Datasheet

DIFFERENTIAL CABLE EQUALIZER AGC LOOP
2.0
1.5
1.0
0.5
0
-0.5
-1.0
Frequency(MHz)
1694FCableAttenuation(dB)
EqualizerGain(dB)
1 10 100
CableAttenuation
VCA821Equalization
R
2
453W
+V
IN
R
G+
R
G-
-V
IN
R
9
432W
C
9
10 Fm
R
8
50W
R
5
50W
R
1
20W
R
10
75W
VCA821
R
21
3kW
C
5
4pF
R
17
6kW
C
6
320 Fm
R
18
13.6kW
C
7
300 Fm
V
G DC
=+1V
75 LoadW
V
G
GND
V
REF
FB
V
OUT
V
IN
V
OUT
VCA821
www.ti.com ....................................................................................................................................... SBOS407B – DECEMBER 2007 – REVISED DECEMBER 2008
A differential cable equalizer can easily be In the typical AGC loop shown in Figure 81 , the
implemented using the VCA821. An example of a OPA695 follows the VCA821 to provide 40dB of
cable equalization for 100 feet of Belden cable 1694F overall gain. The output of the OPA695 is rectified
is illustrated in Figure 78 , with Figure 79 showing the and integrated by an OPA820 to control the gain of
result for this implementation. This implementation the VCA821. when the output level exceeds the
has a maximum error of 0.2dB from dc to 70MHz. reference voltage (V
REF
), the integrator ramps down
reducing the gain of the AGC loop. Conversely, if the
Note that this implementation shows the cable
output is too small, the integrator ramps up increasing
attenuation side-by-side with the equalization in the
the net gain and the output voltage.
same plot. For a given frequency, the equalization
function realized with the VCA821 matches the cable
attenuation. The circuit in Figure 80 is a driver circuit.
To implement a receiver circuit, the signal is received
differentially between the +V
IN
and – V
IN
inputs.
Figure 79. Cable Attenuation versus Equalizer Gain
Figure 80. Differential Cable Equalizer
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