Analysis of the effects of series filtering in coupled-strip sections

Due to the high strip density in present PCB circuits, high speed signal paths are often placed next to other signal strips. These strips, commonly routed over a ground plane, generate coupled microstrip sections. If these high speed signals have high harmonic components, they can easily interfere other signal strips. In order to reduce this high harmonic content, a series impedance is sometimes placed into the signal path to filter it In this paper, the effect of this series filtering is analyzed from a multimodal point of view. A multimodal analysis, very common in EMC, allows a simple interpretation of the phenomena involved in the integrity of the signals propagated through the strips. This multimodal analysis shows that the series filtering over coupled strips can be useless, worsening, even, the interference coupling. In order to test the adequacy of this multimodal approach, two PCB circuits are analyzed, circuitally simulated and measured. The good agreement between circuit simulation and measurements validates this multimodal approach.