A Square Ring Resonator Bandpass Filter With Asymmetrically Loaded Open Circuited Stubs

This work presents the design, fabrication, and measurements of a compact, low loss, bandpass filter, using two asymmetrically loaded open circuited stubs on a microstrip square ring resonator. In the proposed design, a bandpass filter is developed from the stop band characteristics of the ring resonator. The operating frequency of the filter is at 2.4 GHz. The filter has a bandwidth of 1.4 GHz, resulting in a 60% -3 dB fractional bandwidth. The insertion loss is less than 1 dB and the return loss is below -20 dB in the desired passband. A prototype is fabricated to validate the proposed design using Rogers 4003C substrate and an equivalent circuit model with almost identical S-parameters response, is also introduced. The measured results agree well with the simulated results and the equivalent circuit model's response. In order to further improve the band rejection zone, parallel-coupled lines are introduced in replacement of the conventional microstrip lines of the input and output feed lines of the ring resonator. The use of coupled lines improves the band rejection zone of the filter especially for frequencies away from the transmission zeros, and results in insertion loss better than -20 dB.