  We have developed a system for contactless measurement of nonlinear
conductivity in the radio-frequency band, and over a wide temperature range. A
non-resonant circuit is used to electrically excite the sample, and the induced
signal is detected by a resonant circuit whose natural frequency matches higher
harmonics of the excitation. A simple modification of the probe allows
non-resonant detection suitable for stronger signals. Two measurement
procedures are proposed that allow significant excitation power variation, up
to 150 W. The apparatus has been validated trough the measurement of the
nonlinear response at the superconducting transition of a high-Tc
superconductor, and the nematic transition of an iron pnictide.
