Series-cascaded ring resonators for compact microwave filter with high rejection

Very selective fifth order bandpass ring filter is presented based on two series-connected quarter-wavelength side-coupled ring resonators. As the connection between the two quarter-wavelength lines forms a half-wavelength resonator, an extra pole is produced, leading to a total of five poles since each ring resonator presents two poles. This adds to the compactness of the overall filter and improves the selectivity and rejection. Centered at 2 GHz, the filter layout is simulated and implemented on FR4 substrate using microstrips. Filter simulation and measurement results are presented to validate the idea.