The heterodyne effect comes from multiplication of signals. But you really just need to pass their superposition through a nonlinearity, because the Taylor seri

The heterodyne effect comes from multiplication of signals. But you really just need to pass their superposition through a nonlinearity, because the Taylor series expansion will typically have an early term with the multiplication of the two signals' frequencies with a high coefficient. Of course, you'll have lot of artifacts from all the other terms of the expansion, but you can use a bandpass filter to drop all the unwanted terms. Or so I think I remember. It's been a while.