Input-Reflectionless Out-of-Phase 3-dB Bandpass Filtering Couplers

A class of out-of-phase 3-dB bandpass-filtering couplers with input-reflectionless capabilities is presented. To obtain the bandpass-filter (BPF) functionality, identical BPF sections are respectively co-integrated in the coupler signal paths from the input to the direct and coupled ports. Furthermore, a resistivelyterminated bandstop-filter (BSF) section with complementary transfer function with regard to the one of the BPF section is loaded at the coupler input access. In this manner, the RF inputsignal energy that is not transmitted to the direct and coupled terminals is dissipated by the loading resistor of the BSF section. Hence, the input-reflectionless behavior is realized. Optimizationbased first-to-third-order design examples are shown. Moreover, for practical-validation purposes of this RF tri-functional device, a 2-GHz second-order microstrip prototype is built and tested.