Electronic mixer

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An electronic mixer is a device that combines two or more electrical or electronic signals into one or two composite output signals. There are two basic circuits that both use the term mixer, but they are very different types of circuits: additive mixers and multiplying mixers.

Additive mixers add two or more signals together, and this terminology ("mixer") is only used in the realm of audio electronics where audio mixers are used to add together audio frequency signals such as voice signals, music signals, and sound effects.

Multiplying mixers multiply together two time-varying input signals instantaneously (instant-by-instant). If the two input signals are both sinusoids of specified frequencies f1 and f2, then the output of the mixer will contain two new sinsoids that have the sum f1 + f2 frequency and the difference frequency absolute value{f1 - f2}.

Note: Any nonlinear electronic block driven by two signals with frequencies f1 and f2 would generate intermodulation (mixing) products. A multiplier (which is a nonlinear device too) will generate ideally only the sum and difference frequencies, whereas a arbitrary nonlinear block would generate also signals at e.g. 2*f1-3*f2, etc. Therefore in the past often more or less normal nonlinear amplifiers or just single diodes have been used as mixers, instead of a more complex multiplier. A multiplier has usually the advantage of rejecting - at least partly - undesired higher-order intermodulations and larger conversion gain.

Additive mixers

Additive mixers add two or more signals, giving out a composite signal that contains the frequency components of each of the source signals. The simplest additive mixers are simple resistor networks, and thus purely passive, while more complex mixers employ active components such as buffer amplifiers for impedance matching and better isolation.

Product Mixers

Ideal product mixers act as signal multipliers, producing an output signal equal to the product of the two input signals. Product mixers are often used in conjugation with an oscillator in the communications field to modulate signal frequencies. Product mixers can either up-convert or down-convert an input signal frequency, but they are more commonly used to down-convert to a lower frequency to allow for simpler filter designs, as done in superheterodyne receivers. In many typical circuits, the single output signal actually contains multiple waveforms, namely those at the sum and difference of the two input frequencies and harmonic waveforms. The output signal may be obtained by removing the other signal components with a filter. (see intermediate frequency)

Product mixers have been implemented in a wide variety of ways. The most popular are Gilbert cell mixers, diode mixers, diode ring mixers (ring modulation) and switching mixers. Diodes mixers take advantage of the non-linearity of diode devices to produce the desired multiplication in the squared term. It is a very inefficient method as most of the power output is in other unwanted terms which need filtering out. Inexpensive AM radios still use diode mixers.

Electronic mixers are usually made with transistors and/or diodes arranged in a balanced circuit or even a double-balanced circuit. These are readily manufactured by using the technology of either monolithic integrated circuits or hybrid integrated circuits. These are designed for a wide variety of frequency ranges, and they are mass produced to tight toleraces by the hundreds of thousands. These mixers, especially the double-balanced variety, and be bought in large numbers at prices ranging from a dime to a quarter apiece.

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