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 cascaded triplet (CT) unit features flexible control over transmission zeros (TZs), and the cascaded quadruplet (CQ) unit can introduce TZs 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 TZ positions are extracted by the coupling matrix analysis method. A dual-passband high-temperature superconducting bandpass filter is designed with a folded compact dual-spiral microstrip resonator based on the afore mentioned design method. The filter has center frequencies of 2495.75 MHz and 2525.75 MHz, with two equal relative bandwidths of 0.86%@ f 0 and four TZs. The measured results of the filter at a temperature of 77 K show excellent sideband roll-off of ⩾12.8 dB MHz −1 and minimal insertion loss 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 TZs is validated. Furthermore, this study provides an effective approach for designing high-performance ultra-narrowband dual-band filters.