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48 The PCB Design Magazine • April 2014 brooks' bits On the other hand, look at Figure 2. This shows the same three traces, but this time they are only 5 mils above the underlying plane. Now a substantial portion of the electromagnet- ic field is "captured" between the trace and the plane. This significantly reduces the po- tential for EMI radiation off the board. Simply by look- ing at the electromagnetic field we can intuitively see that this configuration will perform better from an EMI standpoint. This graphically illus- trates what is possibly the single most important de- sign rule for signal integrity on PCBs: Rule 1: Route every trace as close as practical to a continuous, related, underlying plane. Next, let's look at Fig- ures 3 and 4. They illustrate a pair of 8 mil traces, 5 mils above the plane, but in Figure 3 they are sepa- rated by 5 mils while in Figure 4 they are sepa- rated by 25 mils. When they are close together they couple into each other, raising the poten- tial for crosstalk problems between them. But when we separate them the fields reduce their ELECTROMAGNETIC FIELDS, PART 3 continues Figure 2: 8 mil trace, 5 mil height, 50 mil separation. Figure 3: 8 mil traces, 5 mil height, 5 mil separation. Figure 4: 8 mil traces, 5 mil height, 25 mil separation.