Planar-Magic-T-Based Dual-Band Bandpass Filters

A new type of RF dual-band bandpass filter (BPF) stage with sharp-rejection filtering capabilities is presented. It consists of a planar-magic-T-based circuit, in which the side arms of the magic-T are loaded with open-circuit-ended multi-section microstrip stubs whereas its S and.ports are taken as the input and output accesses of the proposed dual-band BPF stage. In this manner, by properly selecting the values of the electrical-length and characteristic-impedance design parameters of the dual-band BPF stage, multi-pole dual-band BPF transfer functions with several transmission zeros (TZs) can be obtained. Moreover, higher-order dual-band BPF designs with increased selectivity and out-of-band power-attenuation levels can be realized by means of the in-series cascade connection of multiple replicas of the engineered dual-band BPF stage. For experimental-validation purposes, a planar dual-band BPF prototype is developed and characterized. It exhibits measured lower and upper dual passbands with center frequencies of 1.55 and 2.46 GHz GHz, 3-dB fractional bandwidths of 19.7% and 14.1%, and multiple TZs.