Posted on Nov 27, 2012

The circuit will provide both a sine-and square-wave output for frequencies from below 20 Hz to above 20 kHz. The frequency of oscillation is easily tuned by varying a single resistor. This is a considerable advantage over Wien-Bridge circuits where two elements must be tuned simultaneously to change frequency. Also, the output amplitude is relatively stable when the frequency is changed. An amp is used as a tuned circuit, driven by square wave from a voltage comparator.

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The frequency is controlled by Rl, R2, Cl, C2, and R3, with R3 used for tuning. Tuning the filter does not affect its gain or bandwidth, so the output amplitude does not change with frequency. A comparator is fed with the sine-wave output to obtain a square wave. The square wave is then fed back to the input of the tuned circuit to cause oscillation. Zener diode, Dl, stabilizes the amplitude of the square wave fed back to the filter input. Starting is insured by R6 and C5 which provide de negative feedback around the comparator. This keeps the comparator in the active region. Distortion ranges between 0.75% and 2% depending on the setting of R3. Although greater tuning range can be accomplished by increasing the size of R3 beyond 1 K!l, distortion becomes excessive. Decreasing R3 lower than 50ohm can make the filter oscillate by itself.

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