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Design007-Jun2018

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58 DESIGN007 MAGAZINE I JUNE 2018 Article by Chang Fei Yee KEYSIGHT TECHNOLOGIES This article discusses the impact of stitching vias and discontinued return path or reference on signal integrity during layer transition on high-speed PCBs, particularly in terms of sig- nal reflection and crosstalk. Introduction In electronic systems, signal transmission exists in a closed-loop form. The forward cur- rent propagates from transmitter to receiver through the signal trace. Meanwhile, the return current travels backward from receiver to transmitter through the power or ground plane directly underneath the signal trace that serves as the reference or return path. The path of for- ward current and return current forms a loop inductance. It is important to route the high-speed sig- nal on a continuous reference plane so that the return current can propagate on the desired path beneath the signal trace. In addition to that, whenever there is signal transition from one layer to another through a via, an extra via that connects the reference planes on dif- ferent PCB layers (i.e., stitching via) must be placed near the signal via to provide a continu- ous return path. If the return path is broken due to the absence of stitching via or switching of refer- ence plane from ground to power or vice versa after layer transition on PCB, the "return cur- rent" might detour and propagate on a lon- ger path, which causes the rise of loop induc- tance. This might also lead to the sharing of common r eturn path by different signals that poses high risk of interference among the sig - nals due to higher mutual inductance. This interference results in crosstalk that occurs on the transmitted signal [1] [2] . This phenomenon is proven in the following section with 3DEM simulation.

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