Superconducting Ultrahigh Q Tunable RE Preselector

The application of superconductivity to the problem of reducing radio frequency interference (RFI) is described. A lowloss, extremely narrowband superconducting filter was developed that is tunable from 6.3 to 21 MHz. This filter has an unloaded Q of 350 000 to 600 000 over the tuning band, bandwidth adjustable from 60 Hz to 50 kHz, calibrated external controls for frequency tuning and bandwidth adjustment, and good resettability and stability. An optimum circuit design provides minimum bandwidth variation with frequency tuning and minimum resonant frequency variation with changes in bandwidth. Performance tests on high-frequency receivers showed reductions in intermodulation interference susceptibility of 40 dB and improvements in FSK Teletype signals of 23 dB.