The project I am currently working on is a Theremin and I have decided to incorporate additive synthesis into it. My immediate plan is to be able to select odd harmonics, say the first 4 of them, to be added in order to make the sine wave output into a square wave. I planned to base my circuit on the one contained in the following link:

http://www.techlib.com/files/rfdesign2.pdf

HOWEVER, I have realised that for an input sine wave of varying signal this circuit will not work? My question is am I correct in believing this and is there any way to make it work? Perhaps a heterodyne-esque method of bringing the input signal to a set frequency and then apply the multiplier?

Any help offered is greatly appreciated!