A Ring Resonator Based Wide-Band Band Pass Filter with Single Loaded Stub and Compact Structure

摘要

A compact and simple structure of a wideband band pass filter (BPF) based on ring resonator (RR) topology with only one open circuited stub is proposed. The pass band characteristic of the RR is achieved by placing the output port co-axial to the input port, at a position along the resonator length with maximum electric field. Subsequently wide operating bandwidth is realized by controlling the length of loaded stub due to its degenerate modes. An analytical equation is proposed based on even-odd mode analysis of a simplified transmission line model centrally loaded with stub to theoretically predict the positions of transmission zeros. A cascaded structure of two stub-loaded RRs filter with improved out of band rejection is designed and fabricated at 3 GHz. The insertion loss (IL) and return loss (RL) are better than 1.7 dB and 12.3 dB with a 3-dB fractional bandwidth of 47%. The two transmission zeros at 2.26 GHz and 3.8 GHz show rejections better than 37 dB. Furthermore, the area of the filter is reduced by special placement of the open circuited stubs inside the rings. It is shown that the compact filter is 33% area efficient with comparable performance parameters.