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February 2015 • The PCB Design Magazine 17 EFFECTIvE CHARACTERISTIC IMPEDAnCE continues feature series termination would be the transmission line impedance minus the driver impedance (75-52.7 = 22.3 ohms). 1 For simplicity, a 22-ohm resistor is used since this is a standard value. After running the wizard on this topology, it indicates that the effective Z0 is 64.8 ohms and the recommended termination value is 12.1 ohms (64.8–52.7 ohms = 12.1 ohms). From this example, it is easy to see that the capacitance of the topology is affecting the effective impedance and lowering the recom- mended termination impedance as indicated by the recommended termination resistor (12.1 ohms). Investigating the via Contribution Figure 9 shows the via construction and its transmission line equivalent model (56 ing the effective Z0 increase (recall the effective Z0 results from Figure 5). Simple Series Terminated Circuit Figure 8 expands the previous findings to a simple, often encountered series terminated net topology. The total transmission line length is still approximately one inch divided equally between the three segments. The driver and re- ceiver models are identical to their original con- figuration, and a 22-ohm resistor is added to the circuit. To simulate a more typical situation, the net transitions layers from top to bottom, and back to the top, necessitating two vias (13 mil drill, 24 mil pad). Recall from Figure 2 that the driver impedance is 52.7 ohms as indicated by the wizard. The ideal Figure 7: results of the wizard after the transmission line length is shortened to 0.5 inches. Notice the effective z0 is equal to the impedance z0 parameter of the system—50 ohms.