Audio Sweep Oscillator
This circuit was designed for a sound effect, but may serve other useful purposes when a basic, swept-frequency audio source is required. It produces both square and triangular wave outputs that ascend from about 80 to 730 Hertz in a repeating cycle of about 12 seconds. Appropriate component-value substitutions will yield variations in these parameters.
Operation is as follows: Operational amplifier U2B and its associated components form an astable multivibrator with a period of about 12 seconds. The 39uF timing capacitor is charged abruptly through the diode and 220 ohm resistor, and discharged slowly through the 1M ohm resistor. The exponentially-changing voltage at U2's negative input (pin 6) is buffered by U2C, which also serves as a level shifter. U2C's output (pin 8) is a slowly-descending sawtooth voltage waveform that is used to modulate the frequency of the remaining circuitry, which is a voltage-controlled audio oscillator.
The operation of the voltage controlled oscillator is as follows: Operational amplifier U1A is configured as an integrator, with its input driven from the 30K ohm resistor. The integrator's output (U1A pin 1) feeds a hysteretic comparitor comprised of U1B and two associated resistors. The output of the comparitor switches the voltage at the other end of the 30K resistor, providing positive feedback for the integrator-comparitor pair, causing oscillation. Voltage control of the oscillator's frequency is accomplished by symmetrically clamping the comparitor's output about the oscillator's reference voltage (established by the two 100K ohm resistors). Clamping is accomplished with two precision rectifiers; U1C and its diode for the positive limit, and U1D and its diode for the negative limit. The 3900 ohm resistor permits the voltage clamping to occur without disturbing the comparitor's operation. The clamping voltages are determined by the the slowly-descending sawtooth voltage at U2C's output, with operational amplifier U2A serving as an inverter to derive the positive limit value. As the clamping value changes, the integration current at U1A's summing node (pin 2) changes accordingly, thus modulating the oscillation frequency.
The circuit is largely ratiometric, and so does not require stringent voltage regulation. Regulation may be employed, however, to maintain constant output voltage levels. The nominal output levels, for a 9 volt supply, are 3.5 volts peak-to-peak for the triangle and 7 volts peak-to-peak for the square. Note that the the triangular wave output is not too precise; it exhibits transients and brief discontinuities at the peaks due to U1A's departure from an ideal integrator. However, it is suitable for many applications where wave form purity is not a strict consideration.
To change the oscillator's sweep rate, vary the 1M ohm resistor in U2B's feedback path. The diode in U2B's feedback path may be disconnected for a triangular sweep modulation, or the feedback network may be replaced with other combinations of resistances and diodes to alter the sweep contour as desired. To change the frequency offset, vary the 0.01uF capacitor in U1A's feedback path. Decreasing this capacitor's value will proportionally increase the oscillator frequency. Note that the approximate 9:1 sweep ratio has been optimized for the largest practical value, given the precision and dynamic range of the operational amplifiers.
DANGER: Be careful to observe the polarity of the electrolytic capacitors, especially the 10uF unit in parallel with the battery, which can become hot enough to cause burns, or explode, if reversed. Avoid reversing the battery.
WARNING: The output levels from this circuit are greater than most conventional sources. When connecting this circuit to audio amplifiers and loudspeakers, start with the volume control at minimum, and increase it slowly to a safe level. Always avoid exposure to loud sounds.
January 19, 2001
Source document dated July 14, 1996
Text and image ©2001 by Arthur Harrison
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