Digital Signal Processing Reference
In-Depth Information
CHAPTER 10
Understanding Crosstalk
10.1 Introduction
Crosstalk is the coupling of signal energy from one or more aggressor signals to one
or more victim signals. This causes unintended currents to flow in the victim, which
creates noise voltages.
When designers think of crosstalk, they often only consider how the noise volt-
age causes false triggering at a receiver and so disturbs system timing or reduces
noise margin. Signal integrity engineers take a wider view and recognize that in
addition to those types of problems, crosstalk is capable of causing pulse distortion
and (as described in Chapter 2) damage to the I/O cell if the transient voltages are
large enough.
10.2
How Is Crosstalk Created and What Are Its Characteristics?
The mutual capacitance and inductance appearing between the aggressor and vic-
tim traces [1-6] create the coupling responsible for crosstalk. This coupling was
introduced in Chapter 9 and is shown for two traces in Figure 10.1.
Although only two traces are shown, the mutual capacitance (
C
m
) and mutual
inductance (
L
m
) represent the sum of the electric and magnetic coupling from any
number of aggressors to the victim.
The electric and magnetic coupling causes current to flow in the victim [8], as
shown in Figure 10.2.
By making the loads resistors equal in value to the impedance of the line, Ohm's
law can be used to find the load voltages, and reflections (discussed in Chapter 11)
are eliminated.
The current injected by the mutual capacitance (
I
c
) flows in the victim trace
with half flowing toward the near end and the other half flowing toward the far
end. The current induced by the mutual inductance (
I
l
) does not behave in this way
because Lenz's law causes the victim current to flow in the direction opposite to the
aggressor current (
I
s
). For these things to be so, the victim and aggressors must be
far enough apart that the presence of the victim does not change the aggressors' im-
pedance (and so the voltages coupled to the victim cannot influence the aggressor).
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