ABSTRACT
The applications of superconductors to analogue processing by passive devices are dependent on the very weak microwave power dissipation at the surfaces of these materials. Therefore, the quality factor of microwave resonators in bulk or surface geometry is much larger than that of equal-sized resonators made of normal conductors. Thus by using superconductors, cavities for accelerating particles exhibit a higher conversion efficiency and the transfer functions of planar filters are more attractive. However, the substitution of normal conductors in signal-processing components by superconductors does not allow full advantage to be made of the superconductivity. Components which do not operate suitably with normal conductors, can show excellent characteristics with superconductors. For instance, superconducting thin-film technology makes it feasible to produce planar filters with a narrow bandwidth (less than 5% in relative value). In this case the transmission loss is very low and the frequency response is quasi-rectangular shaped with a high rejection level. With normal conductors, only three-dimensional (3D) geometry is practical to get such a bandwidth without excessive loss and poor selectivity. Moreover, it is possible to fabricate low-loss planar waveguides with narrow superconducting strips. Therefore the superconductors advance the miniaturization of signal-processing devices and this advantage is more significant when the processing circuits are integrated to form subsystems.
