High-Order Quasi-Elliptic-Type Wideband Bandpass Filter With Ultrabroad Input-Reflectionless Stopband Range

A two-layer wideband bandpass filter (BPF) with ultrabroad input-reflectionless stopband range is reported. It consists of a two-port-reflectionless high-order quasi-elliptic-type (QET) wideband BPF and two similar input-reflectionless low-pass filter (LPF) stages. The constituent 2-GHz BPF is shaped by two vertical transitions with shunt resistively terminated T-junctions, which are connected through a microstrip T-junction. It features a two-port-absorptive ninth-order in-band quasi-equiripple BPF response with two close-to-passband transmission zeros (TZs). The building LPF is made up of two in-series-cascaded input-reflectionless LPF units with their first TZs at 6 and 12 GHz, which results in an extended upper stopband beyond 18 GHz. Through this LPF-BPF-LPF approach, an extremely wide input-reflectionless stopband range with increased power attenuation levels in the overall BPF is attained. Its theoretical RF operational principles are detailed. Furthermore, for experimental validation purposes, a two-layer microstrip wideband BPF prototype is simulated, manufactured, and measured. It exhibits a minimum stopband power attenuation level of 29.57 dB, along with minimum input-power-matching levels of 10 and 6.7 dB from dc to 9.85 GHz and from dc to 20 GHz, respectively.