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This paper presents an Iterative Feedback Tuning (IFT) method to guarantee the control system stability throughout the iterations. The theoretical framework is based on the closed-loop stability testing in the context of coprime factor uncertainty for the controller, and it makes use of the small gain theorem for discrete-time systems. Bounds on the gain of the systems involved in the stability analysis are found from nonparametric models in frequency domain. A digitally simulated case study concerning the angular position control of a servo system is included to validate the new stable IFT method.