Octave-tunable constant absolute bandwidth bandstop filter utilizing a novel passively-compensated coupling method

A new passively-compensated coupling structure, compatible with evanescent-mode cavity resonators and capable of producing constant absolute bandwidth over wide tuning ranges, is presented for the first time. It is used to realize a constant absolute bandwidth 2.0 to 4.2 GHz tunable bandstop filter which utilizes evanescent-mode cavity resonators. The filter has high levels of stopband rejection (> 50 dB) across its entire tuning range, and has a 54.5 ± 6.5 MHz 3-dB fractional bandwidth. When compared to the coupling method traditionally used for evanescent-mode resonators, the proposed method reduces the bandwidth variation of this filter from 295% (21 to 83 MHz) to 27% (48 to 61 MHz).