A microstrip quasi-elliptic bandpass filter implemented using stub loaded resonators

Abstract

A highly selective microstrip bandpass filter consists of electromagnetically coupled U-shaped open ring resonators is presented in this paper. The resonators are directly loaded with open-circuited inductive stubs and the feed-lines are interdigital coupled to the resonators. The proposed filter structure provides sufficient degree of freedom to control the centre frequency, fractional bandwidth and transmission zeros. It is shown that the fractional bandwidth of this filter can be adjusted from 15.3 to 9.8% and center frequency from 3.3-3.8 GHz with negligible effect on the stopband characteristics. In addition, the transmission zero f tz2 can be relocated from 3.8 to 4.4 GHz and f tz1 from 3.1 to 2.7 GHz independently. Filter structure is simple, easy and economical to manufacture as fabricated on a conventional dielectric substrate using standard PCB technology. It is found that the simulation and measured results are in good correlation. The sharp selectivity, low losses and wide stop band characteristics make the proposed filter applicable in cognitive radio systems and in high interference environments.