EQUALIZATION - Advanced Signal Integrity for High-Speed Digital Design - page 536

where y k is the input to the symbol detector, c DLE i the coefficients of the feed-
forward filter, c FBF i the coefficients of the feedback filter, and x k the input to the
DFE. The previously detected symbols, b k , are expressed as

0if y k <y threshold
1

b k = (12-33)

if y k >y threshold

where y threshold is the decision threshold.
Decision feedback equalization is often used to remove ISI that is caused by
reflections due to impedance mismatch and may also use zero forcing and LMS
adaptive algorithms [Proakis, 2001]. Note that because of the nonlinearity of the
DFE response, it must be modeled in the time domain.

Example 12-7 DFE Operation for a 10-Gb/s Signaling Link We now apply
a DFE to the differential transmission lines that we have used throughout this
chapter. To guarantee successful operation at 10 Gb/s, the system must meet
minimum differential received specs of 80 mV and 80 ps for the height and width,
respectively. Our system, shown in Figure 12-39, uses 5.057-mA current-mode
transmitters with 5 M output resistance and 0.5 pF output capacitance. The
lines are terminated to ground in 50 , and the receiver has an input capacitance
of 0.5 pF. The model for the differential pair is generated using the methodology
presented in Chapter 10 from the distributed transmission-line parameters listed
below that were calculated at a reference frequency of 1 GHz:

3 . 299

3 . 299 nH / cm

1 . 098

1 . 098 pF / cm

0 . 407 0 . 085

L = C =

0 . 407 0 . 085

509 . 1

509 . 1 m / cm G =

0 . 131

0 . 131 mS / cm

60 . 63 0 . 012

R =

60 . 64 0 . 012

As the figure shows, the system employs the linear equalizer at the transmitter.
The transmit equalization does not include adaptive capability, and is set based on
an expected channel loss characteristic. However, while the transmit equalizer
coefficients open the data eye, they are too strong for this particular system,

i Tx

+

5M Ω 0.5 pF 50 Ω 0.5 pF

+
−

FBF

0.381 m

+

0.65

C DLE =

− 0.35

0.020

C FBF =

0.008

i Tx

5M Ω 0.5 pF 0.5 pF

50 Ω

Figure 12-39 Differential signaling system for Example 12-7.