Series Resonant Converters allow an efficient, galvanically isolated, and load-independent DC/DC conversion, and can be used as isolation DC/DC units in two-stage single-phase AC/DC converters. However, in such systems, the single-phase Active Front End unit injects an undesired second-order harmonic component into the DC side, which requires proper filtering. Unlike other resonant converter topologies, the DC voltage gain characteristics of an SRC is unaffected by switching frequency variations, and cannot be used for harmonic filtering purposes. However, another active filtering mechanism can be employed for a frequency-controlled SRC, which is not based on its static characteristic, but on its dynamical behavior. This unexplored active filtering mechanism is analyzed in this work. In the following analysis, a new equivalent model of a frequency-modulated SRC is first derived, based on averaging its discontinuous conduction mode operation in case of varying switching frequency. Then, based on this average model, the active filtering mechanism based on the switching frequency modulation is explained and discussed, emphasizing its key properties and its differences with the conventional frequency modulation of LLC converters. The analyzed approach has been validated experimentally, demonstrating practical applicability.