Transistor RC Oscillator with Single-Element Tuning

RC oscillators based on two active phase shifters of the all-pass type prove to be tunable over a 10-to-1 frequency range by varying a single resistance. Harmonic distortion can be less than 1 percent over this range. Analysis is made of the variation of Q with tuning and the dependence of phase shift on detailed circuit parameters. Oscillators of this type can be used in telemetry to generate a subcarrier frequency which is a measure of transducer resistance. With temperature compensation, oscillator frequency stability is sufficient to permit the measurement of temperature to ±0.1°C over the range -5°C to 40°C using a thermistor sensor.