Narrowban Lumped-Element Filters Using ap~~itiv~l~-LQad ed nductors

Coupling between microstrip resonators decreases very slowly as a function of the resonator separation. Therefore, it is difficult to realize narrowband filters (e.g. <0.1% fractional bandwidth) in reasonably sized microstrip form due to the very weak coupling values required. In this paper, we report a class of lumped-element filters that uses capacitively-loaded inductors to give frequency-dependent inductance values. A novel frequency- transformation technique is used in the design process. Using this approach, filter bandwidth is determined by the inductance slope of frequency-dependent inductors, E. Large coupling ca- pacitance, thus small coupling element separations, can still be used in narrowband microstrip filters to keep the filter layout compact. We present a S-pole, 0.27% bandwidth YBazCudh high-temperature superconducting thin film microstrip prototype filter at 900 MHz, which has 1.2 dB insertion loss and 20 dB return loss. It was designed with the coupling capacitors of a 1 % bandwidth filter, and then transformed to a 0.27 % fractional bandwidth using an appropriate inductance slope parameter, 2. Measurement showed good agreement with theory.