Types
A device that has a non-linear (square law) characteristic can act as a mixer. Passive mixers use one or more diodes and rely on the non-linear relation between voltage and current to provide the multiplying element. In a passive mixer, the desired output signal is always of lower power than the input signals.
Active mixers use an amplifying device (such as a transistor or vacuum tube) to increase the strength of the product signal. Active mixers improve isolation between the ports, but may have higher noise and more power consumption. An active mixer can be less tolerant of overload.
Mixers may be built of discrete components, may be part of integrated circuits, or can be delivered as hybrid modules.
Mixers may also be classified by their topology. Unbalanced mixers allow some of both input signals to pass through to the output. A single balanced mixer is arranged so that either the local oscillator (LO) or signal input (RF) is suppressed at the output, but not both. A double balanced mixer has symmetrical paths for both inputs, so that neither input signal appears at the output, only the product (IF) signal. Double balanced mixers are more complex and require higher drive levels than unbalanced and single balanced designs. Selection of a mixer type is a trade off for a particular application.
Mixer circuits are characterized by their properties such as conversion gain (or loss), and noise figure.
Nonlinear electronic components that are used as mixers include diodes, transistors biased near cutoff, and at lower frequencies, analog multipliers. Ferromagnetic-core inductors driven into saturation have also been used. In nonlinear optics, crystals with nonlinear characteristics are used to mix two frequencies of laser light to create optical heterodynes.
Read more about this topic: Frequency Mixer
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