Hi -

I came across this circuit the other day, attached below - and was interested in what it might do.
I tried mocking it up, partially, in Falstad:
http://tinyurl.com/y7wqfv7y

The results seem to be more erratic than I would've expected. It seems to claim to be some kind of Sinus Function generator, but I'm not sure what type of signal it would have to get, in order to do that.

Can someone help me understand what this circuit should be doing?

 

Without looking at it in detail, it would appear to be a triangle to sine converter using piece-wise approximation.

 

I'm not sure what type of signal it would have to get, in order to do that.

It requires a triangle wave with good symmetry and amplitude control.

 

an someone help me understand what this circuit should be doing?

That circuit is one form of what's called a "breakpoint shaper" circuit for sine wave approximation using a triangle wave as an input source. It works by effectively reducing the voltage gain of the circuit in steps as the instantaneous voltage of the triangle wave increases (in both the positive and negative directions), to mimic the curvature of a sine wave.

Another form of breakpoint shaper is shown in Fig. 12 on p. 11 of this Texas Instrument app note.