Intel 31244 PCI-X User Manual
26
Routing Guidelines
4.2
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
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.
both backward and forward crosstalk components. Backward crosstalk creates an induced signal on
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 Parasitic Parameters from ANSOFT
Examples of 2D analysis tools include Parasitic Parameters from ANSOFT
*
and XFS from Quad
Design
*
. Crosstalk problems occur when circuit etch lines run in parallel. When board analysis
software is not available, the layout maintains minimum spacing between parallel circuit signals
lines.
lines.
•
A general guideline to use is, that space distance between adjacent signals 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.
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 to specify the height of the above reference plane when laying out
traces and 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.
traces and 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.
•
Avoid slots in the ground plane. Slots increases mutual inductance thus increasing crosstalk.
•
Make sure that ground plane surrounding connector pin fields are not completely cleared out.
When this area is completely cleared out, around the connector pins, 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
When this area is completely cleared out, around the connector pins, 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.
Crosstalk Effects on Trace Distance and Height
A9259-01
Reference Plane
P
aggressor
victim
H
Reduce Crosstalk:
- Maximize P
- Minimize H
Figure 8.
PCB Ground Layout Around Connectors
A9260-01
A. Incorrect method
B. Correct method
Connector
GND PCB Layer
Connector Pins