Independently Controllable Dual-Band Bandpass Filters Using Asymmetric Stepped-Impedance Resonators

This paper presents a dual-band bandpass filter with independent switchability of each passband. By taking advantage of harmonic ratio of asymmetric stepped-impedance resonators, 2.4- and 5.2-GHz resonators are designed. The second resonance of 2.4-GHz resonators is designed to be suppressed by a transmission zero created by extended feed lines. To achieve a wide upper stopband, 5.2-GHz resonators are designed with a scheme that the second peak of these resonators would not appear at a frequency lower than a third peak of 2.4-GHz resonators. Several design graphs for deciding both geometric parameters and transmission zeros are plotted by using the information obtained from electromagnetic simulations. While extracting coupling coefficients between resonators, an external quality factor at 50-Ω port is fixed to determine the precise transmission zeros. Two kinds of feed lines, such as a hook type and a spiral type, are developed, and spiral-type feed lines are chosen for realizing switchable filters. By controlling p-i-n diodes, four states of switchable dual bands are clearly presented in simulations and experiments.