High-temperature superconducting (HTS) microwave bandpass filters (BPFs) have advantages of extreme low insertion loss, high selectivity of passbands, and high rejected-level in stopband. On the other hand, multimode resonators can provide multiple resonances and exhibit the same behaviors with several single-mode resonators, which highly reduce the circuit size. In past few years, we engaged in the development of high performance and compact HTS multi-band microwave BPFs by using multimode resonators, such as the square ring loaded resonator and hairpin ring resonator. Furthermore, differential/balanced filters are widely used in modern communication systems for their good common-mode (CM) rejection capability, which results in higher immunity to the environmental noise and electromagnetic interference as compared with their single-ended counterparts. So, a series of HTS multiband differential/balanced microwave BPFs with deep suppression of common-mode noise signal were developed by adopting multimode resonators. With the advantages of ultra-low in-band insertion losses and high CM suppression, the demonstrated filters are attractive for potential applications in multiband communication systems requiring high-sensitivity and high anti-interference properties.
Keywords: bandpass filter, differential circuits, multimode resonators, multi-band