A Wideband HTS Filter Using Strong Coupling Coplanar Spiral Resonator Structure

A novel coplanar spiral resonator structure, which has a stronger coupling strength than a normal coplanar waveguide (CPW) structure, is introduced. This structure helps to realize the remarkable strong coupling required for low-frequency wideband filters. Although no grounded stubs are deployed to balance the ground plane, the slot-line mode of the filter is not excited. An eight-pole wideband high-temperature superconducting filter, with a fractional bandwidth of 50% and a center frequency of 322 MHz, is developed. The filter is designed and fabricated on a 0.5-mm-thick LaAlO$_3$ substrate with a core size of 41 mm × 22 mm. The measured minimum insertion loss is 0.05 dB and the first spurious peak appears at 842 MHz, which is about 2.6 times the center frequency. The measured results agree well with the simulated results.