Frequency Tuning and Spurious Signal Generation at Microwave Frequencies in Ferroelectric ${\hbox{SrTiO}}_{3}$ Thin-Film Transmission Lines

This paper evaluates the RF tuning and the microwave nonlinear response of SrTiO
₃
thin films by measuring the third harmonics and third-order intermodulation (IMD) products of several coplanar waveguide transmission lines fabricated directly on SrTiO
₃
thin-film samples. From these measurements, we obtained the distributed nonlinear capacitance per unit length as a function of RF bias voltage C(V
_rf
) using an accurate equivalent circuit to model the spurious signal generation. A unique value of C(V
_rf
) is used to describe the third-harmonic generation at frequencies 3f
₁
and 3f
₂
, and the IMD products at frequencies 2f
₁
+f
₂
and 2f
₂
+f
₁
, where f
₁
and f
₂
represent the fundamental frequencies of the two-tone excitation signal. We compare C(Vrf) with the distributed nonlinear capacitance measured as a function of the dc-bias voltage C(V
_dc
) and obtain excellent agreement at 50 and 76 K. From these results, we conclude that full tunability can be achieved on nanosecond time scales, and that spurious signals generated in SrTiO
₃
ferroelectric transmission lines at microwave frequencies can be modeled based on dc-biased measurements