mixer (electronics)

mixer (electronics)
Name	Mixer
Caption	A typical double-balanced mixer circuit diagram.
Classification	Active component, Nonlinear system
Invented	Early 20th century
First produced	c. 1920s
Manufacturer	Various, including Analog Devices, Texas Instruments, Mini-Circuits

mixer (electronics). In electronics, a mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. It is a fundamental component in radio frequency and microwave systems, essential for functions like frequency translation. Its primary operation involves the multiplication of two waveforms, typically a radio frequency signal and a local oscillator signal, to produce sum and difference frequencies.

Function and purpose

The core function of a mixer is frequency conversion, a process critical in virtually all radio receiver and transmitter architectures. In a superheterodyne receiver, invented by Edwin Armstrong, the mixer shifts the desired incoming signal to a fixed, lower intermediate frequency to simplify amplification and filtering. This principle is equally vital in radar systems, satellite communications, and cellular network infrastructure like 5G NR. Beyond reception, mixers are used in modulation for transmitting signals and in frequency synthesizer circuits within equipment from Rohde & Schwarz and Keysight Technologies.

Basic principles and operation

Mixers rely on the mathematical principle that multiplying two sinusoidal signals generates output components at the sum and difference of their input frequencies. If a signal at frequency f_RF and a local oscillator signal at f_LO are applied, the output contains spectral lines at f_RF + f_LO and |f_RF - f_LO|. This process is implemented using devices with nonlinear current-voltage characteristics, such as semiconductor diodes or the transconductance of field-effect transistors and bipolar junction transistors. The desired output, usually the difference frequency, is then isolated using band-pass filters, while unwanted products like the original inputs and the sum frequency are rejected.

Types of mixers

Mixers are categorized by their circuit topology and the components used. A **passive mixer** typically employs Schottky diodes and provides no gain; common configurations include the **single-balanced** and **double-balanced mixer**, with the latter offering better suppression of the local oscillator and input signal feedthrough. The **Gilbert cell** mixer, a type of **active mixer**, uses cross-coupled transistors to provide conversion gain and is widely implemented in monolithic microwave integrated circuits. Other specialized types include **image-reject mixers**, crucial for television broadcasting standards, and **subharmonic mixers** used in millimeter-wave applications for European Space Agency missions.

Key performance parameters

Several figures of merit define mixer performance. **Conversion loss** or **gain** measures the ratio of desired output power to input power. **Isolation**, such as LO-to-RF isolation, specifies how well ports are decoupled. **Linearity** is critical to avoid intermodulation distortion, with the **third-order intercept point** being a key metric. **Noise figure** significantly impacts the sensitivity of systems like the Arecibo Observatory telescope receivers. Dynamic range and **port Voltage Standing Wave Ratio** are also vital, especially in demanding environments like electronic warfare suites deployed by Lockheed Martin.

Applications

Mixers are ubiquitous in wireless technology. Every smartphone contains multiple mixers for frequency division duplex operation in networks like Verizon's LTE infrastructure. In broadcasting, they are used in FM radio transmitters and Digital Video Broadcasting equipment. Scientific applications include radio telescopes such as the Very Large Array and spectrum analyzers from Anritsu. They are also essential in Doppler radar for weather forecasting by the National Weather Service and in phase-locked loop circuits for Global Positioning System satellites operated by the United States Space Force.

Historical development

The earliest mixers were simple crystal detectors used in spark-gap transmitter era receivers. The invention of the triode vacuum tube by Lee De Forest enabled more controllable mixing action. A major leap was the 1918 invention of the superheterodyne principle by Edwin Armstrong, which cemented the mixer's central role. During World War II, advancements driven by MIT Radiation Laboratory research for radar improved diode mixer technology. The invention of the transistor at Bell Labs and later developments in integrated circuit technology by Robert Widlar led to modern active mixers, enabling the miniaturization seen in today's consumer electronics.

Category:Electronic circuits Category:Radio electronics Category:Telecommunication theory