When current passes through a diode, half of the cycle is cut off, as shown in above figure B. This occurs no matter what the frequency, from 60-Hz utility current through RF, as long as the diode capacitance is not too great.
The output wave from the diode looks much different than the input wave. This condition is known as nonlinearity. Whenever there is nonlinearity of any kind in a circuit—that is, whenever the output waveform is shaped differently from the input waveform—there are harmonics in the output. These are waves at integer multiples of the input frequency.
A frequency-multiplier circuit using a semiconductor diode.
Often, nonlinearity is undesirable. Then engineers strive to make the circuit linear, so the output waveform has exactly the same shape as the input waveform. But sometimes harmonics are desired. Then nonlinearity is introduced deliberately to produce frequency multiplication. Diodes are ideal for this purpose. A simple frequency-multiplier circuit is shown in above figure. The output LC circuit is tuned to the desired nth harmonic frequency, nfoo.
For a diode to work as a frequency multiplier, it must be of a type that would also work well as a detector at the same frequencies. This means that the component should act like a rectifier, but not like a capacitor.