Mixer: Blending sound vs. making new frequencies

To most people, a mixer is a kitchen appliance or an audio device that combines multiple sound sources into one output. A sound engineer uses a mixer to balance vocal, instrument and backing track levels before sending them to speakers or a recording device. The user adjusts faders and dials to control volume and tone of each input channel.

In industrial electronics, a mixer is a nonlinear circuit that takes two input signals and produces output signals at new frequencies that did not exist in either input. If one signal oscillates at 10 megahertz and another at 12 megahertz, the mixer generates output at 2 megahertz (the difference) and 22 megahertz (the sum), plus the original frequencies. This frequency conversion is fundamental to radio receivers, telecommunications equipment and radar systems.

The audio mixer and the electronic mixer share a name because both combine inputs. But the audio mixer preserves the original signals, merely adjusting their relative loudness. The electronic mixer actually transforms the signals. When a radio receiver mixes an incoming broadcast signal with a local oscillator signal, it shifts the broadcast down to an intermediate frequency that the receiver can filter and amplify efficiently. This heterodyne process, named for the Greek words for "different" and "power," became essential to radio technology in the early 20th century.

The circuit accomplishes this through a nonlinear element, typically a diode or transistor, where output is not simply a scaled copy of the input. In a linear amplifier, doubling the input doubles the output. In a mixer, the nonlinearity creates harmonics and intermodulation products, and the designer selects which of those new frequencies to pass through and which to filter out. The same nonlinear principle that was a problem in early radio became the basis for frequency conversion.

Modern mixers appear in software-defined radios, signal analyzers and integrated circuits where dozens of mixer stages may be stacked to shift signals across multiple frequency bands. The term has remained distinct from audio mixing, even as both appear in the same piece of equipment: a modern audio interface may contain both a conventional audio mixer for blending channels and, hidden in its RF frontend, an electronic mixer that converts ultrasonic or radio signals into audible range.