High-Order Coaxial Bandpass Filters with Multiple Levels of Transfer Function Tunability

This letter reports on high-order coaxial-resonator-based bandpass filters (BPFs) with multiple levels of transfer function tunability, namely center frequency ( fcen), bandwidth (BW), and intrinsic switching off. They are based on cascaded multi-resonant cells, each exhibiting one pole and two transmission zeros (TZs), and in-line resonators. The proposed concept allows for high-order quasi-elliptic responses without cross couplings that can be reconfigured by only tuning the resonators' fcen. Low-cost integration using 3-D printed cavities and varactor-based PCB tuners is also shown. For proof-of-concept validation purposes, a five-pole/four-TZ BPF with fcen tuning 1.3-1.77 GHz (1.4:1), BW tuning: 62-217 MHz (3.5:1), and an intrinsic switching-off state with >35 dB of isolation was manufactured and measured.