Amplitude fluctuations in a self-oscillating vibroscope system

All systems for maintaining sustained oscillations exhibit, to some degree, variations of amplitude with time, especially after switching on or when subjected to any other disturbance. For automatic vibroscopes which are thermistor-stabilised, the combination of a high Q factor and comparatively low oscillating frequency results in poor damping of the disturbance. This is particularly unsatisfactory since such vibroscopes may be required to provide fast initial rates of sampling, and may have to run unattended for long periods of time. An analysis is made of the system in sufficiently general terms to be applied to a wide range of oscillators stabilised by thermistors for long-term amplitude control. A simple and novel differential operator is introduced. This permits the slowly varying amplitude to be treated independently of the sinusoidal component. The perturbations in amplitude may be approximated by exponentially varying sinusoids. It is shown that `positive¿ third-harmonic distortion in the maintaining amplifier is an important factor contributing to good damping. The use of a suitably chosen voltage-dependent resistor to introduce a calculated amount of positive third-harmonic distortion is shown experimentally to improve damping substantially with negligible effect on the frequency of the sinusoidal component. This technique can give improved damping of the envelope in other systems where the frequency-determining element has a high Q factor and where the long-term stabilisation of amplitude is provided by a temperature-dependent resistor.