So we put a sinewave into both inputs of a four quadrant multiplier and out the other side comes another sinewave at twice the frequency, and offset so that it swings from zero to X volts, not equally either side of zero like the input waveform. Then we put the output through a cap to centre the waveform on zero, and (maybe) through a hi Q tuned circuit to reduce any slight amount of distortion. Then we put this double frequency sinewave through another multiplier so it is now 4X the original frequency, and so on and so on... What interesting property is there about a sinewave that allows us to do this?