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The filter development process, to reduce conducted emissions, is still a nonoptimized problem within the electromagnetic interference improvement of systems. Because the filter attenuation depends on the impedance values of the device under test, numerous attempts might be necessary to find an appropriate filter element. The presented method tries to characterize arbitrary active applications within the time domain for simulation and measurement setups. The evaluated noise sources are described as Y or Z matrices with additional active sources with respect to Thevenin's law. Using a transition from the time into the frequency domain, any circuitry with periodic and stable noise sources can be considered. The application of the method is demonstrated for an automotive buck converter in the frequency range up to 108 MHz. The characterization of the generated noise source model is verified by comparing calculated and measured noise voltages with different filter elements.