Abstract: A system and method are provided for determining an impedance of a physical system based on a ratio of a perturbation to the physical system and a response of the physical system. The determining of the impedance comprises converting a response signal into the frequency domain to identify a magnitude and phase of a first harmonic of the response signal and a magnitude and phase of a third harmonic of the response signal, expressing the magnitude and phase of the first harmonic as a first complex term and expressing the magnitude and phase of the third harmonic as a third complex term, and in determining the impedance, using as the response a sum of at least the first complex term and three times the third complex term. Thereby, a correction may be applied to the impedance measurement to improve accuracy and thereby to allow larger perturbation amplitudes to be used.