A compact and high isolation dual-mode dual-band bandpass filter with tunable transmission zeros

Abstract

A new dual-mode dual-band bandpass filter (BPF) is presented by utilizing a coupled-line with open-circuited quarter-wavelength stub. Classical even- and odd-mode analysis shows that the new dual-band structure has four tunable transmission poles and two tunable and three fixed transmission zeros. The fixed transmission zero at the design frequency divides four transmission poles into two groups, resulting in a dual-band BPF with two transmission poles in each passband. Two tunable transmission zeros can be located either between two passbands to provide a wide and high isolation band or outside two passbands to improve the selectivity of sidebands. As examples, two dual-mode dual-band BPFs, filter A with two transmission zeros between two passbands as well as filter B with two transmission zeros outside two passbands, are designed, fabricated, and measured, of which two passbands are located at the center frequencies of 2.4 and 5.8 GHz for WLAN application, respectively. The simulated and measured results coincide well, validating the design theory.