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Design007-Oct2022

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OCTOBER 2022 I DESIGN007 MAGAZINE 29 readily because their shorter wavelengths are comparable to trace lengths, which act as antennas. Consequently, although the amplitude of the harmonic frequency compo- nents decreases, as the fre- quency increases, the radiated frequency varies depending on the trace characteristics. So, apart from the short 200- mil microstrip fanouts, the emissions of this design are well below that of the FCC/ CISPR Class B limit (lower red line). Whereas, the radia- tion would have been 49.73 dB at 6.76 GHz and 52.10 at 7.8 GHz if it was routed on the outer layers, possibly fail- ing testing. If you plan your routing strategy and stackup design prior to commencing the layout, then design for electromagnetic compliance takes very little extra effort. Figure 3: Routing strategy for DDR3 fly-by configuration. Figure 4: Comparison of radiation from signals routed on the top microstrip layer vs. inner stripline layer.

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