Differential Pairs

A differential pair signal resides on two conducting traces in the presence of a ground plane. To form the differential pair, the transmitter launches a wave that carries equal but opposite voltages on the two traces (e.g., one trace is +1 volt and the other -1 volt). When two traces are routed in close proximity to each other, the waves traveling on those traces exhibits similar amounts of system noise. A differential pair is an effective way to remove noise from a signal because when a receiver processes a differential signal it subtracts the voltages on the two traces. This can dramatically reduce the amount of noise in the differential signal compared to if the traces are driven individually (single-ended signals). In HFSS, a differential pair can be defined for two terminals residing in the same wave port or for terminals that reside in two different wave ports or lumped ports. From the case of wave ports, matched differential pairs can be defined by not renormalizing the wave ports involved in the differential pair definitions.