A model of a device is generated. An input port of the device is stimulated with a large amplitude signal having a central frequency. A first port of the device is perturbed with a small amplitude signal tone. The small amplitude signal tone is at a frequency offset slightly from a harmonic of the central frequency. Spectral component frequencies of a resulting signal from the device are obtained to determine model coefficients for the device. At least some of the spectral component frequencies occur at frequencies offset slightly from harmonics of the central frequency.