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72 DESIGN007 MAGAZINE I OCTOBER 2018 The impact of the number or total length of the serpentine segments and the intra-pair spacing on signal attenuation is discussed in case study 1 and 2. The analysis is performed using 2DEM simulation. The results cover time domain reflectometry (TDR), differential inser- tion loss (Sdd21), eye diagram, and differential to common mode conversion (Scd21). In both case studies, simulation mod- els involve differential pairs for microstrip and stripline with 100-ohm impedance. The microstrip pair has 1-oz. copper thickness, 6-mil trace width, and 5-mil intra-pair spacing with 4.5-mil dielectric thickness. Meanwhile, the pair of symmetrically centered striplines has 1-oz. copper thickness, 5-mil trace width, and 6-mil intra-pair spacing with 9.5-mil dielectric thickness. Medium-loss dielectric is applied as substrate. In channel analysis that generates an eye diagram, a transmit - ter injects a signal with an amplitude of 600 mVpp at one Gbps (i.e., with 35 ps rise/fall time) and 10 Gbps (i.e., with 5 ps rise/fall time). Pre-/de-emphasis and equalization are disabled. Analysis and Results A. Case Study 1 This test case investigates how the num- ber of serpentine segments impacts the signal integrity in term of insertion loss. All the simu- lation models are listed in Table 1. The entire transmission channel is 5 inches long. Each particular serpentine segment is 200 mils long. For microstrip, model 1A does not have serpen- tine routing at all, while models 1B, 1C, and 1D have3, 5 and 9 serpentine segments. Mean- while, for stripline, model 1G does not have ser- pentine routing at all, while models 1H, 1I, and 1J ha ve 3, 5 and 9 serpentine segments. A larger number of segments contributes to the increas - ing length of the entire serpentine portion for the transmission line model. The models' intra- pair spacing for the serpentine segment is set as 2x the non-serpentine portion (i.e., 10 mils for microstrip and 12 mils for stripline). Plots of TDR and Sdd21 for microstrip and stripline are shown in Figures 2 and 3. When the number of serpentine segments or its total portion length increases, more inductive impedance discontinuities are encountered by the signals. This in turn leads to the occur- rence of resonant dips at 10, 30, and 50 GHz for microstrip models. The resonant dip or attenuation gets intensified with a longer total serpentine routing. A similar scenario is expe- rienced by stripline models. Figure 1. Top view of a serpentine on a PCB. Table 1: Simulation models for case study 1.

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