Design of a narrowband HTS filter at 7.4-GHz with improved upper-stopband performance

This paper presents a high-temperature superconducting filter at 7.42 GHz with an improved upper-stopband performance. U-type stepped-impedance resonators are used to relocate the spurious frequency and realize wide stopband response. The compact resonators also reduce the filter size and push away the housing box resonant frequencies. Open stubs are introduced at the midpoints of the resonators to provide additional transmission zeros, which can improve both the band-edge steepness and the out-of-band rejection of the upper stopband. As a result, the spurious response of the filter is relocated to above 18 GHz, and the out-of-band rejection is higher than 70 dB.