A diplexer is a passive device, contains a set of filters, for frequency domain multiplexing of the signals. It combines signals coming from different ports onto a single output port or splits signal from the common port onto different ports. In general the two signals operate at different frequency bands which are disjoint.
Symbolic representation of a typical diplexer is shown in Figure 1. It contains a common feed point (C) and two other ports PL(low frequency) and PH(high frequency). The filter characteristics between ports PL & C and ports PH & C determine the frequency band of the signals applied to port PL and PH. The signals on port PL and port PH pass on to port C, and coexist on port C without interfering with each other. Ideally the ports are isolated, but in practice some of the signal from C leaks back to the other port.
Because of passive nature of the device, the common point can be used to combine or split the signals while providing sufficient isolation between the ports.