Superconducting Linear Microwave Circuits

Applications of superconductivity in microwave technique make possible to develop a) high quality resonators, filters delay lines with small losses and controlled dispersion, b) modulators, switches on the basis of S-N transition (the transition from superconducting to normal state of material under the influence of transport current, laser light etc., c) nonlinear elements (detectors, frequency convertors) on the basis of Josephson or S-I-S junctions. The paragraphs a) and b) will be discussed in this paper. The electric current in superconductor is formed by electrons being in two quantum states: superconducting and normal. As dc-current is connected with the movement only the superconducting electrons, the ac-current is a result of travelling of electrons in both quantum states. Therefore some dissipation of microwave electromagnetic field energy takes place in superconductors allways. The rate of this dissipation is strictly connected with the purity, structure and surface quality of the superconducting samples. The requirement on quality of superconducting materials is sufficiently strong espessialy in the case of microwave applications. This problem turns to be of crucial importance with respect to the new high-Tc superconductors.