Compact microstrip tri-band bandpass filter with mixed electric and magnetic coupling

This paper presents a novel mixed electric and magnetic coupling tri-band bandpass filter (BPF) by using multiple stubs loaded ring resonator. The first three resonant modes of this stubs loaded ring resonator can be separately controlled, resulting in the separately controlled passband frequencies and in-band return losses. Owing to the nature feature of stubs loaded ring resonator, directly coupled configuration and the cancelling effect of mixed electric and magnetic coupling, six transmission zeros can be seen around three passbands, which can improve the band-to-band isolation and passband selectivity significantly. As an example, a second-order tri-band BPF centered at 0.9 / 2.4 / 3.5 GHz are designed and fabricated. The fabricated tri-band BPF has low insertion loss, good return loss and occupies a compact circuit area of 0.06lλg × 0.157λg. Good agreement between the simulated and measured results is obtained.