Hybridly-Integrated Quasi-Elliptic-Type Bandpass Filters with Symmetrical Quasi-Reflectionless Characteristics

A new class of high-quality-factor (Q) and hybridly-integrated quasi-elliptic-type bandpass filters (BPFs) with symmetrical quasi-reflectionless characteristics is reported. Its circuit architecture is based on loading the input and output ports of a quasi-elliptic-type BPF⸺it exhibits two poles and four transmission zeros (TZs)⸺with resistively-terminated bandstop-filter (BSF) branches to absorb the reflected RF power. The quasi-elliptic-type power transmission response is obtained by incorporating multi-resonant cells that exhibit one pole and two TZs and allow for quasi-elliptic-type responses to be realized without the need for cross couplings. A hybrid integration scheme using capacitively-loaded high-Q ceramic coaxial resonators and microstrip-type impedance inverters is proposed for size compactness and low passband insertion loss. The concept is experimentally validated at 3 GHz through the manufacturing and testing of a filter prototype that exhibits a passband with in-band insertion loss (IL) <0.9 dB (corresponds to an effective quality factor of 430) and wide quasi-reflectionless characteristics.