> There are many different Theremins nowadays... Some of them might
> actually produce sine wave. However, the musically more useful
> (including the original Theremin) instruments don't. The reason for
> this is explained here:
>
> http://www.paia.com/ProdArticles/thereton.htm
> "As coupling increases, the two [high frequency] oscillators try to
> pull one another to a common frequency resulting in distortion that
> produces an output rich in harmonics at integral multiples of the
> difference frequency. The waveform may be saw-like or pulse-like
> depending on phase of harmonics..."

A very interesting article with a lot of information I did not know.

Best wishes
Dieter Doepfer