Tunable Multiband Bandpass-to-Bandstop RF Filters

This paper reports on a new class of multiband bandpass-to-bandstop (BP-to-BS) tunable RF filters. The proposed filter concept allows the simultaneous realization of multiband bandpass- and bandstop-type transfer functions that can be independently reconfigured in frequency and in number — through band merging. Switching between the multiband bandpass-filter (BPF) and bandstop-filter (BSF) modes is achieved by changing the coupling path between the source and the load. In both modes, band reconfigurability is obtained by only tuning the resonant frequencies of the filter’s resonators. The proposed spectrally-adaptive multiband BP-to-BS filter concept is presented by means of: i) coupled-resonator synthesis through its coupling-matrix representation and of: ii) an experimental four-resonator microstrip filter prototype (1-to-2 bands in the BPF mode and 1-to-4 bands in the BSF mode) that was designed and tested in the frequency range 0.9–1.6 GHz for proof-of-concept demonstration purposes.