Compact broadband bandpass filters with two transmission zeros

A novel, compact bandpass filter is proposed using stepped-impedance resonators (SIRs). The basic filter structure is composed of two parts, the central SIR sections and the input/output (I/O) SIR sections. The two central SIRs are folded to achieve a compact size and coupled to the I/O SIRs. This new structure has an inherent transmission zero on the upper stopband edge. The skew-symmetric feeding structure is introduced and can provide an additional transmission zero on the lower stopband edge. As a result, the filter with two transmission zeros exhibited extremely sharp rejection skirts around the target passband. The filters with two transmission zeros and four transmission poles have a fractional bandwidth (BW) Δ = 36%. Measured results of experimental circuits show good agreement with simulated responses.