
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Frequency (MHz)
Figure 14. Frequency Response vs. Delay Cap. Value
Delay Cap
1
2
In1
3
In2
4
In3
5
6
7
Delay Cap
DCAP1
GND1
IN1
IN2
IN3
GND2
DCAP2
V_CC
VCC ¹⁴
13
GND4
OUT1 ¹²
11
OUT2
OUT3 ¹⁰ _{Delay Cap}
DCAP3 ⁹
GND3 ⁸
Figure 15. Schematic
peaking
75
peaking
75
peaking
75
220μF
Out1
220μF
Out2
220μF
Out3
© 2007 Fairchild Semiconductor Corporation
FMS6243 Rev. 1.0.0
7
www.fairchildsemi.com