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18 The PCB Design Magazine • October 2014 by two surface roughness (SR) and roughness factor (RF) parameters. It is applied to the con- ductor surface impedance in the model. SR=0.4 um and RF=2 produced perfect good for the generalized modal (GM) insertion loss as shown in Figure 3. FR408HR model identified with the stripline should also work for the microstrip line. It can be validated with the GMS-parameters computed from the measured S-parameters of 2 and 8 inch microstrip line segments and shown in Figure 4. The 6-inch segment model is constructed with the electromagnetic solver to capture the high- frequency dispersion seen as the upward trend in the phase delay in Figure 4. In addition we define solder mask model as the wideband Debye with the values taken from the manufacturer speci - fication: Dk=3.85 and LT=0.02 at 1 GHz. Good match of the phase delay can be observed in Fig- ure 4. To match measured and modeled GM in- sertion loss, the conductor roughness parameters for the microstrip line needed adjustment of the roughness factor from 2 to 3.5. That concludes the material model identification. The process is automated in Simbeor software and typically takes 5–10 minutes. As an optional, but recommended step, GMS- parameters extracted from S-parameters mea - sured for 2- and 6-inch differential links can be further used to confirm (or further adjust) the models extracted with the single-ended lines as demonstrated in [5] . Note that the identified dielectric and con- ductor roughness models are simple frequency- continuous expressions [7] .The models are not just tables of Dk and LT frequency points and do not end where the measured data end. The ex- tracted models are expected to be sufficiently ac- curate from DC up to 40 GHz and well above that frequency—the validation step will confirm it. Step 3: Analysis-to-measurement validation After the S-parameters are measured and pre-qualified and broadband material models SINk OR SWIM AT 28 GBPS continues feature Figure 3: GMS parameters computed from S-parameters measured for 2- and 8-inch stripline segments (red and blue lines) and modeled for 6-inch stripline segment (brown and green lines) with wideband Debye dielectric model with Dk=3.815, LT=0.0117 @ 1 GHz, and Modified Hammerstad conductor roughness model with Sr=0.4 um, rF=2.