Compact Quad-Band Bandpass Filter Based on Stub-Loaded Resonators

摘要

This paper presents a planar quad-band bandpass filter with high out-of-band rejection. The filter is based on intercoupled stub-loaded resonators, where pairs of resonators are electromagnetically coupled to each other and the feedlines. This results in excitation of passbands, where the first and the third passbands are generated by /4 resonators. The second and the fourth passbands are excited by /2 resonators. The proposed technique provides sufficient degree of freedom to control the center frequency and bandwidth of the four passbands. In addition, the five transmission zeros created around the passbands results in a quad-band filter with high selectivity, sharp 3 dB cut-off frequency, high isolation, and low passband insertion-loss. The proposed technique was verified practically. Design methodology and experimental results of the prototype filter are provided.