Narrow-band filter for the frequency range of 1.9 GHz using double-sided YBCO films on 10-mm-square and 20-mm-square LaAlO3 substrates

Narrow-band microstrip hairpin-type filters are designed for wireless-communication applications. We have fabricated the high-T_c superconducting filters by patterning YBa₂Cu₃O_y (YBCO) films deposited on 10-mm-square and 20-mm-square, 0.6-mm-thick LaAlO₃ substrates. The form of hairpin-resonator band-pass filter can be thought of as an alternative version of parallel-couple-resonator filter, except that the open-circuited ends of the resonator are folded back. Our 3-pole 10-mm-square filter has a 2.5-dB maximum insertion loss and 16 MHz bandwidth at 1.94 GHz, while 3-pole 20-mm-square filter has a 0.45-dB maximum insertion loss and 30 MHz bandwidth at 1.94 GHz both with the air-gap height of 3 mm. By mechanically changing the height of air gap between the filter pattern and the top housing plane, the center of frequency of the filter was varied from 1.92 to 2.0 GHz, corresponding to a frequency shift of about 5%. The influences of wafer sizes and housing height on filter performance are discussed.