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This circuit has 3 stages: 1 - An op amp oscillator. In real life there would be no need for the voltage source, but since this simulator uses ideal op amps, we need to introduce a disturbance in the circuit to jump start the oscillation. If you start the sim nothing happens. But give a tiny push on the button and there it goes! 2 - An op amp buffer. This is just here to isolate stage 1 from the input impedance of stage 3, that would mess a bit with the oscillating frequency. 3 - A second order low-pass filter. It’s important to be a second order filter, a first order is not very good at dampening the higher harmonics. Now, stage 1 has two outputs: a square wave at the output of the op amp and a triangular wave at the feedback loop. There is a switch that allows you to change from one to another. You will see, as expe, that the sine wave is more perfect when using the tri wave as imput. A reference oscillator at the bottom can be used to check how close to a perfect sine wave we are getting.
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