Input-Reflectionless Two-Branch Channelized Passive Dual-Band Bandpass Filters

A class of planar input reflectionless quasi-elliptic-type dual-band bandpass filter (BPF) based on fully-passive two-branch channelized principles is reported. When compared to the low-order dual-band BPF units incorporated in its two branches, enhanced filtering selectivity and input-absorptive behavior are attained in the channelized filter. To this aim, the two filter channels are designed to exhibit the same amplitude response but different phase profiles by adding distinct phase-variation blocks at each channel. Thus, the out-of-band RF-signal-power reflections can be efficiently suppressed at the input. Besides, the selectivity of the total transfer function is improved by producing multiple out-of-band transmission zeros (TZs) through the two-channel transversal-signal-interference action at the output. The theoretical foundations of the devised input-reflectionless dual-band BPF and its main design aspects are detailed. Moreover, as practical validation, a microstrip prototype with dual passbands centered at 2.58 GHz and 3.43 GHz is developed and measured.