Miniaturized Two-Pole Lumped BPF with Four Controllable TZs Using Multiple Coupling Paths

Aiming at solving the problem of the second-order filter with no more than two transmission zeros (TZs), a novel dual-mode lumped-element bandpass filter is proposed in this letter to construct four TZs and a second-order passband response. The filter schematic and design formula are established to investigate the passband operation and TZ generation mechanism. Four TZs, generated by three transmission paths, are controllable and can be tuned through adjusting the associated elements, owing to a high level of the design freedom provided by the new dual-mode resonator and flexible I/O feeding circuits. As a demonstration, two filters are realized using hybrid technology for relatively low insertion loss and compact size. The experimental results illustrate the analysis and also exhibit a particular tunable TZ very close to the passband, which is independently allocated either below or above the passband.