An Adjustable Video Cable Equalizer Using The Lt1256

Linear Technology Corporation

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An Adjustable Video Cable Equalizer Using The LT1256
Design Note 92
Frank Cox
This design note presents a voltage controlled cable equal-
izer based on the LT1256 video fader. The circuit features
ease of adjustment, simplicity and the capability for remote
control. The amount of equalization can be adjusted con-
tinuously from the maximum allowed by the passive com-
ponents to none at all. While the example shows video, this
high performance equalizer can be used in any system
using long runs of coaxial cable or twisted pair to transmit
analog signals.
A voltage or current controlled equalizer is essential in
systems which automatically set cable compensation. In
systems where cable equalization is set manually, a voltage
controlled equalizer is still preferred as it does not require
routing the signal path to the control. Instead, only a DC
control voltage passes from the front panel to the equalizer.
Automatic equalization is possible for properly character-
ized video cables. Maximum equalization is set to coincide
with the maximum length of cable expected; the equalizer
is controlled by a servo loop. One method of generating the
necessary control voltage is to sample the color burst
+V
10k
V
FS
LT1256 PIN 12)
U3
(2.5)
V
C
LT1256 PIN 3)
CONTROL VOLTAGE
VIDEO
INPUT
75Ω
1
13
amplitude and compare this with a reference voltage using
a summing integrator. Since the frequency roll-off of the
cable is known (and fixed for a given cable) only the amount
of equalization needs to be adjusted.
In many applications color video is transmitted down long
runs of coaxial or twisted-pair cable. Losses in the cable
increase with signal frequency and cable length. The type
of cable will determine the rate of high frequency loss.
Color information in NTSC video is contained primarily in
the high frequency portion of the spectrum. Besides caus-
ing a loss of detail in the picture, excessive high frequency
loss will make reliable decoding of the composite color
signal more difficult or impossible. Most commercial dis-
tribution amplifiers have provisions for equalizing the cable
losses, but many times these units come at a high cost.
Figure 1 is a complete schematic of the cable equalizer. The
LT1256 (U1) is a two input, one output 40MHz current
feedback amplifier with a linear control circuit that sets the
and LTC are registered trademarks and LT is a trademark of
Linear Technology Corporation.
1.5k
+
8
3
10k
14
+
U2
6
75Ω
+
2
VIDEO
OUTPUT
1k
2
U1
1.5k
240Ω
50pF
12k
14.3k
1.5k
OVERALL GAIN
ADJUST
500Ω
110pF
100pF
20k
50pF
1.43k
DN92 • F01
Figure 1. LT1256 Cable Equalizer
12/94/92