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high speed PCB and EMC design -(3)  

2007-05-07 00:48:55|  分类: 无线通信 |  标签: |举报 |字号 订阅

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Signal Integrity – Reducing Crosstalk and Distortion

1. Self shielding occurs when the return current is allowed to surround the outgoing current, as in a coaxial cable.

2. Traces on adjacent layers should be oriented perpendicular to one another.

3. Ringing may indicate excessive wiring inductance.

4. Rounding may indicate excessive capacitance.

5. Unintended sharp transitions in signal level may indicate reflections due to impedance mismatches.

6. Separate high current, low frequency (kHz) return paths (ground) from low current return paths. Connect the high and low current returns together at only one point.

7. Connect “guard traces” to the return plane on both ends so that the traces can serve as additional signal return paths.

8. Do not allow any electrically floating metal.

High Frequency Transmission Lines

1. A trace may need to be treated as a transmission line when trace length > rambda/20, or propagation delay > (pulse rise time)/4.

2. Use a Z0> 40Ω to minimize the drive current and a Z0 < 120Ω to reduce emission and susceptibility.

3. Unintended sharp transitions in signal level may indicate reflections due to impedance mismatches.

4. The time delay for the arrival of a reflection may indicate the distance to the impedance mismatch.

9. Connect all metal fill areas to the return plane.

10. Connect all unused integrated circuit gate inputs to either return (ground) or +VDC.

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