Microwave signal-processing applications of HTS films

Abstract

The low surface resistance (Rs) of high‐temperature superconductors (HTS) will lead to the development of passive microwave devices for application in radar, electronic warfare, and satellite systems with performance significantly better than achieved with normal conductors. In particular, delay line based devices such as phase shifters, convolvers, and correlators will have low lossses and multi‐GHz bandwidths. Low‐loss filters which presently occupy cubic feet in waveguide will be fabricated in compact microstrip or stripline, and ultra‐high Q resonators which currently require liquid helium refrigeration will be operated at around 77 K. Measurement of Rs of HTS is important both for device design and for optimization of the film growth process. Several approaches have been developed which provide data over a wide range of frequency and temperature, including stripline, cacity, and dielectric resonator techniques. HTS films for microwave applications should have at least Rs(HTS(<Rs(Cu)/10, at the same temperature and frequency, on both the top and bottom film surfaces. In order to achieve this the film should be epitaxial with their c‐axis normal to the film. YBCO films with c‐axis normal produced by laser ablation have the lowest reported surface resistance of 8 mΩ at 86.7 GHz and 77 K, which is as good as Nb at the same reduced temperature.