Multifunctional and Reconfigurable Bandpass Filters with Continuously-Tunable Attenuation or Quasi-Reflectionless Behavior

This brief presents a novel class of multifunctional and reconfigurable bandpass filters (BPFs) that either demonstrate: i) a continuously tunable attenuation levels within their passband, functioning as co-designed bandpass filtering attenuators (BPFAs), or ii) a quasi-reflectionless bandpass behavior. In the BPFA mode, the filters achieve continuously tunable attenuation characteristics by utilizing PIN diodes to load two capacitively-loaded resonators resistively. The quasi-reflectionless BPF mode is realized by connecting two coupled-line bandstop filter sections, loaded with PIN diodes on their input and output terminals. To validate the proposed concept experimentally, two prototypes (two-cell and three-cell BPFs) were designed, fabricated, and measured. The results demonstrate a BPFA mode with a wide passband spanning from 1.44 to 2.56 GHz and tunable attenuation levels ranging from 3.43 to 20.31 dB. In the reflectionless BPF mode, they exhibit a 10-dB reflection fractional bandwidth (RFBW) of 200% and an out-of-band rejection exceeding 38 dB.