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Design007-Nov2020

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NOVEMBER 2020 I DESIGN007 MAGAZINE 49 Finally, one should avoid routing high-speed signals on the outer microstrip layers of a mul- tilayer PCB. This can decrease radiation by up to 10 dB. Plus, inner stripline traces are less susceptible to outside noise as they are sand- wiched between two planes. The four constraints to keep in mind are as follows: 1. Keep the mark to space ratio of the waveform equal as this eliminates all the even harmonics leaving only the odd harmonics. 2. Route high-speed signals out from the center of the board where possible as any radiation will be in the opposite direction and will tend to cancel out. 3. Route high-speed signals between the planes. Fan-out close to the driver (200 mils), dropping to an inner plane, and route back up to the load again with a short fan-out. 4. Use the same reference plane for the return signals, as this reduces the loop area and hence radiation. Let's make a comparison of the radiated noise between the top layer and inner layer routing, as in Figure 3. As you can see (with all other factors being equal), in this case, the trace routed on inner layer 3 exhibits 4–10 dB less noise than the trace routed on the top layer. The high-fre- quency components radiate more readily because their shorter wavelengths are compa- rable to trace lengths, which act as antennas. You do not need to do any manual routing yourself to get an acceptable route of the non- critical nets. Of course, matched delay, differ- ential pairs, and other critical signals should be routed with the precision they demand. This is the process: 1. Start by placing all the components by functionality, selecting the desired component on the schematic, and placing it in the best location/rotation to aid the routing. 2. Similarly, when routing, select a chip on the schematic (the nets are highlighted on the PCB) and then fan-out with the router. Figure 3: Comparison of radiation from microstrip and stripline routing.

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