Design of Quasi-elliptic Filter for Optimizing the Band-Edge Selectivity

A new technique is presented for maximizing the band-edge selectivity of elliptic filters without increasing filter order, and then the equal ripple parameter of quasi-elliptic function can be calculated when the band-edge selectivity of the elliptic function and quasi-elliptic function are the same. The quasi-elliptic function based on the method not only keeps the bandwidth invariable, but effectively narrows the transition band. Finally, a novel quasi-elliptic micro strip filter composed of four spiral stepped-impedance resonators (SIR) is designed. The simulated and measured results show that the filter has good performance, and can effectively inhibit the second harmonics.