Abstract: The analysis of the nonlinear signals or systems is insufficient using the power spectrum and autocorrelation function. We do not have complete information about such signals or systems, because of the loss of the phase information in the power spectrum and autocorrelation function. The analysis of the nonlinear phenomena is more effective using higher order statistics (HOS). In this paper, the basic definitions and properties of HOS are discussed. It is defined the quadratic phase coupling phenomenon (ǪPC) and numerical results aimed at analysing phase coupling depending on the frequency and amplitude changes are presented.