Issue link: https://iconnect007.uberflip.com/i/461203
16 The PCB Design Magazine • February 2015 mission line to 0.5 inches. Notice now that the effective Z0 is equal to the transmission line Z0 parameter of the system—50 ohms. From this it is reasonable to conclude that the simulator no longer considers this to be a distributive system since the length of the interconnect is shorter than some factor when compared to the driver transition time. 1/6 th Rule Now the interconnect length is changed to be 1/6 th of the driver transition time, which according to the wizard is 855ps. This transi- tion time would equate to an interconnect de- lay of approximately 142.5ps. Using the origi- nal transmission line delay factor of 155ps/ inch, the line length is calculated to be ap- proximately 0.919 inches. After running this simulation, the wizard indicates that the field solver still considers the system lumped, and therefore, not considering the interconnect length in the effective Z0 calculations. If the interconnect length is set just slightly longer than this 1/6 th factor (e.g., 0.925 inches), the simulator switches over to a distributive sys- tem and the effective Z0 parameter decreases to 48.1 ohms from 50 ohms (lumped system calculation). Also notice that 48.1 ohms is calculated for a transmission line length of 0.925 inches, which is slightly larger than for the one-inch line (46.5 ohms). Since the line length is slight- ly less, the total distributed capacitance is also less, thus the driver's loading is decreased, mak- EFFECTIvE CHARACTERISTIC IMPEDAnCE continues feature Figure 6: The important parameters for the 50 ohm transmission line.