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60 DESIGN007 MAGAZINE I SEPTEMBER 2018 Article by Chang Fei Yee KEYSIGHT TECHNOLOGIES This article discusses the impact of signal routing and return path or reference on cross- talk in multi-board interconnect. The inves- tigation is performed with 3DEM simulation using Keysight EMPro. Subsequently, crosstalk in frequency and time domain are observed, along with surface current density on the return path. Introduction In an electronic system, the signal transmis- sion exists in a closed-loop form. The forward current propagates from transmitter to receiver through the signal trace. On the other hand, for a single PCB, the return current travels backward from receiver to transmitter through the ground plane closest to the signal trace. Meanwhile, for multi-board interconnect (e.g., connectivity through flex or ribbon cable), the return current travels back to the transmitter through the ground or return wire, preferably as close as possible to the signal wire. The path of forward current and return current forms a loop inductance. It is important to provide sufficient ground or return wire in multi-board interconnect. Otherwise, the "return current" might detour and propagate on a longer path that leads to the sharing of common return wire or path by different signals that poses high risk of inter- ference or coupling among the signals due to higher mutual inductance. This interference results in signal crosstalk. This phenomenon is proven in the following section with 3DEM simulation [1] . Analyzing Crosstalk with 3DEM Modeling To investigate the impact of signal routing and the return path on multi-board signal integrity, three test models of 3DEM were constructed using Keysight EMPro. In test case 1 (Figure 1), two microstrip signal traces (shown in brown) with 50 ohm characteristic impedance in sin- gle-ended mode on board "A" are connected to board "B" using flex cable. The signal traces on each board are 100 mils long, 1.2 mils thick and 5 mils wide. The solid ground plane exist

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