Design of the wide stopband dual-band bandpass filter using stepped-impedance resonators with complex coupled line

This paper presents a dual-band bandpass filter (BPF) using stepped-impedance resonators (SIRs) with enhanced stopband characteristics. The proposed filter can easily suppress its spurious responses by controlling the complex coupled line which consists of the electric-coupled parts of the two SIRs and feed lines. In order to improve the attenuation characteristics between the two passbands, source-load cross coupling is also introduced. Two-pole dual-band BPF with fractional bandwidths of 3.4% and 2.0% at the center frequencies of 1.8 GHz and 2.6 GHz is designed, simulated and measured. Transmission zeros near the passbands are generated and consequently enhanced skirt characteristics are achieved. The spurious responses are suppressed up to five times of the first passband center frequency.