Ultra-compact microwave filters using kinetic inductance microstrip

Multipole microwave filters were designed and fabricated using microstrip transmission line sections that consisted of two very thin films of sputtered NbN separated by another very thin film of sputtered Si. Since the thicknesses of all three films were much less than the superconducting penetration depth, the kinetic inductance was significantly greater than the magnetic inductance. As a result, the phase velocity of a microstrip transmission line is much less than the free-space speed of light. Since resonant structures are reduced in size proportionately, the size and weight of microstrip circuit can be greatly reduced. Prototype filters consisting of four open-circuited half-wavelength microstrip stubs separated by full-wavelength microstrip sections were measured. The circuits were connected to 34-mil-diameter coaxial cable via an intermediate coplanar waveguide section. Passbands of 4 GHz, separated by 3 GHz reject bands, were measured in a structure which occupied less than 0.5 cm², including the coplanar waveguide transitions. Higher-order passbands, although possessing an increased insertion loss, maintain filter passband characteristics through 20.0 GHz