Design of a Wideband Bandpass Filter with a High Band Selectivity and a Wide Stopband

In this paper, a wideband bandpass filter (WBPF) with a wide stopband by using a multimode stepped impedance resonator (SIR) and tapping a stepped-impedance open-circuited stub is designed and implemented for the 3.1-4.9 GHz low band specification. The wide passband performance with a wide stopband is formed by allocating the first three resonant modes of the multimode SIR, and the band selectivity is further improved by using the stepped-impedance open-circuited stubs to have sharp attenuation rates at both lower and upper sides of the passband. Moreover, the insertion loss of the designed filter is improved by two aperture-backed structures under the inter-digital input/output ports. The fabricated WBPF exhibits a high performance, including the bandwidth of 3.3-5 GHz (3-dB fractional bandwidth of 45%), low insertion loss of 1 +/- 0.3 dB at center frequency f(0) of 4 GHz, and a wide stopband from 5.1 GHz to 11 GHz. Experimental results also show a good agreement with the simulated ones.