Frequency mixer
In its most common application, two signals are applied to a mixer, and it produces new signals at the sum and difference of the original frequencies.
Mixers are widely used to shift signals from one frequency range to another, a process known as heterodyning, for convenience in transmission or further signal processing.
Frequency mixers are also used to modulate a carrier signal in radio transmitters.
The essential characteristic of a mixer is that it produces a component in its output which is the product of the two input signals.
Both active and passive circuits can realize mixers.
Passive mixers use one or more diodes and rely on their nonlinear current–voltage relationship to provide the multiplying element.
Active mixers use an amplifying device (such as a transistor or vacuum tube) that may increase the strength of the product signal.
Active mixers improve isolation between the ports, but may have higher noise and more power consumption.
Mixers may be built of discrete components, may be part of integrated circuits, or can be delivered as hybrid modules.
[2] Mixer circuits are characterized by their properties such as conversion gain (or loss), noise figure and nonlinearity.
[3] Nonlinear electronic components that are used as mixers include diodes and transistors biased near cutoff.
Linear, time-varying devices, such as analog multipliers, provide superior performance, as it is only in true multipliers that the output amplitude is proportional to the input amplitude, as required for linear conversion.
In nonlinear optics, crystals with nonlinear characteristics are used to mix two frequencies of laser light to create optical heterodynes.
A diode can be used to create a simple unbalanced mixer.
through an ideal semiconductor diode is primarily an exponential function of the voltage
Then, disregarding the constants in the diode equation, the output voltage will be proportional to: In addition to the original two signals
If two sinusoids of different frequencies are fed as input into the diode, such that
becomes: Expanding the square term yields: According to the prosthaphaeresis product to sum identity
A narrowband filter may be used to remove undesired frequencies from the output signal.
[4] Another form of mixer operates by switching, which is equivalent to multiplication of an input signal by a square wave.
In a double-balanced mixer, the (smaller) input signal is alternately inverted or non inverted according to the phase of the local oscillator (LO).
That is, the input signal is effectively multiplied by a square wave that alternates between +1 and -1 at the LO rate.
In a single-balanced switching mixer, the input signal is alternately passed or blocked.
The input signal is thus effectively multiplied by a square wave that alternates between 0 and +1.
This results in frequency components of the input signal being present in the output together with the product,[5] since the multiplying signal can be viewed as a square wave with a DC offset (i.e. a zero frequency component).
The aim of a switching mixer is to achieve the linear operation by means of hard switching, driven by the local oscillator.
The advantage of a switching mixer is that it can achieve (with the same effort) a lower noise figure (NF) and larger conversion gain.
The mixer circuit can be used not only to shift the frequency of an input signal as in a receiver, but also as a product detector, modulator, phase detector or frequency multiplier.
[6] For example, a communications receiver might contain two mixer stages for conversion of the input signal to an intermediate frequency and another mixer employed as a detector for demodulation of the signal.
This article incorporates public domain material from Federal Standard 1037C.
