Compact dual-band stepped-impedance resonator filter with separate coupling paths

A new compact microstrip dual-band bandpass filter (BPF) based on multi-mode λ/4 stepped-impedance resonators (SIRs) with separate coupling paths is investigated. The dual-band BPF with a second-order bandpass response is composed of only two λ/4 SIRs, resulting in a small circuit size. A coupling gap and a common short-ended stub between the two coupled SIRs are used to create the electric and magnetic couplings with separate coupling paths. The bandwidths of the dual-band BPF can be well controlled by adjusting the coupling strengths of the two coupling paths. Thus, a greater design flexibility for the dual-band BPF can be obtained. For demonstration, a second-order dual-band BPF with a compact size of 0.14 λ_g × 0.16 λ_g has been designed and fabricated. A good agreement is achieved between simulation and measurement.