高温超导滤波器频带偏移研究——以L波段滤波器为例

Research on frequency offset of HTS filter taking L-band filter as demo

基于阶跃阻抗谐振器(SIR)设计了一款工作于L波段的十四阶高温超导带通滤波器,借助交叉耦合结构在通带两侧引入两对传输零点以提高带边陡度,使得滤波器的60 dB/3 dB矩形度优于1.25。滤波器制备在双面涂覆YBCO薄膜、厚度为0.500 mm的MgO基片上。70 K温度下的测试结果表明:其带外抑制优于80 dB,插入损耗优于0.31 dB,反射损耗优于14.8 dB,通带范围为1.235~1.271 GHz。与设计指标1.250~1.286 GHz相比,通带范围往低频端整体偏移15 MHz。梳理滤波器的研制流程,经分析后认为频带偏移主要源于光刻、刻蚀中线条变窄效应和超导薄膜在超导状态下的动态电感效应。

Based on step impedance resonators(SIRs),a 14-pole high-temperature superconducting(HTS)bandpass fil⁃ter operating in L-band was designed.Two pairs of transmission zeros were supplemented on both sides of the passband of the filter by means of two cross-coupling structures to improve the band-edge steepness of the filter.These employed means ensure the 60 dB/3 dB rectangularity of the filter is smaller than 1.25.The designed filter was fabricated on a MgO substrate coated with YBCO films on its both sides and with a thickness of 0.500 mm.The measured features of the filter at 70 K indicate that the out-of-band rejection is larger than 80 dB,the insertion loss is smaller than 0.31 dB,the reflection loss is larger than 14.8 dB,and the passband is 1.235 to 1.271 GHz.Compared with the designed passband of 1.250 to 1.286 GHz,the measured passband moves 15 MHz to the lower frequency.After analyzing the fabrication process of the filter,the frequency offset of the fabricated filter is thought to originate from the narrowed microstrip lines in the fabrication process of both lithography and etching and the kinetic inductance effects of the superconducting film in the superconducting state.