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26 DESIGN007 MAGAZINE I OCTOBER 2022 Electromagnetic (EM) energ y propagates through the dielectric materials of a multi- layer PCB guided by the signal traces between the planes, for inner stripline layers (Figure 1), but it acts slightly differently on the outer microstrip layers. Microstrip layers generally have a solid ground reference plane on one side but allow radiation from the boundless surface into the air. A well-thought-out routing strat- egy can avoid up to 10 dB of radiation from the substrate. Embedding signals between the planes reduces these emissions, and suscepti- bility to radiation, as well as providing electro- static discharge protection. So, not only can Routing Strategies to Minimize Radiation one prevent noise from being radiated but also reduce the possibility of being affected by an external source. Studies conducted by Hewlett-Packard have found that there are up to 20 dB greater emis- sions from edge-located traces compared to traces located in the centre of the board on outer layers. Yet the same test performed on buried traces indicated no change as the traces were placed nearer the PCB edges. is implies that it is best to keep well away from the edge of the board when routing on the outer microstrip layers. e impedance changes as the reference plane decreases in the area beneath the trace. On a multilayer PCB, critical signals should be routed on a stripline layer adjacent to a solid reference plane to reduce radiation. e spac- ing between the signal trace and the return plane should be as small as possible to increase coupling and reduce the loop area. e three constraints to keep in mind: • Keep the mark-to-space ratio of the waveform equal as this eliminates all the even harmonics. • Route high-speed signals between the planes, 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. • Use the same reference plane (GND if possible) for the return signal, as this reduces the loop area and hence radiation. Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Figure 1: Microstrip EM fields (top) and stripline EM fields (bottom).

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