In an effort to keep costs low, the vLabtool only has one high resolution DDS sine wave generator incorporated onto the base board. However, certain electronics experiments(such as Amplitude modulation) require two waveforms.
Therefore, a fixed frequency quadrature oscillator was added at the cost of an op-amp and a few passive components. The output waveform is bipolar, with a fixed amplitude.
This fairly common circuit generates a sine wave and a cosine at the same frequency. D4,D5 are 4v7 zener diodes.