Balanced dual-band bandpass filter with deep common mode attenuation using multi-mode ring resonators

This paper proposes a balanced filter design method based on multi-mode ring resonators. By optimizing the resonator structure, independent control of center frequencies of the dual passbands is achieved. Additionally, parallel-coupled feeding is adopted, and the external quality factor is adjusted by tuning the spacing between the feed lines and resonators, enabling independent optimization of the bandwidth for both passbands. Experimental results show that the filter achieves a return loss of less than −17 dB within the dual passbands, an inter-passband isolation exceeding −40 dB. The minimum common-mode mode (CM) suppression of the differential mode (DM) passbands are 41.9 dB and 42.2 dB. The fabricated and tested filter demonstrates good agreement between simulation and measurement results.