Abstract-This paper gives closed-form equations for the intermodulation and third harmonic signals generated in a nonlinear transmission line with distributed quadratic nonlinearities in the conductor and dielectric. Although the formulation developed is general, it is intended to be used in planar devices combining hightemperature superconductor (HTS) and oxide ferroelectrics. The analysis in this paper shows that the intermodulation and third harmonic signals produced by an HTS tend to cancel those of a ferroelectric, and that full cancellation is theoretically possible. This opens the way for using HTS/ferroelectric multilayers, not (only) for their tunable or phase-shifting properties, but for highly linear spurious-free planar HTS devices.
-This work evaluates the microwave nonlinear properties of ferroelectric BaSrTiO thin films by measuring the frequency response of several coplanar transmission lines and interdigital capacitor structures as a function of the applied electric field from 150 Hz to 40 GHz. From these measurements, we obtain the distributed nonlinear capacitance C(VdC) as a function of dc bias. We also measure the harmonic generation at microwave frequencies in ferroelectric transmission lines, and use an accurate circuit model to obtain C(Vrf), the nonlinear capacitance as a function of rf bias. Information about the tuning speed of the film is obtained from a comparison between the two nonlinear capacitances. Characterization of this mechanism is also required to assess the spurious signal generation in ferroelectric-based devices.
The microwave properties of Ca-doped YBaCuO ͑YBCO͒ thin films ͑350 nmഛ t ഛ 400 nm͒ have been investigated, using 30% Ca-substituted YBCO ͑Y 0.7 Ca 0.3 Ba 2 Cu 3 O 7−␦ ͒ surface layers, as such layers have been shown to enhance the critical-current density of films with high-angle grain boundaries for dc and lowfrequency currents. This study is an extension to the microwave range, with the aim of obtaining conditions that improve the transport properties of high-temperature superconductors. The effect of the substitution was observed to be dependent on the thickness of the overlayer. A small but significant improvement in the nonlinear part of the surface impedance ͑Z s ͒ and intermodulation distortion ͑IMD͒ was observed for a sample with a 20-nm-thick Y 0.7 Ca 0.3 Ba 2 Cu 3 O 7−␦ surface layer. These results indicate that some potential exists to improve the nonlinear microwave properties by Ca substitution. On the other hand, samples with a 40-nm-thick Y 0.7 Ca 0.3 Ba 2 Cu 3 O 7−␦ surface layer exhibit a degradation of their microwave properties. The possible causes of improvement and degradation are presented in view of the deposition procedure and the oxygen content of the films.
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