Hidden schematic

摘要

I've invited my good friend, Bruce Archambeault, PhD, creator of the IBM EMC-rule-checking program "EMSAT," to share his thoughts about ground. Your schematic is lying. It shows only the intended flow of signal power. It masks details crucial to the operation of unintentional signal pathways, such as crosstalk and EMI. To understand these factors, you must uncover the hidden schematic operating behind your logic diagram. The hidden schematic represents, among other things, the flow of physical current in your pc board. Current never flows "one way," as logic diagrams show; it always makes a loop out to the load and returning to the source. For EMI purposes, the returning current path is as important as the intended signal path. As long as you keep the out' going and returning signal paths closely aligned, the radiation patterns from the two currents cancel. Interruptions to the returning current path have serious negative effects on the EMI/EMC performance of a pc board. Even slight deviations in the returning current path can dramatically increase emissions.