Abstract
We have developed a bandwidth and a center frequency tuning method for use in high-temperature superconducting microstrip filters. A pair of waveguide is placed between the resonators, and the bandwidth is adjusted by changing the switch states of the waveguides. Additional electrical pads are placed open ends of the resonators for tuning the center frequency. Pads are also placed around the input/output coupled-line elements to enable the external quality factors to be adjusted, thereby reducing the insertion loss caused by tuning. A prototype three-pole tunable bandpass filter was fabricated by depositing YBa2Cu3O7 thin film on an MgO substrate and has a measured center frequency of 4.83GHz and bandwidth of 121MHz. Use of the waveguides to adjust the coupling coefficients, the electrical pads to adjust the effective lengths of the resonators and the pads to adjust the external quality factors resulted in 120-MHz bandwidth tuning and 400-MHz center frequency tuning without increased insertion loss.
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