Design of Multifunctional Bandpass Filter with Tunable Attenuation and Reflectionless Behavior

This paper reports on a new class of a multifunctional bandpass filter (BPF) that can be reconfigured to exhibit: i) continuously-tunable attenuation levels in its passband performing the function of a co-designed bandpass filtering attenuator (BPFA) and ii) quasi-reflectionless behaviour. The reconfigurable attenuation characteristics in the BPFA mode of operation are obtained by resistively loading two of its capacitively-loaded resonators with PIN diodes. The highly-selectivity quasi-reflectionless BPF mode is materialized by shunt connecting two coupled-line bandstop filter sections on both input and output terminals that are also loaded with PIN diodes. For proof-of-concept demonstration purposes, an experimental prototype was EM simulated, manufactured, and measured. It demonstrated a broad BPFA mode with a passband between 1.44 to 2.56 GHz and tunable attenuation between 3.43 to 20.31 dB. In the reflectionless mode, it exhibited a 10-dB reflection fractional bandwidth (RFBW) of 200 % and out-of-band rejection of more than 38 dB.