RF mixer

A mixer is the gearbox of a radio: it shifts a signal from one frequency to another without changing the information it carries. You feed it your signal (RF) and a clean reference tone from a local oscillator (LO); the mixer multiplies them and produces new tones at the sum and difference of the two frequencies. Picking off the difference moves a hard-to-handle 2.4 GHz signal down to, say, a comfortable 100 MHz intermediate frequency where filters and amplifiers are cheap and precise.

Mixing works because multiplying two sine waves mathematically creates sum and difference frequencies — a deliberate, useful non-linearity, unlike the accidental distortion engineers fight elsewhere. This 'heterodyne' trick, the heart of nearly every radio since the 1920s, lets a receiver tune across a huge band while its sharp filtering stays at one fixed frequency. The price is the 'image frequency,' an unwanted second band that also folds down onto the IF and must be filtered out before mixing. A mixer's quality is judged by its conversion loss or gain, the isolation between its ports, and its linearity (IP3).