Intel IXP46X, IXP45X manual Cross-Talk, EMI-Design Considerations

Intel® IXP45X and Intel® IXP46X Product Line of Network Processors—Category

•Wherever possible, use a perfect symmetry within a differential pair.

•Minimize the number vias.

•Avoid routing other signals close by or in parallel to the differential pair, maintaining no less than 50 mil to any other signal.

•Maintain control impedance for each differential pair to 90 Ω +/- 15 Ω.

•Use high value ferrite beads (100 MHz/60 Ω – 100 MHz/240 Ω).

5.2.6Cross-Talk

Cross-talk is caused by capacitance and inductance coupling between signals. It is composed of both backward and forward cross-talk components.

Backward cross-talk creates an induced signal on the network that propagates in the opposite direction of the aggressor signal. Forward cross-talk creates a signal that propagates in the same direction as the aggressor signal.

Circuit board analysis software should be used to analyze your board layout for cross- talk problems.

•To effectively route signals on the PCB, signals are grouped (address, data, etc.).

—The space between groups can be 3 w (where w is the width of the traces).

—Space within a group can be just 1 w.

—Space between clock signals or clock to any other signal should be 3 w. The coupled noise between adjacent traces decreases by the square of the distance between the adjacent traces.

5.2.7EMI-Design Considerations

It is strongly recommended that good electromagnetic interference (EMI) design practices be followed when designing with the IXP45X/IXP46X network processors.

•Information on spread-spectrum clocking is available in Intel® IXP4XX Product Line of Network Processors and IXC1100 Control Plane Processor: Spread-Spectrum Clocking to Reduce EMI Application Note.

•Place high-current devices as closely as possible to the power sources.

•Proper termination of signals can reduce reflections, which may emit a high- frequency component that may contribute to more EMI than the original signal itself.

•Ferrite beads may be used to add high frequency loss to a circuit without introducing power loss at DC and low frequencies. They are effective when used to absorb high-frequency oscillations from switching transients or parasitic resonances within a circuit.

•Keep rise and fall times as slow as possible. Signals with fast rise and fall times contain many high-frequency harmonics which may radiate significantly.

•A solid ground is essential at the I/O connector to chassis and ground plane.

•Keep the power plane shorter than the ground plane by at least 5x the spacing between the power and ground planes.

•Stitch together all ground planes around the edge to the board every 100 to 200 mil. This helps reduce EMI radiating out of the board from inner layers.

Intel® IXP45X and Intel® IXP46X Product Line of Network Processors
HDD	February 2007
68	Document Number: 305261; Revision: 004