Magnetically tunable superconducting resonators and filters

Compact, low-loss, tunable filters are needed for overload protection in the front end of many microwave-frequency systems. We have demonstrated magnetically tunable superconducting resonators and filters comprising microwave circuits coupled to ferrite substrates in monolithic structures using niobium at 4 K and hybrid resonator structures using YBCO at 77 K. A three-pole 1% bandwidth filter with 10-GHz center frequency and 1-dB insertion loss is described. In these devices the tunability results from changes in the magnetization of the ferrite rather than changes in the ferrimagnetic resonance frequency as in conventional YIG filters. Tunability data plotted us a function of magnetic field are fitted to the hysteresis theory developed previously and indicate that a tuning range of 13% is achievable. We have demonstrated switching times of less than one microsecond in structures incorporating closed magnetic paths in the form of a circular toroid.