Superconducting spiral filters with quasi-elliptic characteristic for radio astronomy

To produce a filter small enough to fit a 2-in wafer at 408 MHz while maintaining high-quality performance, half-wavelength single spiral microstrip resonators are introduced. New coupling structures make both positive and negative coupling available by changing the directions of spiral winding. An eight-pole high-temperature superconducting bandpass spiral filter with 3.7% bandwidth at 408-MHz band is presented for radio astronomy applications at the Jodrell Bank Radio Observatory, Macclesfield, Cheshire, U.K. A quasi-elliptic characteristic with four transmission zeros is realized by adding three cross-couplings to the standard Chebyshev filter. The filter shown is designed and fabricated on a 32 mm × 18 mm × 0.508 mm MgO substrate. The untuned measured results of the filter at 30 K show a maximum passband insertion loss 0.35 dB (ripple 0.27 dB), a minimum return loss 13.2 dB, and minimum out-of-band rejection of 65 dB, which have good agreement with its electromagnetic full-wave simulation results.