Design of a Dual-Band Superconducting Filter for 5G Mobile Communication

High-performance filters based on high-temperature superconducting (HTS) thin film technology have been developed for 5G mobile communications applications. However, a design that enables the resonant frequencies and passband bandwidths of the dual-band bandpass filter to be easily specified is lacking. This paper presents the design of a central load stub step impedance resonator based on a T-type resonator. The feasibility of achieving a wide stopband was analyzed using a structural diagram. The filter incorporates a dual-path coupling structure to adjust the internal coupling and a dual-feed external coupling structure to control the external quality factor. A fourth-order dual-passband filter was then fabricated on a double-sided YBCO HTS thin film with the size of 15.7 mm × 9.4 mm (0.47 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> × 0.28 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub>) and tested at 55 K. It achieved center frequencies of 3550 MHz (2.8% bandwidth) and 4850 MHz (2.0% bandwidth), meeting the N78 and N79 bands of 5G technology. The filter also generated four transmission zeros on both sides of the two passbands and achieved an out-of-band suppression greater than 80 dB, demonstrating excellent performance. The stopband rejection was greater than 50 dB for frequencies of up to 11 GHz. Because of its ease of use, design will facilitate the adoption of HTS filters in communications systems.