Variable frequency audio generator


This circuit was created to be used to test amplification stages. It produces both square and triangular wave outputs that rise from approx 80 a 730 Hertz in a repeated cycle of approx 6 seconds. Appropriate substitutions of the component value will produce variations in these parameters.

Operation is as follows: The U1A op-amp and its associated components form an astable multivibrator with a period of approximately 6 seconds. The timing capacitor C1 is charged abruptly through the diode D1 and the resistance from R4 and discharged slowly through the resistance R5. The voltage that changes exponentially at the negative input of U1A is buffered by U1B, which also acts as a level variator. The release of U1B (pin 7) it is a sawtooth voltage waveform that slowly drops and is used to modulate the frequency of the remaining circuit, which is a voltage controlled audio oscillator.

The operation of the voltage controlled oscillator is as follows: The U2C operational amplifier is configured as an integrator, with its input piloted by resistance R13. The exit of the integrator (pin 8) it supplies a hysteresis comparator composed of U2D and two associated resistors. The comparator output switches the voltage at the other end of the resistor R13, providing positive feedback for the integrator-comparator pair, causing oscillations. The control of the voltage of the oscillator frequency is obtained by symmetrically blocking the comparator output on the reference voltage of the oscillator (established by U1C). Locking is accomplished with two precision rectifiers; U2B and its diode for the positive limit and U1D and its diode for the negative limit. The resistor R16 from 3900 Ohm allows you to block the voltage without disturbing the operation of the comparator. The tightening voltages are determined by the sawtooth voltage that slowly drops at the output of U1B, with the operational amplifier U2A which acts as an inverter to derive the positive limit value. As the lockout value changes, the integration current in the adder node of U2C (pin 9) changes accordingly, thus modulating the oscillation frequency.

The circuit does not require strict voltage regulation. However, the adjustment can be used to maintain constant output voltage levels. The nominal output levels, for a diet of 12 volt, I'm 5 peak-to-peak volts for the triangle e 9 peak-to-peak volt for the square. Note that the triangular wave output is not too precise; shows transients and short discontinuities at the peaks due to the departure of U2C from an ideal integrator. However, it is suitable for many applications where the purity of the waveform is not a rigorous consideration.

To change the scan speed of the oscillator, vary the resistance R5 in the feedback path of U1A. Diode D1 in the feedback path can be disconnected for a triangular sweep modulation. To change the frequency offset, vary the capacitor C5 in the feedback path of U2C. Decreasing the value of this capacitor will proportionally increase the frequency of the oscillator. Note that the approximate sweep ratio 9: 1 has been optimized for maximum practical value, considering the precision and dynamic range of the operational amplifiers used.

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