Nonlinear buffer layers relevant for reduced nonlinear effects in HTS microwave devices

Abstract

Microwave devices made of a High-Temperature Superconductor (HTS) exhibit a nonlinear response as the microwave power increases. The HTS nonlinearities generate a nonlinear inductance Ld(irf) and a nonlinear resistance Rd(irf) in a device. Ld(irf) and Rd(irf) are responsible for an increase of the device loss, a small frequency dispersion as well as the generation of spurious signals like Intermodulation Distortion (IMD). Nevertheless, the HTS nonlinearities in a microwave device can be reduced using a nonlinear dielectric like a ParaElectric Material (PEM). This assumption has recently been demonstrated theoretically. In a microwave device made of a HTS and a PEM, the nonlinear contribution to the capacitance Cd(vrf) from the PEM acts oppositely to the nonlinear contribution to Ld(irf). This may cancel the effect of the HTS inductive nonlinearities. The PEM also produces a nonlinear conductance Gd(vrf) in a device. All these nonlinear terms contribute to the IMD output power and the nonlinear quality factor (Q0) of a resonant passive microwave device. In this paper, the dependence of the different nonlinear contributions on frequency and applied dc bias voltage (Vdc) is investigated. The relevance to employ PEM in order to reduce the nonlinearities in HTS microwave devices is discussed.