Quasi-Elliptic Multi-Band Filters with Center-Frequency and Bandwidth Tunability

A class of tunable multi-band bandpass filters (BPFs) that is based on a quasi-bandpass topology is presented. Unlike other approaches, it features a quasi-elliptic-type filtering response with controllable passbands in terms of center frequency and bandwidth. Additional transmission zeros (TZs) are produced through cross couplings that are introduced between adjacent quasi-bandpass sections through their non-resonating nodes (NRNs). These TZs are synchronously tuned with passband reconfiguration. The lack of direct interaction among the resonating nodes of this multi-band BPF architecture results in a transfer function with high tuning robustness. Furthermore, when compared to prior-art coupled-resonator multi-band BPF designs, a reduced number of inverters are needed when synthesizing a large number of transmission bands. For the first time, the coupling-matrix formalism of the engineered multi-band BPF is expounded. In addition, a microstrip dual-band BPF prototype with mechanically-variable capacitors is manufactured and characterized for experimental demonstration purposes.