Systems and methods for testing radio frequency FET switches at high RF voltages. Embodiments utilize an impedance transformer, or resonator, to step up the available voltage from an RF signal generator and amplifier to a device under test (DUT). The resonator reduces the RF power required to test at higher voltages, resulting in lower cost and other benefits. When a DUT begins to exhibit excessive non-linear distortion, resonance is lost, applied RF test signal power is reflected back as a reflected signal, and current to the DUT is starved by the resonator, protecting the DUT from destructive power levels. Measuring the amplitude of the reflected signal at the harmonic frequencies of the RF test signal allows detection of a harmonic knee point for selected reflected signal harmonics, and consequently allows determination of the power level of the RF test signal at which excessive non-linear distortion occurs.