8.3 Multiconductor Transmission Lines

It's known from elementary transmission line theory that to avoid reflections from the far end, a single wire transmission line should be terminated at the far end to a reference (generally ground or to a reference voltage, often called V _tt) in an impedance equal to Z _o. However, we've just seen that a system of two wires supports two propagation modes, each with their own impedance. In general, a system of N conductors has N modes of propagation ^[5], each with unique impedance. This suggests that a single resistor to ground at the far end of each line would not properly terminate a multiconductor line under all switching conditions. In fact, a network of resistors between each conductor and V _tt is necessary for ideal termination ^[6].

While quite acceptable as a way to terminate a pair of wires (such as a differential pair, covered in Section 8.4.2), applying this scheme to a wide bus consisting of many conductors is not feasible at the PWB level. Instead, the general practice for far-end termination is to use a single resistor with a value equal to Z _o connected V _tt (which could be ground). Although not ideal, a single resistor to can adequately terminate a multiconductor transmission line, especially if the spacing between traces is large. Large spacing means that the coupling coefficient k is small, which implies that Z _o ? Z _oo ?