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The Quadrature Oscillator

The quadrature oscillator is another kind of phase shift oscillator. A quadrature oscillator circuit. The difference is that the quadrature oscillator uses an op amp integrator to obtain a full 90° phase shift from a single RC segment, and still produce a usable output voltage.

Because of that 90° phase shift, we only need two op amps to produce both sine and cosine wave outputs, as shown in the circuit to the right. At the same time, however, we do still need three RC segments as shown. Frequency of operation will be ω = 2πf = 1/RC if all components are matched.

There are a number of variations on this circuit, but the basic operation is still the same. Amplitude control can be an issue, and too high an amplitude leads to distortion of the waveform. To limit the output amplitude, some circuits use a pair of back-to-back Zener diodes or some equivalent circuit to clip the signal fed back from the Cosine output to the Sine integrator. Because all RC segments are also low-pass filters, the distortion produced by the clipping action is significantly reduced, and the output signals are both good quality sine and cosine waves.

At the same time, if the loop gain is insufficient, oscillations will cease, or will not start when power is applied. To assure sufficient gain, resistor R1 is often made slightly smaller than R.

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