A Compact Superconducting Filter at 6.5 MHz Using Capacitor-Loaded Spiral-in-Spiral-Out Resonators

Chip-capacitor-loaded spiral-in-spiral-out (SISO) resonators are proposed for superconducting filters at the HF band. Interdigital capacitors are used between resonators to provide large coupling values at the fundamental mode and small coupling values at harmonic modes. A compact four-pole high-temperature superconducting filter centered at 6.5 MHz is demonstrated, with a 8% fractional bandwidth, 0.13 dB insertion loss, and 14.3 dB return loss. The spurious response of the filter is below -40 dB up to about 13 times of the fundamental mode.