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14 The PCB Design Magazine • February 2015 Transmission Line's Effect on Effective Characteristic Impedance The length of the transmission line also had an effect on the results of the wizard. Recall from Figure 1-1 that the driver and receiver are con- nected by a 1 inch 50 ohm transmission line. Figure 6 below shows the transmission line pa- rameters calculated by the field solver where the inductance (L) and capacitance (C) are the re- sults of the entire one-inch of transmission line. Depending on which source is referenced, publications differ on when the interconnect is considered to be a lumped or distributive sys- tem. 1 This distributive property is based on the drivers transition rate as compared to the delay of the interconnect. The field solver appears to using a 1/6 th rule as based on the experiment outlined next. For this experiment, the length of the trans- mission line is shortened to see where the wiz- ard starts to ignore its effect on the effective Z0 parameter. The receiver IBIS model is set back to its original configuration with C_comp being equal to 0.5pF and the pin RLC parameters set to equal those shown in Figure 4. Recall from Figure 2 that the effective Z0 was equal to 46.5 ohms by this configuration. Now the transmission line is shortened to 0.5 inches to see the effect on the wizard. Figure 7 shows the results after shortening the trans- EFFECTIvE CHARACTERISTIC IMPEDAnCE continues feature Figure 5: The wizard results after changing the capacitance parameter of the pin to 10pF. Notice how the effective z0 is considerably lower than previously shown.