Abstract
A system is described which excites continuous oscillations in a resonator at two frequencies simultaneously. The resonator, typically in the form of a disc, can have modes which are close together in frequency and a double mode oscillator facilitates measurement of the difference in frequency. Such a measurement can be used to accurately determine Poisson ratio for the material of the disc. The resonator is excited by means of a nonlinear feedback loop which is designed to give stable double mode oscillations with frequencies close to the natural modes of the resonator. Several results from describing function theory are used to predict the behaviour of the system and the analysis is compared with the results of an analogue computer simulation.
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