Digital Signal Processing Reference
In-Depth Information
C
R
1
R
1
In
Out
C
L
R
1
R
1
In
Out
R
2
R
2
L
L
In
Out
R
1
R
1
C
(a)
(b)
Figure 12-20
Alternate passive CTLE implementation: (a) single-ended; (b) differential.
for both singled-ended and differential signals. Analysis of this equalizer is left
as a problem at the end of the chapter. Passive equalizers offer the advantage
of improved performance with no additional power consumption. In addition,
multi-Gb/s data rates drive smaller, higher-frequency passive devices, making
integration into the silicon an attractive design option. However, they demand
tighter control of the components values than for typical digital applications.
In addition, the frequency response of passive equalizer circuits is not directly
tunable without additional active control circuitry, which will tend to degrade the
power benefit.
12.2.2 Active CTLEs
Equalizers can also be constructed with active components (amplifiers) to provide
some signal gain. This type of equalizer is often done using a split-path approach,
as shown in Figure 12-21 [Liu and Ling, 2004]. The incoming signal is fed into
a unity-gain path and a high-frequency boost path, which are then summed to
create the output. The transfer function for this equalizer is
1
+
R
2
/R
3
H(f)
=
1
/
2
πf R
1
C
1
+
1
(12-18)
1
+
The high-pass filter has a voltage gain of 1
+
R
2
/R
3
, with a corner frequency
equal to 1/
R
1
C
1
. The equalizer transfer function for a high-pass filter voltage
gain of
3
.
5 dB and a 5-GHz corner frequency is plotted along with that of the
passive equalizer in Figure 12-22. The magnitude plots for the two equalizers
show a very similar shape, with an offset of approximately 5 dB. The resulting
−
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