4 GHz band HTS receiving-filters for a compact cryogenic receiver front-end

We report on the development of 4 GHz band high-temperature superconducting (HTS) filters for a compact cryogenic receiver front-end. For the application to the next-generation communications system with higher bit rate data transmission, 9-pole and 15-pole microstrip hairpin bandpass filters with a center frequency of 4 GHz and a relatively wide bandwidth of 100 MHz were designed and fabricated. A 9-pole bandpass filter using YBCO superconducting films on MgO dielectric substrate performed the frequency responses corresponding closely to the simulated results. By using LaAlO₃ substrate with higher dielectric constant, a compact, more poles, 15, bandpass filter with steeper skirt frequency characteristics was obtained. However, the measured less return loss response around the center frequency of the in-band implies that a precise coupling-control between adjacent resonators and also non-adjacent resonators is required. Moreover, the receiving-circuits comprising each fabricated YBCO bandpass filter sample and a 4 GHz band cryogenic low-noise amplifier were measured at 70 K in terms of S-parameters and noise figure (NF). The results which reflect each of the filter performance indicated high selectivity, high gain, and low NF.