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Design007-June2019

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78 DESIGN007 MAGAZINE I JUNE 2019 Article by Chang Fei Yee KEYSIGHT TECHNOLOGIES This article discusses the impact of stitching capacitors [1] in mitigating signal crosstalk due to return path discontinuity during layer transi- tion on PCB. The investigation was performed with 3DEM simulation using Keysight EMPro. 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. On the contrary, the return current travels backward from receiv- er to transmitter through the power or ground plane right underneath the signal trace that serves as the reference or return path. The path of the forward current and return cur- rent forms a loop inductance. It is important to route the high-speed signal on a continu- ous reference plane so that the return current can propagate on the desired path, directly be- neath the signal trace. When the return path is broken due to the switching of reference planes with different potential, (e.g., from ground to power or vice versa after layer transition on PCB), the return current might detour and propagate on a lon- ger path, which causes a rise in loop induc- tance. This might lead to the sharing of a com- mon return path by different signals that pose a high risk of interference among the signals due to higher mutual inductance. This inter- ference results in signal crosstalk. To mitigate the crosstalk due to return path discontinuity (RPD), stitching capacitors are mounted on the PCB to serve as a bridge between the two refer- ence planes of interest on different PCB layers. Analysis of Signal Crosstalk To investigate the impact of stitching capacitor in mitigating signal crosstalk due to RPD during layer transition on PCB, three simulation mod- els of 3DEM are constructed. In model 1A (Fig- ure 1), two signal traces with 50 ohm charac- teristic impedance in single-ended mode on the top PCB layer transition to the bottom layer us- ing vias. Each segment of the signal traces on top and bottom layers is 100 mils long and 5 mils wide. Meanwhile, the diameters of the via barrel and pad are 5 mils and 7 mils respectively.

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