Behavior of Lossy Spiral Inductors and Their Applications to the Design of Tunable Band Reject Filters

In this paper, we use a unique behavior of spiral inductors, designed intentionally to have a large parasitic capacitance, in the realization of a tunable band reject filter. It is shown that the conductivity of the metal strips forming the inductor have a significant impact on how the spiral inductor behaves as an inductor or a capacitor, regardless of the operating frequency. The concept is used to demonstrate a band reject filter made out of niobium operating at 4K, whose conductivity can be changed with the application of a dc current. The same concept can be employed using other materials whose conductivity can be controlled either electrically, thermally or optically.