On a Class of Capacitively Tuned Transducer Oscillators

A class of transducer oscillators is characterized by a tuning method that exchanges short-term for long-term stability, and accepts both single and differential tuning sensor capacitances. When the tuning method is applied to a restricted class of RC oscillators, the relation between tuning capacitance and oscillator period can be made linear. The effect of amplifier phase shift and distortion is analyzed and a design procedure given. In one application, an oscillator used with an air-pressure sensor is tuned from 25 to 50 ms (corresponding to 40 to 20 Hz) with a differential tuning capacitance varying from -2 to + 7 pF. The short-term stability is 80 ppm when the average of 100 periods is measured. The deviation from linearity is less than 1.5 percent of the total range.