  We describe a method of analysis which allows for reconstructing the
nonlinear disturbance of a high Q harmonic oscillator. When the oscillator is
driven with two or more frequencies, the nonlinearity causes intermodulation of
the drives, resulting in a complicated spectral response. Analysis of this
spectrum allows one to approximate the nonlinearity. The method, which is
generally applicable to measurements based on resonant detection, increases the
information content of the measurement without requiring large detection
bandwidth, and optimally uses the enhanced sensitivity near resonance to
extract information and minimize error due to detector noise.

Mesoscale and Nanoscale Physics