CHAPTER ELEVEN
Superconducting Filters
Since the discovery of high-temperature superconductors (HTS) in 1986 [1], high-temperature superconductivity has been at the forefront of advanced filter technology and has been changing the way we design communication systems, electronic systems, medical instrumentation, and military microwave systems [2,–7]. Superconducting filters play an important role in many applications, especially those for mobile communication systems [8–28], satellite communications [29–40], and radio astronomy and radars [41–53]. Most superconducting filters are simply microstrip structures using HTS thin films [54–104]. Some typical superconducting filters are described in this chapter. In addition, this chapter will cover some important properties of superconductors and substrates for growing HTS films [105–110], which are essential for the design of HTS microstrip filters.
11.1 HIGH-TEMPERATURE SUPERCONDUCTING (HTS) MATERIALS
11.1.1 Typical HTS Materials
Superconductors are materials that exhibit a zero intrinsic resistance to direct current (dc) flow when cooled below a certain temperature. The temperature at which the intrinsic resistance undergoes an abrupt change is referred to as the critical temperature or transition temperature, denoted by Tc. For alternating current (ac) flow, the resistance does not go to zero below Tc, but increases with increasing frequency. However, at typical RF/microwave frequencies (in the cellular band, for example), the resistance ...
