Oscillator Circuit for a Vibrating Capacitor Driven by an RF Electric Field

The vibrating capacitor to be discussed consists of a metalized membrane, clamped at its edges, between two electrodes. The membrane is driven by an RF electric field (309 kHz), which is amplitude modulated at the natural frequency (6 kHz) of the membrane. This RF field is caused by a voltage applied to one of the electrodes, which, together with the membrane, forms the driving capacitor. The other electrode, and the membrane, constitute the vibrating capacitor. A special RF oscillator has been designed, of which an essential part is the driving capacitor. This capacitor causes the RF voltage to be amplitude modulated at the natural frequency of the membrane. The oscillation conditions of this electromechanical system are derived from the equation of motion of the membrane and the electrical properties of the driving circuit. An oscillator circuit with optimum performance is designed with the aid of a computer. Finally, the computed results are verified by measurements.