I/O Controller Hub 2
R
168 Intel
®
Pentium
®
4 Processor / Intel
®
850 Chipset Family Platform Design Guide
Figure 121. Trace Routing
45 degrees
Trace
45
degrees
9.9.2.1.1 Trace Geometry and Length
The key factors in controlling trace EMI radiation are the trace length and the ratio of trace-width
to trace-height above the ground plane. To minimize trace inductance, high-speed signals and
signal layers that are close to a ground or power plane should be as short and wide as practical.
Ideally, this trace width to height above the ground plane ratio is between 1:1 and 3:1. To maintain
trace impedance, the width of the trace should be modified when changing from one board layer to
another if the two layers are not equidistant from the power or ground plane. Differential trace
impedances should be controlled to be ~100 Ω. It is necessary to compensate for trace-to-trace
edge coupling, which can lower the differential impedance by 10 Ω, when the traces within a pair
are closer than 0.030 inches (edge to edge).
Traces between decoupling and I/O filter capacitors should be as short and wide as practical. Long
and thin traces are more inductive and would reduce the intended effect of decoupling capacitors.
Also for similar reasons, traces to I/O signals and signal terminations should be as short as
possible. Vias to the decoupling capacitors should be sufficiently large in diameter to decrease
series inductance. Additionally, the PLC should not be closer than one inch to the connector/
magnetics/ edge of the board.
9.9.2.1.2 Signal Isolation
Some rules to follow for signal isolation:
• Separate and group signals by function on separate layers if possible. Maintain a gap of
70 mils between all differential pairs (Phoneline and Ethernet) and other nets, but group
associated differential pairs together. Note: Over the length of the trace run, each differential
pair should be at least 0.3 inches away from any parallel signal traces.
• Physically group together all components associated with one clock trace to reduce trace
length and radiation.
• Isolate I/O signals from high speed signals to minimize crosstalk, which can increase EMI
emission and susceptibility to EMI from other signals.
• Avoid routing high-speed LAN or Phoneline traces near other high-frequency signals
associated with a video controller, cache controller, processor, or other similar devices.
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