The paper deals with the frequency modelling of EMC phenomena using equivalent circuit (with frequency perturbation sources for the modelling of the switching cells of the static converters) in the context of the sizing by optimization of EMC filters. The paper focusses on the way to obtained automatically a frequency model, including the exact computation of its jacobian, useful in the context of design by deterministic gradient optimization. It mainly focuses on selectivity (optimization contextualization) in the modelling with its interests compared to a generic modelling (without knowing with variables will be constrained in the optimization process). The approach is applied to the optimization of EMC filter for power electronics aircraft context.