Describing function is the term used on non-linear transfer elements for the frequency response while only taking into account the fundamental oscillation of the , recorded or calculated for various constant amplitudes of the sinusoidal input signal.
During the determination of the frequency response of non-linear transfer elements, the curve shape and amplitude of the fundamental oscillation of the mostly anharmonic oscillations of the output variable are also non-linearly dependent on the amplitude of the input oscillation. The determination of the fundamental oscillation of the output oscillation based on Fourier analysis or a suitable measuring method represents a certain linearisation of the non-linear element (see DIN 19 226) (VDI-VDE 2600).