Ultra-compact microwave filters using kinetic inductance microstrip

This paper reports on multi-pole microwave filters designed and fabricated using microstrip transmission line sections which consist of two very thin films of sputtered niobium nitride (NbN) separated by another very thin film of sputtered Si. Since the thicknesses of all three films are much less than the superconducting penetration depth, the kinetic inductance is 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 circuits can be greatly reduced. Prototype filters consisting of four open circuit half-wavelength microstrip stubs separated by full-wavelength microstrip sections have been measured. The circuits are connected to 34 mil diameter coaxial cable via an intermediate coplanar waveguide section. Passbands of 4 GHz separated by 3 GHz reject bands have been measured in a structure which occupies 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