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40 The PCB Design Magazine • December 2015 for routing. This provided some mutual induc- tive coupling between the wide power and ground traces and saved on board area. How- ever, it meant that the return current had to flow all the way around the board perimeter, creating a large loop area. Fortunately, the PDP- 11/40 was manufactured way before the era of FCC-mandated radiation guidelines. Needless to say, this is not a good supply configuration for high-speed design. Don't try this at home! The power and ground grid configuration of Figure 3 also saves on board area, but at the expense of increased mutual inductance. In this case, the ground (GND) traces are horizontal on the bottom of the board, while the power (VCC) traces are vertical on the top side. Con- necting the two supplies, at every intersection, with a decoupling capacitor forms a cross-hatch pattern. Current returns equally well, to its source, along either the ground or power traces. The down-side here is that the capacitors used should be of particularly good quality (low ESL) as the return current must traverse several ca- pacitors to return to the source. If you are limit- ed to a double sided board, then this is the best approach for providing power to the network of chips. But, solid copper planes, in a multilayer configuration, are of course a much, much bet- ter solution for high-speed design. Although it is true that a plane has signifi- cantly less inductance than a trace, plane in- ductance is not negligible. The mechanism by which a plane reduces the inductance is by al- lowing the current to spread out, effectively creating numerous parallel paths. But at high speeds, return currents flow the path of least in- ductance which tends to direct the current di- rectly below the signal trace. Also, due to skin effect, high-frequency cur- rents cannot penetrate a plane, and therefore, all currents in conductors are surface currents. This effect will begin to occur at frequencies above 30 MHz for 1 oz. copper layers in a PCB. Therefore, at high frequencies, a plane in a PCB is really beyond design PLANE CRAzY, PART 1 Figure 3: Power and ground grid configuration.

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