A data collection method is presented, by which identification of high order linear models is achieved. A closed loop experiment with a controller is performed. Frequency characteristics are used and parameters of the model, i.e. the critical operating mode values are obtained. The critical operating mode is realized by two state controller. A system of non-linear equations is compiled, according to the parameters of the model by using the analytical expressions of the amplitude-frequency and phase-frequency characteristics. A method for solving the system is proposed.