The present invention includes a method for generating a model of a circuit having an input port and an output port. The method determines an amplitude for current leaving the output port at a frequency ωk when a signal that includes a carrier at ωj modulated by a signal Vj(t) is input to the input port, wherein ωk is a harmonic of ωj. The determined amplitude is used to determine values for a set of constants, ak, such that a function fk(V,ak) provides an estimate of the current, Ik(t), leaving the output port at a frequency ωk when a signal having the form V ⁡ ( t ) = Re ⁢ ∑ k = 1 , H ⁢ V k ⁡ ( t ) ⁢ exp ⁡ ( jω k ⁢ t ) is input to the input port. Here Vk(t) is a component of a set of values V. The fk(V,ak) are used to provide a simulator component adapted for use in a circuit simulator.