1. A method of estimating a figure of merit for a nonlinear device under test, comprising: developing from a first sub-sampling of input signals and corresponding output signals for said device a nonlinear behavioral model fitted mathematical function for said device containing terms depending on linear and nonlinear combinations of said input signals and linearly on a set of parameter coefficients; deriving from said nonlinear behavioral model fitted mathematical function a figure of merit having terms depending on linear and nonlinear combinations of said input signals and linearly on a set of parameter coefficients; before said nonlinear behavioral model is applied, pre-computing in advance for a substantially fixed value of said input signal all terms in said nonlinear model that depend on nonlinear combinations of said input signal; and evaluating said parameter coefficients, such that said nonlinear behavioral model is optimized.