Multilayered Wideband Balun Bandpass Filters Designed with Input-Reflectionless Response

In this paper, a wideband input-reflectionless balun bandpass filter based on a two-layered substrate is proposed. Through a shunt 50-Ω-terminated microstrip resonator connected at the unbalanced port, the initially-presented three-pole wideband filtering response of a reflective-type balun can be modified into a wideband filtering response with input-reflectionless behavior. Based on the three-port circuit topology of this conceived wideband filtering balun, a two-port equivalent circuit model, which is a 50-to-100-Ω impedance transformer network, can be derived and its relevant operational principle is then discussed. To experimentally validate the engineered design approach, a wideband input-reflectionless balun bandpass filter prototype centered at 2 GHz is developed and characterized. It features a measured amplitude imbalance smaller than 0.16 dB and phase imbalance lower than 1° between the two output ports within its 98.8% 3-dB fractional bandwidth (FBW), as well as input-power-matching levels above 14.62 dB from 0 to 3.88 GHz.