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36 Intel
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31154 133 MHz PCI Bridge Design Guide Design Guide
Routing Guidelines
6.1 Crosstalk
Crosstalk is caused by capacitive and inductive coupling between signals. Crosstalk is composed of
both backward and forward crosstalk components. Backward crosstalk creates an induced signal on
a victim network that propagates in the opposite direction of the aggressor signal. Forward
crosstalk creates a signal that propagates in the same direction as the aggressor signal.
Circuit-board analysis software is used to analyze your board layout for crosstalk problems.
Examples of 2D analysis tools include Ansoft* Parasitic Parameters* and Quad Design* XFS*.
Crosstalk problems occur when circuit etch lines run in parallel. When board analysis software is
not available, the layout must be designed to maintain at least the minimum recommended spacing
for bus interfaces:
• As a general guideline, the distance between adjacent signals must be a least 3.3 times the
distance from signal trace to the nearest return plane. The coupled noise between adjacent
traces decreases by the square of the distance between the adjacent traces.
• It is also recommended that you specify the height of the above-referenced plane when laying
out traces and that you provide this parameter to the PCB manufacturer. By moving traces
closer to the nearest reference plane, the coupled noise decreases by the square of the distance
to the reference plane.
These design guidelines are illustrated in Figure 5:
Additional crosstalk guidelines include the following:
• Avoid slots in the ground plane. Slots increase mutual inductance and thus increase crosstalk.
• Ensure that the ground plane surrounding the connector-pin fields is not completely cleared
out. When the area around the connector pins is completely cleared out, all the return current
must flow together around the pin field, increasing crosstalk. The preferred method of laying
out a connector in the GND layer is shown in Figure 7.
Figure 6. Crosstalk Effects on Trace Distance and Height
A9259-01
Reference Plane
P
aggressor victim
H
Reduce Crosstalk:
- Maximize P
- Minimize H