Very compact spiral resonator implementation of narrow-band superconducting quasi-elliptic filters

Spiral superconducting filters have been improved by implementing quasi-elliptic characteristics. The better fabrication tolerance, simulation accuracy, power handling and quality factor have been retained. A structure for the required negative cross-coupling is introduced, and 0.86% band-width filters with about 1750 MHz centre frequency are demonstrated, including one with tuning screws.