In this paper, a method combining the Proper Orthogonal Decomposition (POD) and a time-frequency analysis (TFA) is presented. The POD method is a tool adapted to the study of multi-degrees of freedom systems, which can give some insight of the behaviour of complex nonlinear systems, whereas TFA is suitable for scalar signal with multi-frequency components. The method presented here is of principal interest in the case of swept-sine excitation of complex multi-DOF systems. The method can be used to extract the Non-linear Normal Modes (NNM). It is illustrated on a 3-degrees-of-freedom system.