Cascaded fourth-order mixed-coupled bandpass filter with good frequency selectivity

A compact fourth-order bandpass filter with good frequency selectivity is presented. The proposed BPF consists of four cascaded stepped impedance resonators (SIRs). Mixed electric/magnetic (E/M) coupling are introduced in the proposed filter to generate transmission zeros (TZs) near the passband for sharp frequency selectivity. The method on the generation of transmission zeros caused by mixed coupling introduced in the filter is analyzed. A new filter with 15.4% fractional bandwidth (FBW) has been designed and fabricated to verify the validity of the proposed method. Three TZs are introduced near the passband, which is at 1.21GHz, 1.72GHz, 3.03GHz. Measured results are in good agreement with the electromagnetic simulation.