Oscillator Circuits

In many applications, the resultant changes in the capacitance of capacitive transducers can be measured with a suitable ac bridge such as Wein bridge or Schering bridge. However, in a majority of cases, improvised versions of bridges are used as oscillator circuits for capacitive signal processing. The transducer is configured as a part of the oscillatory circuit that causes changes in the frequency of the oscillations. This change in frequency is scaled to be a measure of the magnitude of the physical variable.

As part of the oscillator circuits, the capacitive transducer has excellent frequency response and can measure both static and dynamic phenomena. Its disadvantages include sensitivity to temperature variations and the possibility of erratic or distorted signals due to long lead lengths. Also, the receiving instrumentation may be large and complex, and it often includes a second fixed-frequency oscillator for heterodyning purposes. The difference frequency thus produced can be read by an appropriate output device such as an electronic counter.

References

1.J. P. Bentley, Principles of Measurement Systems, 2nd ed., United Kingdom: Longman Scientific and Technical, 1988.

2.E. O. Doebelin, Measurement Systems: Application and Design, 4th ed., New York: McGraw-Hill, 1990.

3.J. P. Holman, Experimental Methods for Engineers, 5th ed., New York: McGraw-Hill, 1989.

4.F. T. Noth and G. C. M. Meijer, A Low-Cost, Smart Capacitive Position Sensor, IEEE Trans. Instrum. Meas., 41, 1041-1044, 1992.

Appendix to Section 6.3

List of Manufacturers

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