Bandpass filters using multi-mode square ring resonators

Abstract

A novel scheme of a multi-mode square ring resonator (MMSRR) is proposed in this study. To analyse the characteristics of the proposed resonator, the odd- and even-mode based method is used, which shows that the basic type of resonator has five resonant modes and three transmission zeros (TZs). The frequencies of the transmission poles and TZs can be changed by varying the sizes of the transmission lines of the resonator flexibly. By varying properly the parameters of the resonator, a dual-band bandpass filter (BPF) is designed with central frequencies of 1.78 and 3.04 GHz, respectively. To explore further the performance of the proposed MMSRR, two single-band BPFs with controllable bandwidth are designed. The bandwidth of the filter can be tuned in the range of 138.6 MHz by changing only the characteristic impedance of middle microstrips, which is much higher than previous works. Furthermore, to improve the band-to-band isolation of dual-band filters, a stub is imposed in the middle of the proposed resonator, which can improve significantly the band-to-band isolation. Simulation and measurement results show that the isolation is > 26.3 dB. The results of the simulation and measurement are in good agreement.