Miniaturized Ultra-Narrowband Superconducting Microstrip Filter With Stable Coupling Using Optimized Twin Spiral-In-Spiral-Out Resonators

A twin spiral-in–spiral-out (SISO) resonator has been proposed in the design of a compact ultra-narrowband high temperature superconducting filter. We systematically investigated the relationship between the coupling characteristics and the structure of the twin SISO resonator. Coupling strength and polarity can be effectively changed by adjusting the resonator structural ratio. This method of optimizing the ratio of the twin SISO resonator structure has an advantage of achieving stable and weak coupling, even if the resonator distance is relatively narrow, thereby promoting the miniaturization of the filter. Moreover, the three methods of optimizing ratio, distance, and staggering of adjacent resonators could be regarded as three degrees of freedom for adjusting the adjacent coupling to meet design needs. To demonstrate this method, we designed and fabricated a six-order superconducting filter with a fractional bandwidth of 0.18% at 2258 MHz based on the optimized twin SISO resonator. The resulting measurement shows a good agreement with the simulation.