take the diode transfer function and show where the non-linear multiplication arises - as we did a few weeks back. You will need to recall power series for exponential function exp(x) from calculus and some trigonometry ideas as well.
The diode transfer relation to start is: I in diode = Is*[exp(k*V across diode) - 1]
Show an appropriate number of terms of the series, mix in some algebra, simplify and ID the multiplication.
Now, what happens when the voltage you drop on a diode is the sum of a low frequency and high frequency sine wave? Stir in some trig to show what frequencies should be in the spectrum.
Then, create a graph of the spectrum and its features you expect to see.