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A Low-Distortion Acoustic-Measurement Oscillator Using Semiconductor Junctions as Variable-Tuning Elements

This oscillator is of neither the function-generator nor the beat-frequency type. It is basically an RC oscillator, in which the R's are formed by transistor junctions, whose d.c. bias voltage is varied to change the frequency. The inherently exponential current/voltage relationship for such junctions results in an accurately logarithmic scale shape. The provision of accurate compensation for the effects on frequency of variations in ambient temperature is of vital importance, but a highly satisfactory result is achieved. The circuit lends itself to the provision of auto-sweep and warble facilities, and is currently in use by two leading British firms for production testing of loudspeakers and microphones repecctively. The waveform purity - in particular the absence of high-order harmonics - is an attractive feature for such purposes. The circuit inherently provides two equal-amplitude outputs in phase-quadrature, which lends itself to the operation of an economical type of tracking-filter system.

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