Wideband Filtering Rat-Race Coupler with Shared Triple-Mode Resonator

In this paper, a filtering rat-race coupler (FRC) is presented, which features the merits of a wideband filtering response, good impedance matching at both input and output ports, good isolation, high-frequency selectivity, small in-band phase difference error, a wide stopband, and small size. It was composed of two leading triple-mode resonators, one core section, and two & lambda;/2 open-circuited transmission lines (TLs). The core section consisted of three & lambda;/4 TLs and one & lambda;/4 short-circuited parallel coupled line (CL). The leading triple-mode resonator consisted of one & lambda;/4 short-circuited stub and two & lambda;/4 TLs. By loading the triple-mode resonator as the shared component and combining it with the core section, a wideband filtering response with four transmission poles could be achieved. In addition, two transmission zeros were generated by loading the & lambda;/2 open-circuited TLs to the output ports of the FRC. The working principles of the proposed FRC are analyzed, and equations are derived with parametric analysis. For demonstration, a prototype operating at the center frequency of 1.4 GHz was designed, fabricated, and measured. Measurement results show that a fractional bandwidth of more than 60% was achieved under the criteria of 10 dB return loss and 20 dB isolation. In this bandwidth, the amplitude imbalances and phase difference errors were less than 1 dB and 10 & DEG;, respectively. Additionally, it also exhibited high selectivity and a wide stopband.