Frequency mixer
Encyclopedia
In electronics
a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 - f2 of the original frequencies. Other frequency components may also be produced in a practical frequency mixer.
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. For example, a key component of a superheterodyne receiver
is a mixer used to move received signals to a common intermediate frequency
. Frequency mixers are also used to modulate
a carrier frequency in radio transmitter
s.
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 the input signal power to pass through to the output. A single-balanced mixer is arranged so that the local oscillator, (or RF) signal port, cancels and cannot pass through to the output. A doubly balanced mixer has symmetrical paths for both inputs, and will have no output if either input signal is not present.
Selection of a mixer type is a trade off for a particular application. Mixer circuits are characterized by conversion gain, and noise figure
. Balanced and double-balanced designs allow less of the input signals to feed through to the output.
Nonlinear electronic components that are used as mixers include diode
s, transistor
s biased near cutoff, and at lower frequencies, analog multiplier
s. 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
.
can be used to create a simple unbalanced mixer. This type of mixer produces the original frequencies as well as their sum and their difference. The importance of the diode is that it is non-linear (or non-Ohmic
), which means its response (current) is not proportional to its input (voltage). The diode therefore does not reproduce the frequencies of its driving voltage in the current through it, which allows the desired frequency manipulation. Certain other non-linear devices such as tunnel diode
s or Gunn diode
s can be utilized similarly.
The current I through an ideal diode as a function of the voltage V across it is given by
where what is important is that V appears in es exponent. The exponential can be expanded
as
and can be approximated for small x (that is, small voltages) by the first few terms of that series:
Suppose that the sum of the two input signals
is applied to a diode, and that an output voltage is generated that is proportional to the current through the diode (perhaps by providing the voltage that is present across a resistor
in series with the diode). Then, disregarding the constants in the diode equation, the output voltage will have the form
The first term on the right is the original two signals, as expected, followed by the square of the sum, which can be rewritten as
, where the multiplied signal is obvious. The ellipsis represents all the higher powers of the sum which we assume to be negligible
for small signals.
The aim of a switching mixer is to achieve linear operation over the signal level, and hard switching driven by the local oscillator. Mathematically the switching mixer is not much different from a multiplying mixer, just because instead of the LO sine wave term we would use the signum function. In the frequency domain the switching mixer operation leads to the usual sum and difference frequencies, but also to further terms e.g. +-3*fLO, +-5*fLO, etc.
The advantage of a switching mixer is that it can achieve - with the same effort - a lower noise figure (NF) and larger conversion gain. This come because the switching diodes or transistors act either like a low resistor (switch closed) or large resistor (switch open) and in both cases only minimum noise is added. From the circuit perspective many multiplying mixers can be used as switching mixers, just by increasing the LO amplitude. So RF engineers simply talk about mixers, and mean switching mixers.
, modulator, phase detector
or frequency multiplier. 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.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 - f2 of the original frequencies. Other frequency components may also be produced in a practical frequency mixer.
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. For example, a key component of a superheterodyne receiver
Superheterodyne receiver
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
is a mixer used to move received signals to a common intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
. Frequency mixers are also used to modulate
Modulate
Modulate is Bob Mould's fifth solo album, released in 2002. Although a few tracks on his previous release, The Last Dog and Pony Show, had featured tape loops and samples, Mould shocked his fans with such a dramatic embrace of electronica...
a carrier frequency in radio transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
s.
Types
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 can 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 the input signal power to pass through to the output. A single-balanced mixer is arranged so that the local oscillator, (or RF) signal port, cancels and cannot pass through to the output. A doubly balanced mixer has symmetrical paths for both inputs, and will have no output if either input signal is not present.
Selection of a mixer type is a trade off for a particular application. Mixer circuits are characterized by conversion gain, and noise figure
Noise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...
. Balanced and double-balanced designs allow less of the input signals to feed through to the output.
Nonlinear electronic components that are used as mixers include diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
s, transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s biased near cutoff, and at lower frequencies, analog multiplier
Analog multiplier
In electronics, an analog multiplier is a device which takes two analog signals and produces an output which is their product. Such circuits can be used to implement related functions such as squares , and square roots....
s. Ferromagnetic-core inductors
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
driven into saturation
Saturation (magnetic)
Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off...
have also been used. In nonlinear optics
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
, crystals with nonlinear characteristics are used to mix two frequencies of laser light to create optical heterodynes
Optical heterodyne detection
Optical heterodyne detection is an important special case of heterodyne detection. In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with a reference "local oscillator" that is set at a close-by frequency...
.
Diode
A diodeDiode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
can be used to create a simple unbalanced mixer. This type of mixer produces the original frequencies as well as their sum and their difference. The importance of the diode is that it is non-linear (or non-Ohmic
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
), which means its response (current) is not proportional to its input (voltage). The diode therefore does not reproduce the frequencies of its driving voltage in the current through it, which allows the desired frequency manipulation. Certain other non-linear devices such as tunnel diode
Tunnel diode
A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanical effects....
s or Gunn diode
Gunn diode
A Gunn diode, also known as a transferred electron device , is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions...
s can be utilized similarly.
The current I through an ideal diode as a function of the voltage V across it is given by
where what is important is that V appears in es exponent. The exponential can be expanded
Taylor series
In mathematics, a Taylor series is a representation of a function as an infinite sum of terms that are calculated from the values of the function's derivatives at a single point....
as
and can be approximated for small x (that is, small voltages) by the first few terms of that series:
Suppose that the sum of the two input signals
is applied to a diode, and that an output voltage is generated that is proportional to the current through the diode (perhaps by providing the voltage that is present across a resistorResistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
in series with the diode). Then, disregarding the constants in the diode equation, the output voltage will have the form
The first term on the right is the original two signals, as expected, followed by the square of the sum, which can be rewritten as
, where the multiplied signal is obvious. The ellipsis represents all the higher powers of the sum which we assume to be negligibleNegligible
Negligible refers to the quantities so small that they can be ignored when studying the larger effect. Although related to the more mathematical concepts of infinitesimal, the idea of negligibility is particularly useful in practical disciplines like physics, chemistry, mechanical and electronic...
for small signals.
Switching
Another form of mixer operates by switching, with the smaller input signal being passed inverted or uninverted according to the phase of the local oscillator (LO). This would be typical of the normal operating mode of a packaged double balanced mixer module such as an SBL-1, with the local oscillator drive considerably higher than the signal amplitude.The aim of a switching mixer is to achieve linear operation over the signal level, and hard switching driven by the local oscillator. Mathematically the switching mixer is not much different from a multiplying mixer, just because instead of the LO sine wave term we would use the signum function. In the frequency domain the switching mixer operation leads to the usual sum and difference frequencies, but also to further terms e.g. +-3*fLO, +-5*fLO, etc.
The advantage of a switching mixer is that it can achieve - with the same effort - a lower noise figure (NF) and larger conversion gain. This come because the switching diodes or transistors act either like a low resistor (switch closed) or large resistor (switch open) and in both cases only minimum noise is added. From the circuit perspective many multiplying mixers can be used as switching mixers, just by increasing the LO amplitude. So RF engineers simply talk about mixers, and mean switching mixers.
Applications
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 detectorProduct detector
A product detector is a type of demodulator used for AM and SSB signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator, hence the name...
, modulator, phase detector
Phase detector
A phase detector or phase comparator is a frequency mixer, analog multiplier or logic circuit that generates a voltage signal which represents the difference in phase between two signal inputs...
or frequency multiplier. For example a communications receiver
Communications receiver
A communications receiver is a type of radio receiver used as a component of a radio communication link.-Features:Commercial communications receivers are characterised by high stability and reliability of performance, and are generally adapted for remote control and monitoring...
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.
See also
- Frequency multiplierFrequency multiplierIn electronics, a frequency multiplier is an electronic circuit that generates an output signal whose output frequency is a harmonic of its input frequency. Frequency multipliers consist of a nonlinear circuit that distorts the input signal and consequently generates harmonics of the input signal...
- Optical heterodyne detectionOptical heterodyne detectionOptical heterodyne detection is an important special case of heterodyne detection. In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with a reference "local oscillator" that is set at a close-by frequency...
- Gilbert cellGilbert cellIn electronics, the Gilbert cell is a transistor circuit used as an analog multiplier and frequency mixer, first described by Barrie Gilbert in 1968. The advantage of this circuit is the output current is an accurate multiplication of the base currents of both inputs...
- IntermodulationIntermodulationIntermodulation or intermodulation distortion is the amplitude modulation of signals containing two or more different frequencies in a system with nonlinearities...
- Pentagrid converterPentagrid converterThe pentagrid converter is a radio receiving valve with five grids used as the frequency mixer stage of a superheterodyne radio receiver....
- Product detectorProduct detectorA product detector is a type of demodulator used for AM and SSB signals. Rather than converting the envelope of the signal into the decoded waveform like an envelope detector, the product detector takes the product of the modulated signal and a local oscillator, hence the name...
- Ring modulationRing modulationRing modulation is a signal-processing effect in electronics, an implementation of amplitude modulation or frequency mixing, performed by multiplying two signals, where one is typically a sine-wave or another simple waveform. It is referred to as "ring" modulation because the analog circuit of...
- Rusty bolt effectRusty bolt effectThe rusty bolt effect is a description of radio interference due to interactions with dirty connections or corroded parts. It is more properly known as passive intermodulation, and can result from a variety of different causes such as ferromagnetic conduction metals, nonlinear microwave absorbers...
- Subharmonic mixer
- Third-order intercept pointThird-order intercept pointIn telecommunications, a third-order intercept point is a measure for weakly nonlinear systems and devices, for example receivers, linear amplifiers and mixers. It is based on the idea that the device nonlinearity can be modeled using a low-order polynomial, derived by means of Taylor series...