User's Manual

Intel
®
820 Chipset Design Guide 3-17
Advanced System Bus Design
Additional aggressors are possible in the z-direction, if adjacent signal layers are not routed in
mutually perpendicular directions. Because cross-talk-coupling coefficients decrease rapidly with
increasing separation, it is rarely necessary to consider aggressors that are at least five line widths
separated from the victim. The maximum cross-talk occurs when all the aggressors are switching in
the same direction at the same time.
There is cross-talk internal to the IC packages, which can also affect the signal quality.
Backward cross-talk is present in both stripline and microstrip geometry’s (see Figure 3-5). A way
to remember which geometry is stripline and which is microstrip is that a stripline geometry
requires stripping a layer away to see the signal lines. The backward coupled amplitude is
proportional to the backward cross-talk coefficient, the aggressor’s signal amplitude, and the
coupled length of the network up to a maximum that is dependent on the rise/fall time of the
aggressor’s signal. Backward cross-talk reaches a maximum (and remains constant) when the
Figure 3-4. Aggressor and Victim Networks
Figure 3-5. Transmission Line Geometry: (A) Microstrip (B) Stripline
Aggressor
Zo
Signal propagates in both
directions on aggressor line.
Zo
Victim
Zo
Zo
AC Ground Plane
A. Microstrip
B. Stripline
Dielectric,
ε
r
Signal Lines
Signal Lines
Dielectric,
ε
r
W
Sp
t
li d