Electronic mixer
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An electronic mixer is a device that combines two or more electronic signals into one composite output signal. There are two basic types of mixer. Additive mixers add two signals together, and are used for such applications as audio mixing. Multiplying mixers multiply the signals together, and produce an output containing both original signals, and new signals that have the sum and difference of the frequency of the original signals.
[edit] Additive mixers
Additive mixers add two or more signals, outputting 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.
[edit] 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 it is more common to down-convert to a lower frequency to allow for easier filter design as is practiced in the Superheterodyne receiver. 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 ideal signal may be obtained by removing the other signal components with a filter.
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. A diode mixer has two or more signals going into a diode. 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.
Gilbert cell mixers are just an arrangement of transistors that multiplies the two signals. The switching mixers (below) pass more power and usually insert less distortion.
Switching mixers use an array of Field effect transistors or (in older days) vacuum tubes. These are used as electronic switches, to permit the signal to go one direction, then the other. They are controlled by the signal being mixed. They are especially popular with digitally-controlled radios.
