Mixed-Technology Quasi-Reflectionless Planar Bandpass Filters

This paper reports on the RF design of mixed-technology quasi-reflectionless planar bandpass filters (BPFs). They are based on a hybrid integration scheme in which microstrip resonators are effectively combined with lumped-element ones for size compactness. By loading the input and output ports of a first-order BPF with first-order bandstop-filter (BSF) sections that exhibit complementary transfer function with regard to the BPF one, a symmetric quasi-reflectionless behaviour can be obtained at both accesses of the overall filter. The first-order quasi-absorptive BPF cell can be extended to higher-rejection realizations by cascading in series multiple first-order stages and merging their interconnecting BSF sections. For experimental-validation purposes, two quasi-absorptive BPF prototypes (one-and two-stage schemes, respectively) centred at 2 GHz were designed., manufactured., and measured. They exhibit return-loss (RL) levels higher than 10 dB over fractional bandwidths (FBW s) of 117% and 111%, respectively.