The main oscillator is made fromtwo opamps. One integrator and a komparator.

The integrator output rises until the threshold on the komparator is reached -> komparator changes output polarity -> integrator output sinks.

An Opamp in the middle is needed as a inverter since the output of the integrator is also inverted.

To set the frequency we can either change the gain of the integrator, the feedback of the komparator, or most important for later -> the input voltage levels of the integrator.

Blue: Squarewave output

Green: Triangle signal

Red: Komparator, positive input