Enhanced performance of HTS hairpin type bandpass filter by interdigitated inner poles

To improve the performance of the hairpin type bandpass filter, we designed a new style hairpin type HTS 2-pole microstrip bandpass filter by using interdigitated inner poles. The filter was designed to operate at 5.8 GHz. The interdigitated inner pole has 29 fingers where each finger had a dimension of 0.4 mm length and 0.1 mm width. The gap distance between the two fingers was 0.1 mm. In simulations, the new style filter showed a significant improvement in the performance over the regular hairpin type filter. To fabricate the filters by using superconductive YBa₂Cu₃O_7-x films, we deposited the films on the sapphire substrates by using CeO₂ thin layer as a buffer layer. We used ArF pulsed laser deposition system to fabricate the films. We used ArF to obtain the smoother film surface instead of KrF that has been used more widely. We also rotated the substrates during the film growth to obtain the better film uniformity. Fabricated YBa₂Cu₃O_7-x films showed a critical temperature of 87.8 K. The packaged filter with the size of 13.7 mm×3.3 mm showed lower center frequency, narrower bandwidth, and less loss compared to the same size regular hairpin type filter, by 14.5%, 29.6%, 0.55 dB, respectively.