Frequency mixer

Lore & Background

The names detector, heterodyne, mixer, and converter have overlapped historically, and their meanings have varied with time and context. In early radio literature, devices that combined two signals to produce new frequencies were often described as heterodyne detectors or simply detectors, since an important early use was producing audible beat notes from continuous-wave signals. As superheterodyne receivers became common, the frequency-changing stage was often called a converter, especially when oscillator and mixing functions were combined in one vacuum tube or transistor. In modern usage, mixer is the general term for a circuit that translates frequencies by combining two input signals in a nonlinear manner.

Early mixers commonly worked by adding the two input signals and applying the combined waveform to a nonlinear device such as a crystal detector, diode, or vacuum tube. Because of the nonlinear transfer characteristic, the output contained sum and difference frequencies together with harmonics and intermodulation products. Early analyses distinguished between linear-law operation, where the device characteristic is approximately proportional, and square-law operation, where the output is proportional to the square of the input, producing strong second-order mixing products. The 1924 RCA Radiola superheterodynes used the Houck mixer, which employed the second harmonic of the local-oscillator for frequency conversion.

Mixers may be classified by topology as unbalanced, single balanced, or double balanced. An unbalanced mixer allows both input signals to pass through and appear as components in the output. A single balanced mixer suppresses one input signal at the output, typically the local-oscillator. A double balanced mixer suppresses both inputs from the output and passes primarily the sum and difference products. The diode-ring mixer is a common form of double balanced mixer and operates approximately as a switching multiplier when driven by a strong LO signal.

Reader's Guide

Frequency mixers are fundamental to modern radio electronics, enabling the frequency translation that underpins superheterodyne receivers and transmitters. In a superheterodyne receiver, a mixer combines the incoming radio-frequency signal with a local-oscillator to produce a fixed intermediate frequency, allowing convenient filtering and amplification. The same principle in reverse translates an intermediate-frequency signal to the final transmit frequency in a superheterodyne transmitter. Mixers were also important in carrier telephony, where multiple telephone conversations could share one circuit by shifting voice channels to higher-frequency slots. In electronic music, nonlinear mixing is commonly known as ring modulation, creating new tones not present in either original signal. The mixer circuit can be used not only to shift frequency but also as a product detector, modulator, phase detector, or frequency multiplier. Selection of a mixer type involves trade-offs: passive mixers use diodes and produce output of lower power than the inputs, while active mixers use amplifying devices that may increase the strength of the product signal but can have higher noise, more distortion, and power consumption. Double balanced mixers, such as the diode-ring mixer using Schottky diodes, are widely used for their low feedthrough, broad frequency range, and good large-signal performance.

Did You Know?

• A superheterodyne receiver tuned to 1000 kHz with a 1455 kHz oscillator produces a 455 kHz intermediate frequency.
• Bell System research in the late 1920s examined double-balanced mixers using copper-oxide rectifiers.