A Low Distortion Audio-Frequency Oscillator

In the first portion of this paper, which deals with the theory of negative resistance oscillators, it is shown that for small harmonic content the form of the negative resistance characteristic in the vicinity of the operating point should be such that the average negative resistance increases with amplitude of oscillation. It is then shown that Turner´s Kallirotron circuit has negative resistance characteristics of the desired form. The final part of the paper describes an audio-frequency oscillator based on Turner´s circuit. The second and third harmonic content of the output may be kept below 0.2 per cent by the use of low resistance coils and diode automatic amplitude control. Higher harmonics are negligible. At a thousand cycles per second the frequency drift relative to a tuning fork oscillator does not exceed 0.04 cycle. The frequency change caused by a 22½-volt change in plate supply voltage is 0.04 cycle.