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62 DESIGN007 MAGAZINE I AUGUST 2018 a trace width of .0045", intra-pair spacing of .008", and substrate thickness of .005" between trace and upper/lower reference plane, to achieve characteristic impedance of 91.2 ohms. Meanwhile, model 4B has the same parameter setting as 4A, except the substrate thickness between the trace and upper/lower reference plane is increased to .007", to achieve charac- teristic impedance of 103.2 ohms (i.e., within ±10% tolerance of the nominal 100 ohms). By field solving the four-port simulation topology depicted in Figure 3 and differen- tial mode conversion, far-end crosstalk of the transmission line models in Table 2 are plot- ted in Figure 4 (i.e., from 1MHz to 30GHz). Across the wideband up to 30GHz, crosstalk for model 3A is about 3dB lower versus 3B. Similarly, crosstalk for model 4A is at least 2.5dB lower versus 4B. This result indicates that for both microstrip and stripline in differential mode, thinner substrate or dielec- tric between a PCB trace and an adjacent reference plane reduces the signal crosstalk. Summary The study presented here proves that crosstalk can be minimized by reducing the dielectric thickness between a PCB trace and the adjacent reference plane. This simple implementation serves as an additional way to further reduce uninten- tional coupling, alongside other good PCB lay- out practices such as increasing the coupled distance and decreasing the coupled length between adjacent transmission lines. DESIGN007 References 1. Barry Olney, Beyond Design, Signal Integ- rity, Part 2, November 2014. 2. Altera, High-Speed Board Layout Guide- lines. Chang Fei Yee is a hardware engineer with Keysight Technologies. His respon- sibilities include embedded system hardware development, and signal and power integrity analysis. Figure 4: Simulated plots of far-end crosstalk for transmission line models listed in Table 2. Register now for Mentor's free monthly Advanced Technology Webinar Series, to be held August 30, 2018, from 8:30 – 9:30 a.m. The topic is panelization with the Xpedition Flow. Xpedition Fablink simplifies the process of creat- ing a panel design. Driven by the PCB, users can keep panel data up to date and easily place multiple PCBs in one panel if necessary. Panel-level DRC checks ensure that panels are correct and ready to manu- facture. The presenter is Kyle Lake of Oasis Sales, formerly a corporate marketing engineer with Mentor. To learn more or to register, click here. Source: Mentor Graphics Free Webinar: Panelization with the Xpedition Flow

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