Reconfigurable Multi-Band Microwave Filters

An original and simple approach to the design of fully reconfigurable multi-band microwave bandpass filters (BPFs) with an arbitrary number of passbands is reported in this paper. It exploits the use of an innovative quasi-BPF configuration made up of different sets of controllable mono-frequency resonators to separately shape each tunable passband. Thus, high-selectivity multi-band bandpass filtering transfer functions exhibiting independent control in terms of center frequency, bandwidth, and transmission zeros can be synthesized. Furthermore, as an unprecedented frequency-agility feature of the proposed reconfigurable multi-band BPF structure when compared to the state-of-the-art, its passbands can be merged together to form broader, and for certain realizations, higher order transmission bands. This allows even more degrees of reconfiguration to be achieved in the devised circuit, which can also operate as ultra-wideband BPF with flexible in-band notches or self-equalized flat-group-delay quasi-elliptic-type BPF. The theoretical foundations of the described reconfigurable multi-band BPF scheme, along with guidelines for its design and a triple-passband filter synthesis example based on the coupled-node formalism, are expounded. In addition, as an experimental proof-of-concept, two microstrip prototypes with high- Q tuning implemented through mechanically variable capacitors are manufactured and tested. They are a wideband dual-band BPF and a quadruple-band BPF with narrow-bandwidth passbands.