On the development of superconducting microstrip filters for mobile communications applications

This paper presents recent developments of an eightpole planar high-temperature superconducting (HTS) bandpass filter with a quasi-elliptic function response. A novel planar filter configuration that allows a pair of transmission zeros to be placed at the band edges is described. The miniature HTS filter has a fraction bandwidth less than 1% and is designed for mobile communication base-station applications to increase sensitivity and selectivity. Design considerations including filter characteristics, design approach, sensitivity analysis and unloaded quality factor of resonators are addressed. The filter was fabricated using double-sided YBCO thin film on an MgO substrate of size 0.3 x 22.5 x 39 mm. Very good experimental results were obtained with the filter cooled using liquid nitrogen. The minimum passband loss was measured to be approximately 1 dB. The passband width at points l dB down from the minimum loss point was 12.8 MHz for a center frequency of 1738.5 MHz. High selectivity was achieved with a 30-dB rejection bandwidth of 16 MHz.