NCS - 11a - What is describing function

Describing function is a quasi linear approximation to a nonlinearity. This is obtained by a ration of the fundamental component of the of the output to nonlinearity (for sinusoidal input) and the sinusoidal input to the nonlinearity. The fundamental component of the periodic output of the nonlinearity can be easily obtained by Fourier series expansion. It comes out that for odd nonlinearities, the describing function is function of only amplitude of the input and is independent of its frequency.