Compact dual-band bandpass filter with controllable bandwidths

A compact dual-band bandpass filter with controllable bandwidths and good selectivity is presented in this paper. The proposed filter employs two sets of resonators operating at diverse frequencies to generate two passbands. One set of resonators is utilized to not only generate the upper passband but also feed the lower passband resonator, which is a two-mode resonator, as well as realizing source-load coupling for the lower passband. The feed and resonators couplings vary as the operation band changes, thus it could provide sufficient degrees of freedom to control the center frequencies and bandwidth of the two passbands, respectively. Five transmission zeros can be created close to the passband edges, resulting in high skirt selectivity. To validate the design and analysis, a dualband bandpass filter centered at 2.45GHz/5.27GHz for WLAN application is fabricated and measured.