Single-/Multi-Band Bandpass Filters and Duplexers With Fully Reconfigurable Transfer-Function Characteristics

This paper reports on the RF design of fully reconfigurable quasi-elliptic-type single- and multi-band microwave bandpass filters (BPFs) and duplexers. They are based on series-cascaded resonators and multi-resonant/multi-band cells that produce frequency-reconfigurable poles and transmission zeros (TZs). By appropriately tuning the resonant frequencies of their constituent resonators, multiple levels of transfer-function adaptivity can be obtained. These capabilities include center-frequency and bandwidth tuning, as well as intrinsic RF switching-off (i.e., without the use of RF switches). As opposed to conventional BPF tuning mechanisms, the aforementioned reconfiguration characteristics avoid the use of tunable couplings. The basic operating principles of the engineered resonator-cascade BPF concept are presented through a three-pole/two-TZ example. Afterward, methods for expanding the proposed filter design approach to higher order, multi-band, and multi-port (e.g., duplexers) realizations are explained through coupling-matrix-based analysis. For practical-demonstration purposes, four L-band tunable microstrip prototypes were developed and measured. They include: 1) a five-pole/four-TZ BPF; 2) a five-pole/two-TZ BPF; 3) a dual-band BPF with two three-pole/two-TZ passbands; and 4) an RF duplexer.