Design of high-order coupling ultra-narrowband HTS symmetric dual-band bandpass filter based on controlled TZs by CT and CQ units

Abstract By employing the synthesis method of the generalized Chebyshev function filter and the frequency-domain symmetric transformation of the transmission and reflection function response, the transformation from a single-passband filter to a symmetric dual-passband filter in the frequency domain is successfully achieved. The CT (Cascaded Triplet) unit features flexible control over transmission zeros, and the CQ (Cascaded Quadruplet) unit can introduce transmission zeros in pairs in a single step. Based on the topology of CT and CQ units, a typical topological structure model and the corresponding coupling matrix of the dual-passband filter with specified transmission zero positions are extracted by the coupling matrix analysis method. A dual-passband high-temperature superconducting (HTS) bandpass filter is designed with a folded compact dual-spiral microstrip resonator based on the aforementioned design method. The filter has center frequencies of 2495.75 MHz and 2525.75 MHz, with two equal relative bandwidths of 0.86%@f0 and four transmission zeros. The measured results of the filter at a temperature of 77K show excellent sideband roll-off of ≥ 12.8 dB/MHz and minimal insertion loss (IL) of ≤ 0.2 dB, which is consistent with the theoretical model and full-wave electromagnetic simulation frequency response. Thus, the effectiveness of the proposed topology and design method in achieving dual-passband characteristics and introducing in-band transmission zeros is validated. Furthermore, this study provides an effective approach for designing high-performance ultra-narrowband dual-band filters.