A Low-Distortion Acoustic-Measurement Oscillator Using Semiconductor Junctions as Variable-Tuning Elements
The oscillator described is neither of the function-generator type nor of the beat-frequency type. It is basically an RC oscillator, in which the resistances are formed by transistor jnctions whose dc 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 autosweep and warble facilities, and is currently in use by two leading British firms for production testing of loudspeakers and microphones, respectively. 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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