Spectral (frequency domain) illustration of signal mixing.
When two waves having different frequencies are combined in a nonlinear circuit, new waves are produced at frequencies equal to the sum and difference of the frequencies of the input waves. Diodes can provide this nonlinearity.
Suppose there are two signals with frequencies f1 and f2. For mathematical convenience, let’s assign f2 to the wave with the higher frequency, and f1 to the wave with the lower frequency. If these signals are combined in a nonlinear circuit, new waves result. One of them has a frequency of f2 + f1, and the other has a frequency of f2 − f1. These sum and difference frequencies are known as beat frequencies. The signals themselves are called mixing products or heterodynes (above figure).
Above figure, incidentally, is an illustration of a frequency domain display. The amplitude (on the vertical scale or axis) is shown as a function of the frequency (on the horizontal scale or axis). This sort of display is what engineers see when they look at the screen of a lab instrument known as a spectrum analyzer. In contrast, an ordinary oscilloscope displays amplitude (on the vertical scale or axis) as a function of time (on the horizontal scale or axis). The oscilloscope provides a time domain display.