Varactor-Tuned Narrow-Band High Temperature Superconducting Notch Filter

A tunable narrow band high temperature superconducting (HTS) notch filter is presented. HTS folded square open loop resonator loaded with hyper-abrupt GaAs varactor is used to reach high tunability and avoid the via-hole grounding. Measured results show that the notch frequency can be tuned from 286 to 485 MHz (tuning range of 51.6%), and the attenuation from 26 to 70 dB when the bias voltage of the varactors is from 1 to 20 V. The -3 dB bandwidth is between 6.9 and 7.9 MHz, and the out-of-band insertion loss is less than 0.2 dB. Furthermore, the reliability of manufacture techniques of the filter is proved by mechanical vibration and shock tests.