Ultra-Compact Superconducting Narrow-Band FiltersUsing Single- and Twin-Spiral Resonators

Superconducting filters using spiral resonators have been demonstrated. Compared with meander lines, errors in track width have a smaller effect on the total inductance; fabrication tolerance, simulation accuracy, power handling, and quality factor are consequently improved. The coils do not require air bridges, which are difficult to produce in superconductors. The devices can be designed using a standard procedure based on the coupling coefficients between resonators and coupling to source and load. Fourth- and sixth-order Chebyshev filters employing single and twin spirals are demonstrated for 0.86% bandwidth filters with center frequency of approximately 1750 MHz. A new coupling structure has been introduced for the single-spiral resonators. Despite the very small size of approximately 1.7 x 1.7 mm , the resonators have a quality factor of approximately 30 000 at 77 K and the filters can handle approximately 20 mW.