Audio Oscillator

A low distortion audio frequency sine wave can be easily generated by passing the output of a simple square wave oscillator through a sharp cut off low pass filter to attenuate the odd harmonic components. The output level of the sine wave is precisely defined by the rail voltage and the gain or loss in the filter. A problem is that most filters h
Audio Oscillator - schematic

ave a fixed cut off frequency hence such a sine wave source is restricted to a small frequency range. There is, however, one type of integrated circuit package containing a switched capacitor fitter in which the cut off frequency can be controlled by the frequency of a clock running at a multiple of the cut off frequency. The circuit described in this article makes use of a switched capacitor low pass filter type MF6-50 (a sixth order Butterworth) which operates with a clock frequency 50 times its cut off frequency. By controlling the frequency of the clock, the cut off frequency can be set to a range of values extending to above 20 kHz. Using this filter, the circuit forms a variable frequency sine wave oscillator which can be tuned, at constant output level, over a frequency range of 2 Hz to 20 kHz with harmonic components less than 0. 1 percent of the fundamental frequency amplitude, that is, more than 60 dB below that amplitude. As the sine wave is formed from a square wave, the square wave is also available as an alternative output. The basis of the system is shown in the block diagram, Figure 1. A clock (fck), tunable within the range of 112 Hz to 1. 12 MHz drives both the switched capacitor fitter and a divide by 56 counter which gives square wave output in the range of 2 Hz to 20 kHz. The counter output is fed to the input of the filter which has a cut off` frequency (fc) equal to fck divided by 50, that is, 12...

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