I assume that you are posting in the analog forum you want an analog multiplier. Do you want to multiply two analog signals or produce a higher harmonic of a sine wave. For the first there is the original Gilbert multiplier from his 1968 paper. He took the four quadrant switching mixer which dates back to valve/tube days and put predistorters on one set of inputs.
If you want to get a higher frequency, use a nonlinear biasing of a transistor. Class C works very well for harmonic generation. If you want to just double, a push pull diode circuit very much like a full wave transformer mains rectifier circuit works well.