A novel closed-loop method of obtaining the frequency response of a physical system is presented. It is assumed that the physical system to be examined may be excited by a known sinusoidal input signal and that the resulting output response may be measured. The method described herein employs this input-output information together with a steepest descent computing procedure to obtain the in-phase and quadrature components of the system’s response. The method may be mechanized on either an analog or digital computer and may be applied to either linear or nonlinear systems. Examples illustrating the application of the procedure to a variety of linear and nonlinear systems are given.

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