Digital Circuit Boards: Mach 1 GHz

4.17 Trace Geometries that Reduce Termination Resistor Counts

Consider a wide trace that has a characteristic impedance of 25 ohms. If the line splits into two 50-ohm paths of equal length and the lines are unterminated, the reflections will arrive back at the split at the same time and the line will behave as one 25-ohm line. This configuration takes one series termination resistor.

This same principle can be used to form three or more logic paths. The length of each transmission path must be held constant. At each split, the parallel characteristic impedances must match. An example of a three-way split is shown in Figure 4.6.

Figure 4.6 A branching transmission line. Note: The 50-ohm lines should be the same length.

There is a net saving in surface area as there are no spaces between traces over a portion of the run and there are fewer terminating resistors.