Miniaturized ultra-narrowband HTS linear phase filter with CQ units sharing quarter-wavelength resonators

This paper proposes a method for designing a miniaturized ultra-narrowband high-temperature superconducting (HTS) linear phase filter based on the resonator-shared cascaded quadruplet (CQ) units constructed by the quarter-wavelength microstrip line. Two types of CQ unit with different coupling configurations are utilized to introduce transmission zeros and realize the linear phase. The introduction of transmission zeros increases the transition band roll-off rate, and the in-band group delay characteristic is improved through the phase compensation. The number of resonators and the filter size further decrease by sharing a resonator between two adjacent CQ units. Finally, a ten-order compact ultra-narrowband linear phase HTS filter with three CQ units is designed to demonstrate the feasibility. The central frequency of the filter is 450 MHz with the fractional bandwidth of 2.22%, and the dimension is 38.7 mm × 12.5 mm (0.141λ_g0 × 0.047λ_g0, λ_g0 is the guided wavelength at the central frequency f₀). The measured results show an excellent performance in the insertion loss, the out-of-band rejection, the return loss, and the passband selectivity. Moreover, the bandwidth with a group delay ripple within 30 ns occupies 70% of the entire passband, highlighting the linear phase feature. In addition, the first parasitic passband is located at 3.8 times of the fundamental frequencies. The test results are in good agreement with the simulated ones.