Design of Four-Port Planar Filter Circuits With Multiplexer Operation

This paper presents a design method of four-port microstrip filter circuits that have increased filter passbands for multiplexer operation using a simple feed structure. The proposed circuits can be used as different diplexers or even triplexers according to different application requirements. The stub-loaded resonator is used as the basic element to design each filter channel. A simple feed structure with asymmetrical U-shaped feedlines is deployed using the discriminating coupling method. As a result, for each filter channel, specific transmission zero is introduced in other channel frequency bands, thus achieving high channel isolation. Firstly, a compact four-port filter circuit is proposed with four diplexer functions. By exciting one port as the input while two adjacent ports as the output, two filter passbands can be utilized for duplexing channels. Then, without adding extra ports, a four-port five-passband filter circuit is designed. It features a high integration level since it can be used as either diplexers or triplexers, once the corresponding ports are excited. For proof of concept, two design examples are fabricated. The prototypes show good agreement between the simulated and measured results, verifying the feasibility of the proposed design methodology. In addition, the operating frequency and bandwidth of each filter passband can be tuned to desired values. These characteristics are promising in multi-band base-station antenna array feed systems.