simple way to add harmonics to a fundamental signal

[Martin Czech]

> Hi,
>    I'm currently designing a VCO, and I want to add 3 or 4 harmonics to the fundamental signal. Right now the only way that I've found is to add one oscillator for
> every harmonics, but since my oscillator will be complex (square,triangle,sin,saw,etc)
> the circuit will be very complex and painful to make (it will be a 8 notes polyphonic synth). I thought that there may be a simple way to do this, for example sending the fundamental signal to a part of the circuit that would add the harmonics. Anyway I have no clue of what I should look for. Anyone have some suggestions or schematics ?
> 

2x, 4x frequency may be obtained by full wave rectifying (proper dc offset
after each stage).  Input a tri wave, or sine. The result will still
have some fundamental left, but in a mix with the fundamental, this is ok.

3x is harder to obtain. You could try to feed a sine into a Tchebycheff
polynome function.  3x would require a 3rd-order polynome.

The question is how to get the x**3, x**2 functions needed.
Multipliers are expensive, that's why I thought about a nonlinear diode network.

m.c.