Relationship between crossmodulation and intermodulation

When the frequency dependence of the 3rd-order distortions in a nonlinear system is taken into account, it is found that the well known fixed relationship between the (2q-p) inter-modulation product and the crossmodulation no longer exists. Owing to the same frequency dependence, the crossmodulation from n `unwanted¿ signals, e.g. in a c.a.t.v. system, is not simply n times the crossmodulation from a single unwanted signal, but the n components have to be added vectorially.