The first reports on experimental investigation of nonlinearities in BaxSr1-xTiO3 based ceramics date back to 1950s and early 1960s. DiDomenico et al report the first successful demonstration of a 3rd harmonic generator in 1962. Recently 2nd harmonic generation was demonstrated ; by applying DC bias. Samoilova et al. reported frequency conversion (harmonic generation) experiment in a Coplanar Waveguide (CPW) with a ferroelectric film. Where as Booth et al  uses a CPW with Ba0.3Sr0.7TiO3 film; the non-linear capacitance C by using two approaches: i) third harmonic generation; and ii) C(VDC) measurements at microwave frequencies. It was shown that the non-linear capacitance C determined from C(VDC) measurements is in excellent agreement with the one found from third harmonic generation experiment; this leads to the conclusion that ferroelectric devices might be tuned with nanosecond speeds.
To study nonlinearities Kozyrev et al designed a resonator loaded by a ferroelectric capacitor; using both a single signal as well as intermediation distortion; which confirmed previous results. Kozyrev et al also gives a simple theory for estimation of overheating of the ferroelectrics films at high power levels; and it was shown that the power handling capability of a ferroelectric tuneable filter may be as high as 3.5 W. Experiments and more detailed analysis of the microwave heating on nonlinearities are given elsewhere.
In this work; we used simple; on-chip microprobe reflection measurements to determine microwave power dependence of the capacitance of parallel-plate ferroelectric varactors. It is shown that the nonlinear parameters estimated from this experiment are in good agreement with the nonlinear parameters found from C(VDC) measurements at low frequencies. Hence; nonlinear parameters found from simple C(VDC) measurements may be used in the development of microwave devices; such as harmonic generators and mixers.