Magnetically tunable superconducting resonators and filters

Compact, low-loss, tunable filters are needed for overload protection in th e front end of many microwave-frequency systems. We have demonstrated magne tically tunable superconducting resonators and filters comprising microwave circuits coupled to ferrite substrates in monolithic structures using niob ium at 4 K; and hybrid resonator structures using YBCO at 77 K. A three-pol e 1% bandwidth filter with 10-GHz center frequency and l-dB insertion loss is described. In these devices the tunability results from changes in the m agnetization of the ferrite rather than changes in the ferrimagnetic resona nce frequency as in conventional MG filters. Tunability data plotted as a f unction of magnetic field are fitted to the hysteresis theory developed pre viously and indicate that a tuning range of 13% is achievable. We have demo nstrated switching times of less than one microsecond in structures incorpo rating closed magnetic paths in the form of a circular toroid.