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A typical power-line filter is composed, essentially, by common-mode chokes, X-class and Y-class capacitors. A good characterization of these components is needed to develop a technique to design the optimal power-line filter for an electric or electronic device by finding their best values. In this paper, a new methodology to characterize the behavior of power-line filter capacitors is presented. This methodology is based on a model where common-mode and differential-mode interference are separated into different ports in order to facilitate the study of the propagation phenomena. The methodology is used to explain modal conversion inside impedance networks with X-class and Y-class capacitors, to predict the common-mode and differential-mode emissions when these capacitors are connected to electric or electronic devices, and to improve the classical methodology of power-line filter implementation finding the optimal capacitances of the impedance networks. This new methodology has been successfully tested by using real measurements from capacitors and electric devices.