Application of High Temperature Superconductors in Passive Microwave Devices

Abstract

The front end of a microwave communications system is assembled from circuit elements comprising antennae, local oscillator, mixer and filters. Interconnection is by transmission line, into which delay may be incorporated. All of these components can, with advantage, be implemented in superconducting material. Superconductors, when operating below their cross-over frequency, have values of surface resistance lower than those for normal metals such as copper and silver. This results in lower insertion loss for microwave components, allowing the design of smaller, more compact, yet at the same time more complicated, devices. The non-dispersive nature of superconductors can also be an advantage in enabling very high bandwidth signal processing. High temperature superconductors (HTS) allow operating temperatures in the liquid nitrogen range, not too far below the ambient temperature in many communications satellites. The Oxford microwave programme has concentrated on the implementation of all of the above devices in thin-film HTS, mainly Tl-2212, on a variety of substrates. The film fabrication, and performance of delay lines, antenna-mixers, resonators, filters and a voltage-controlled oscillator, are described.