Design of two-stage UIR and SIR bandpass filters with an elliptic function-like response

Two-stage bandpass filters with a sharp transition band are presented. The two stages are cascaded by a direct coupling scheme, as well as a three-line structure. Each half-wave resonator is trapped with a quarter-wave open stub at its center and a transmission zero can be created. The positions of the two zeroes can be easily tuned to locate close to the passband, so that an elliptic function-like response can be obtained. Both uniform impedance resonators (UIRs) and stepped impedance resonators (SIRs) are employed in the design. Experimental results show a close agreement with the design.