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30 The PCB Design Magazine • February 2014 In a previous Beyond Design, Impedance Matching: Terminations, I discussed various ter- mination strategies and concluded that a series terminator is best for high-speed transmission lines. Different terminating strategies have ad- vantages and disadvantages depending on the application, but in general, series termination is excellent for point-to-point routes, one load per net. In summary, series termination reduces ringing and ground bounce. But, what if there are a number of loads— how should these transmission lines be routed? For perfect transfer of energy and to eliminate reflections, the impedance of the source must equal the impedance of the trace(s) to the load. Bifurcated transmission lines—traces that are split into two or more T-sections—are some- times used to distribute signals to multiple loads. The impedance of the bifurcated line is not constant along the trace route, as the traces branching from the T-section are virtually in parallel when you consider the equivalent AC circuit. In this case, proper termination has not been provided and an impedance discontinuity can be seen at the branch point. In Figure 1, a 50 ohm signal from the driver is split into two transmission lines of 50 ohms and then into the loads. At branch (A), the two 50 ohm traces in parallel equate to a 25 ohm equivalent trace, and a mismatch in impedance. Figure 2 illus- trates the resultant waveform of the unmatched transmission line. Bifurcated transmission lines with matched impedances are a better choice. Rather than having the traces all the same width, the branch traces are reduced in order to match the imped- ance at the branch. The impedance of the traces after the branch then becomes 2 x Zo or 100 by Barry olney in-CiRCuiT DeSign PTY lTD | AuSTRAliA BEyoND DESIGN feature column Figure 1: An unmatched bifurcated transmission line. Effective Routing of Multiple Loads