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While Reginald Fessenden worked for the U.S. Weather Bureau in the early 1900s, he was determined to advance radio principles to the point where a network of coastal radio stations could transmit weather information over long distances. Studiously working for his $3,000 a year, Fessenden invented a principle of combining two radio signals to form a reduced composite of the signals in the audible spectrum—a process known as “heterodyning.” Building on Fessenden’s work, Edwin Armstrong later developed the first super-heterodyne receiver.

Modern mixers have moved far beyond this heritage to multiply or divide RF signals well into the millimeter-wave range. Yet the advanced behavior of these mixers requires increasing complexity in an industry that demands never-ending performance enhancements.

Mixers are used throughout the RF industry wherever significant frequency translation is needed. Using the dynamics of a nonlinear node and clever circuitry, which forces some linear behavior from the mixing device, two input signals can be multiplied or divided (Fig. 1). Diodes, Schottky diodes, bipolar-junction transistors (BJTs), or field-effect transistors (FETs) can be used as nonlinear elements. 

Creating an upconverted or downconverted output signal requires the use of a local-oscillator (LO) input signal, intermediate-frequency (IF) input/output signal, and an RF input/output signal. The most common mixers use the switching action of the LO to drive a nonlinear junction in and out of conduction, thereby clipping the RF signal. As no device is ideal, several unwanted frequency products are generated by the mixing action. One figure of merit for mixers reveals how well these products are suppressed. Other figures of merit include spurious-free dynamic range (SFDR), noise figure (NF), input third-order intercept point (IIP3), the 1-dB compression point, conversion gain, and isolation (Fig. 2).

Among the various types of mixers, there are two main categories: passive and active mixers. Passive, or current-commutating, mixers generally have higher 1-dB compression points, a lower NF, and a higher IIP3. Active mixers tend to have much lower power consumption and even a potential increase in conversion gain—although they trade off lower power for lower linearity. Both passive and active mixers can be divided into several classes: single-device, single-balanced, double-balanced, and triple-balanced (double-double-balanced mixer).

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