Reading time ( words)
In a previous Beyond Design, Impedance Matching: Terminations, I discussed various termination strategies and concluded that a series terminator is best for high-speed transmission lines. Different terminating strategies have advantages 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 sometimes 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 illustrates the resultant waveform of the unmatched transmission line.
Editor's Note: This column originally appeared in the February 2014 issue of The PCB Design Magazine.
