Adaptive-transfer-function bandpass filters using reconfigurable evanescent-mode-cavity resonator cascades

An original architecture of a fully-reconfigurable bandpass filter (BPF) is presented. It consists of two series-cascaded-through inter-stage connecting lines-spectrally-overlapped second-order BPFs and a filtering cell that generates two out-of-band transmission zeros (TZs) for selectivity enhancement. The three inter-cascaded filtering blocks comprise six frequency-tunable resonators that interact by means of static impedance inverters. By controlling the natural frequencies of the resonators in all filtering units through variable-reactance elements, an overall frequency-agile bandpass transfer function in terms of center frequency, bandwidth, and TZs is attained. Furthermore, since the passband-width is controlled without altering the inter-resonator couplings, benefits are obtained in terms of lower in-band insertion loss and larger bandwidth tuning ratio that can be practically reduced to zero (i.e., intrinsic switching-off state). The operating principles of the devised filter-cascade concept are expounded. Also, an evanescent-mode cavity-resonator prototype with tunable response in the range 2.9-3.5 GHz is built and tested for experimental verification. To the best of the authors' knowledge, the developed BPF circuit demonstrates one of the highest electronic-reconfiguration capabilities reported up to date for 3-D BPFs.