High-temperature superconductor (HTS) materials are very promising for high-frequency passive devices because of their extremely low loss at the microwave region. A coplanar waveguide (CPW) is one of the simplest planar transmission lines and is especially well suited to HTS films because only one side of the substrate needs to be coated before patterning. Furthermore, since the size of the CPW device can be reduced in scale without changing its characteristic impedance, it is easy to miniaturize the HTS bandpass filter (BPF) and make it compatible with the monolithic microwave integrated circuits (MMICs). In this paper, HTS Y-Ba-Cu-O (YBCO) films were prepared on MgO polycrystalline substrates (50×25×1mm
3) by the sol-gel process. Because this process requires no high vacuum, no special equipment and no large target, a YBCO film is easy to fabricate, at a very low cost, on a large substrate. The temperature dependences of the surface resistance (
Rs) at different positions of the YBCO film around 9.9GHz were observed. The value of average
Rs was 2.9mW at 30K. The large uniform YBCO film was prepared by the sol-gel process. CPW 2-pole band-pass filters (BPFs) (center frequency is around 2GHz) with attenuation poles are prepared by using YBCO sol-gel film. The electrode configuration of YBCO BPFs was patterned by the wet-etching technique with a photo mask. Microwave characteristics of our YBCO BPF with attenuation poles at both sides of the passband are better than that of Cu BPF. To realize a sharper cutoff and a narrow bandwidth BPFs, 3-pole BPF with attenuation poles were also prepared. It has an insertion loss of 2.41dB at the center frequency 1.98GHz, and the fractional bandwidth is 2.43%. These characteristics are better than those of Cu BPF (insertion loss is 5.505dB at 1.93GHz, and the fractional bandwidth is 2.84%). In our results, the possibility of realizing superconducting coplanar filters with attenuation poles is revealed.
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