MixingWhen two waves having different frequencies are combined in a nonlinear circuit, newfrequencies are produced. These new waves are at the sum and difference frequenciesof the original waves. You’ve probably noticed this mixing, also called heterodyning, ifyou’ve ever heard two loud, sine wave tones at the same time.Suppose there are two signals with frequencies fl and f2. For mathematical conve-nience, assign f2 to the wave with the higher frequency. If these signals are combined ina nonlinear circuit, new waves will result. One of them will have a frequency f2− fl, andthe other will be at f2 + f1. These are known as beat frequencies. The signals are calledmixing products (Fig. 20-4).Mixing37320-4Spectral (frequency-domain) illustration of mixing.Frequency designators are discussed in the text.Figure 20-4 is a frequency domain graph. Amplitude (on the vertical scale) isshown as a function of frequency (on the horizontal scale). This kind of display is whatengineers see when they look at the screen of a spectrum analyzer. Most of the graphsyou’ve seen so far have been time domain graphs, in which things are shown as a func-tion of time. The screen of an oscilloscope normally shows things in the time domain.How do you get the nonlinearity necessary to obtain a mixer circuit? There are var-ious different schemes, but one common way is—you guessed it—to use diodes. Mixercircuits are discussed in chapter 27.