Quasi-Elliptic-Type Multiplexer Design Without Cross Coupling

A method to design quasi-elliptic-type multiplexers without cross coupling in its channelizing bandpass filters is reported. It is based on the use of a multi-transmission-zero (TZ)/multipole filtering cell in the junction of the multiplexer channels. This filtering cell increases the orders of the multiplexer channels by one in relation to the isolated filters, whereas its TZs are shared by all channels. In this manner, an enhanced order sharp-rejection multiplexer can be synthesized. The operational foundations of the proposed quasi-elliptic-type multiplexer approach, which is general for symmetrical and asymmetrical arbitrary-order designs in any RF technology, are described under a coupling-matrix-based formalism. Moreover, for practical-validation purposes, a third-order microstrip duplexer prototype with channels centered at 2.15 and 2.65 GHz is manufactured and tested.