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60 DESIGN007 MAGAZINE I JUNE 2018 Analysis of Signal Reflection and Crosstalk with 3DEM Modeling To investigate the impact of stitching via and discontinued return path on high speed signal fidelity, three test models of 3DEM are con- structed using Keysight EMPro. In test case 1 (the 3DEM structure in Figure 1), two signal traces with 50 ohm characteristic impedance in single ended mode on top PCB layer are transitioned to bottom layer using vias. Each segment of the signal traces on both top and bottom layers is 100 mil long and 5 mil wide. Meanwhile, the diameter of the via barrel and pad is 5 mil and 7 mil respectively. The PCB stackup shown in Figure 2 is applied in this 3D model, where solid planes exist on layer 2 and 3. All the four copper layers have 1.2 mil thickness. FR-4 material is used as the PCB substrate. The two signal traces are sepa- rated 15 mil apart (i.e., triple the signal trace width for minimum crosstalk). A stitching via is placed 20 mil off each signal via to connect electrically the two reference planes on layer 2 and 3. From a crosstalk perspective, port 1 and port 2 terminations are assumed to be the transmit- ting and receiving ends, respectively, of the aggressor line. Meanwhile, port 3 and port 4 terminations are assumed to be the transmit- ting and receiving end, respectively, of the vic- tim line. In test case 2 (Figure 3), the 3DEM structure is left with only a single stitching via electri- cally connecting the two reference planes on layer 2 and 3. The rest of the portion is the same as test case 1. On the other hand, in test case 3 (Figure 4), voids are placed on the layer 3 reference plane to break the electrical connectivity between the planes on layer 2 and 3 though stitching vias. This is to simulate the effect of the discontin - ued return path on signal reflection and cross- Figure 2: PCB stackup similar to our first example. Figure 1: 3DEM structure in test case 1. Figure 3: 3DEM structure in test case 2.

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