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Design007-Sept2018

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62 DESIGN007 MAGAZINE I SEPTEMBER 2018 3 mils beneath the signals on each board. FR-4 is used as the PCB dielectric substrate. The two signal traces on each board are 15 mils apart (i.e., triple of the signal trace width for mini- mum crosstalk). The ground trace (5 mils in width, shown in grey) is placed 5 mils away to the right of each signal trace to electrically connect the ground or reference plane of each board. On the other hand, the signal and ground traces over the flex cable are 500 mils long. From a crosstalk perspective, port 1 and port 2 represent the transmitting and receiving end, respectively, of the aggressor line. Meanwhile, port 3 and port 4 represent the transmitting and receiving end, respectively, of the vic- tim line. In test case 2 (Figure 2), the 3DEM structure is similar to test case 1, except the ground trace between the two signal traces are placed 5 mils away to the left of the port 1 signal. This is to simulate the negative impact of ground trace removal on the forward signal crosstalk vs. test case 1. But in test case 3 (Figure 3), the 3DEM structure is left with only a single ground wire connecting electrically the reference planes of board "A" and "B." The rest of this example is the same as test case 2. This is to simulate the effect of insufficient ground or return wire on crosstalk vs. test case 2. The S-parameters of the 3DEM models in these test cases are plotted in Figure 4 (i.e., spans from 1MHz to 8GHz). S41 parameter represents far-end crosstalk (FEXT). A more severe signal crosstalk is indicated by smaller absolute value in dB. With reference to Figure 4, across the wide band range, the most severe FEXT is experienced in test case 3, followed by case 2 and the least severe in case 1. 100 mil Board A dielectric Board A ground layer Signal over cable 500 mil Board B dielectric Board B ground layer S G S G PORT 1 PORT 3 100 mil PORT 2 PORT 4 S S G G S S G Figure 1: The 3DEM structure in test case 1. Figure 3: The 3DEM structure in test case 3. Figure 4: Simulated plots of S41 (FEXT). Figure 2: The 3DEM structure in test case 2.

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