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74 DESIGN007 MAGAZINE I OCTOBER 2018 5% is caused by the one-inch total serpentine length. With the same total serpentine length, signal transmission at 10 Gbps or higher-speed grade gets attenuated at a larger magnitude versus one Gbps or lower-speed grade. The plots of differential to common mode conversion (Scd21) for microstrip and strip- line are shown in Figure 4. A smaller absolute magnitude of Scd21 in dB indicates differen- tial is more easily converted to common mode, which is encountered by a transmission chan- nel with longer serpentine portion on the PCB. This weakens the immunity of the channel against common-mode noise or crosstalk. B. Case Study 2 This test case observes how the intra-pair gap of serpentine segments impacts the sig- nal integrity in term of insertion loss. All the simulation models are listed in Table 3. The entire transmission channel is 5 inches long. Each particular serpentine segment is 200 mils long and all models have five serpentine seg - ments, equivalent to a one-inch total serpen- tine length. For microstrip, model 2A does not have serpentine routing at all, while mod- els 2B, 2C, and 2D have intra-pair air gap of serpentine segment of 1.5x, 2x, and 3x ver- sus non-serpentine. Meanwhile, for stripline, model 2E does not have serpentine routing at all, while models 2F, 2G, and 2H have an intra- pair air gap of serpentine segment of 1.5x, 2x, and 3x versus non-serpentine. Plots of TDR and Sdd21 for microstrip and stripline are shown in Figures 2 and 3. When the number of serpentine segments or its Figure 4: Scd21 for microstrip (left) and stripline (right). Table 3: Simulation models for case study 2.