Copuling of Traces
CHAPTER 11.3 Coupling of the traces
Two nearby traces create coupling. Changing electric field due to the rate of change of voltage dV/dt, on one trace induces a capacitive coupling on a nearby trace. A changing magnetic field due to changing current dI/dt on one trace induces inductive coupling on a nearby trace. The capacitive or the inductive coupling induce undesired distortions in the form of cross talk in the nearby traces. A trace carrying higher edge rate signal has the potential to create more cross talk. Keeping high edge rate trace far away helps reduce the cross talk, as does reducing the trace to ground plane separation. It is part of PCB designers to follow rules to avoid cross talk created due to high speed traces. The task involves identification of high speed traces, calculation of amount of tolerable cross talk, formulation of the rules and adherence to the rules. PCB Designers are often required to keep the design as compact as possible. To reduces the cross talk, on the other hand, it is required that the traces be spread out. It is the job of the PCB designer to balance the contradictory requirements. Identifying the high speed signals, calculating the tolerable crosstalk, forming and following the design rule is the key to designing a PCB free from cross talk errors.
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