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54 DESIGN007 MAGAZINE I DECEMBER 2018 On a multilayer PCB, critical signals should be routed on a stripline (inner layer) adjacent to a solid reference plane to reduce radiation. The spacing between the signal trace and return plane should be as small as possible to increase coupling and reduce loop area. The three constraints to keep in mind include: 1. Making the mark to space ration of the waveform equal because this eliminates all the even harmonics 2. Routing high-speed signals between the planes and fanout out close to the driver (200 mils), dropping to an inner layer and route back up to the load again with a short fanout 3. Using the same reference plane (GND if possible) for the return signal because it reduces the loop area and radiation As you can see from Figure 3, the trace routed on inner layer three exhibits between four to 10 dB less noise than the trace routed on the top layer. Note that there are radiating harmon- ics above 40 dB on the top layer routing. Also, high-frequency components radiate more read- ily because their shorter wavelengths are com- parable to trace lengths, which act as antennas. Figure 2: Cross-probing part of an address bus from schematic to router. (Source: Mentor Graphics, PADS) Figure 3: Comparison of signals routed on the top versus inner layer three. (Source: HyperLynx)

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