Miniaturized Superconducting K-Band Bandpass Filter

A novel high-temperature superconducting (HTS) microstrip K-band filter is designed, fabricated, and measured. The Chebyshev filter has five poles and is designed for the center frequency of 20.45 GHz with a percentage bandwidth of 2.5%. The proposed miniaturized resonator minimizes the unwanted coupling between nonadjacent resonators, which is typically pronounced at high frequencies. The layout is patterned on a 0.5-mm-thick MgO substrate. The dimension of the fabricated filter is 7.5 mm × 3 mm. The filter employs a via-less Coplanar Waveguide to microstrip transition and the layout is configured to allow ease of adjusting and tuning. The filter with via-less Coplanar Waveguide input/output pads is measured in an RF cryogenic probe station at 77 and 4 K to investigate variation of center frequency with operating temperatures. Simulation and measured results of the filter show a spurious-free response up to 35 GHz.