K-band dual-band superconducting bandpass filter using uneven coupled SIRs and stub-loaded combining network

Abstract A K-band millimeter dual-band superconducting filter with uneven coupled step-impedance resonators (SIRs) and stub-loaded combining network is proposed in this paper. The novel uneven internally coupled SIR introduces a notch to the classical internally coupled SIR, enhancing the Q-factor for narrow-band filter design. The performance of the uneven coupled SIR is theoretically analyzed, and two single-passband superconducting filters based on the proposed resonator are designed and demonstrated its high Q-factor. A novel stub-loaded combining network is then investigated theoretically to integrate the two single passband filters into a dual-band filter. To validate the design concept, a dual-band superconducting filter is designed on a magnesium oxide (MgO) substrate using the yttrium barium copper oxide superconducting film. The measured results reveal insertion losses of 0.91–1.2 dB of the first passband at 24.21–24.41 GHz with the bandwidth of 0.82% and 0.43–1.3 dB of the second passband at 24.67–25.2 GHz with the bandwidth of 2.12%, respectively. Both passbands exhibit return loss levels exceeding 8 dB, a maximum out-of-band suppression of approximately 65 dB, and a rejection level of about 35 dB between the two passbands.