A circulator is a passive, non-reciprocal three- or four-port device that only allows a microwave or radio-frequency signal to exit through the port directly after the one it entered. Optical circulators have similar behavior. Ports are where an external waveguide or transmission line, such as a microstrip line or a coaxial cable, connects to the device. For a three-port circulator, a signal applied to port 1 only comes out of port 2; a signal applied to port 2 only comes out of port 3; a signal applied to port 3 only comes out of port 1, and so on. An ideal three-port circulator has the following scattering matrix:
ANSI and IEC standard schematic symbol for a circulator (with each waveguide or transmission line port drawn as a single line, rather than as a pair of conductors)
Types[edit]
Depending on the materials involved, circulators fall into two main categories: ferrite circulators and nonferrite circulators.
Ferrite[edit]
Ferrite circulators are radio-frequency circulators which employ magnetized microwave ferrite materials. They fall into two main classes: differential phase shift circulators and junction circulators, both of which are based on cancellation of waves propagating over two different paths in or near magnetized ferrite material. Waveguide circulators may be of either type, while more compact devices based on stripline are usually of the junction type.[1][2] Two or more junction circulators can be combined in a single component to give four or more ports. Typically permanent magnets produce a static magnetic bias in the microwave ferrite material. Ferrimagnetic garnet crystal is used in optical circulators.
Though ferrite circulators can provide good "forward" signal circulation while suppressing greatly the "reverse" circulation, their major shortcomings, especially at low frequencies, are the bulky sizes and the narrow bandwidths.
Nonferrite[edit]
Early work on nonferrite circulators includes active circulators using transistors that are non-reciprocal in nature.[3] In contrast to ferrite circulators which are passive devices, active circulators require power. Major issues associated with transistor-based active circulators are the power limitation and the signal-to-noise degradation,[4] which are critical when it is used as a duplexer for sustaining the strong transmit power and clean reception of the signal from the antenna.
Varactors offer one solution. One study employed a structure similar to a time-varying transmission line with the effective nonreciprocity triggered by a one-direction propagating carrier pump.[5] This is like an AC-powered active circulator. The research claimed to be able to achieve positive gain and low noise for receiving path and broadband nonreciprocity. Another study used resonance with nonreciprocity triggered by angular-momentum biasing, which more closely mimics the way that signals passively circulate in a ferrite circulator.[6]
In 1964, Mohr presented and experimentally demonstrated a circulator based on transmission lines and switches.[7] In April, 2016 a research team significantly extended this concept, presenting an integrated circuit circulator based on N-path filter concepts.[8][9] It offers the potential for full-duplex communication (transmitting and receiving at the same time with a single shared antenna over a single frequency). The device uses capacitors and a clock and is much smaller than conventional devices.[10]
Applications[edit]
Isolator[edit]
When one port of a three-port circulator is terminated in a matched load, it can be used as an isolator, since a signal can travel in only one direction between the remaining ports.[11] An isolator is used to shield equipment on its input side from the effects of conditions on its output side; for example, to prevent a microwave source being detuned by a mismatched load.
Duplexer[edit]
In radar, circulators are used as a type of duplexer, to route signals from the transmitter to the antenna and from the antenna to the receiver, without allowing signals to pass directly from transmitter to receiver. The alternative type of duplexer is a transmit-receive switch (TR switch) that alternates between connecting the antenna to the transmitter and to the receiver. The use of chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received pulses, however, requiring a circulator for this function.
In the future-generation cellular communication, people talk about full-duplex radios, where signals can be simultaneously transmitted and received at the same frequency. Given the currently limited, crowded spectrum resource, full-duplexing can directly benefit the wireless communication by twice of the data throughput speed. Currently, the wireless communication is still performed with "half-duplex", where either the signals are transmitted or received at different time frames, if at the same frequency (typically in radar), or the signals are simultaneously transmitted and received at different frequencies (realized by a set of filters called a diplexer).
Reflection amplifier[edit]
A reflection amplifier is a type of microwave amplifier circuit utilizing negative differential resistance diodes such as tunnel diodes and Gunn diodes. Negative differential resistance diodes can amplify signals, and often perform better at microwave frequencies than two-port devices. However, since the diode is a one-port (two terminal) device, a nonreciprocal component is needed to separate the outgoing amplified signal from the incoming input signal. By using a 3-port circulator with the signal input connected to one port, the biased diode connected to a second, and the output load connected to the third, the output and input can be uncoupled.
https://en.wikipedia.org/wiki/Circulator
An isolator is a two-port device that transmits microwave or radio frequency power in one direction only. Due to internal behavior, the propagation in one direction is allowed while the other direction is blocked. The non-reciprocity observed in these devices usually comes from the interaction between the propagating wave and the material, which can be different with respect to the direction of propagation.
It is used to shield equipment on its input side, from the effects of conditions on its output side; for example, to prevent a microwave source being detuned by a mismatched load.
-in-WG16.jpg)
Non-reciprocity[edit]
An isolator is a non-reciprocal device, with a non-symmetric scattering matrix. An ideal isolator transmits all the power entering port 1 to port 2, while absorbing all the power entering port 2, so that to within a phase-factor its S-matrix is
To achieve non-reciprocity, an isolator must necessarily incorporate a non-reciprocal material. At microwave frequencies, this material is usually a ferrite which is biased by a static magnetic field[1] but can be a self-biasied material.[2] The ferrite is positioned within the isolator such that the microwave signal presents it with a rotating magnetic field, with the rotation axis aligned with the direction of the static bias field. The behaviour of the ferrite depends on the sense of rotation with respect to the bias field, and hence is different for microwave signals travelling in opposite directions. Depending on the exact operating conditions, the signal travelling in one direction may either be phase-shifted, displaced from the ferrite or absorbed.
Types[edit]
Most common types of ferrite-based isolators are classified into four categories: terminated circulators, Faraday rotation isolators, field-displacement isolators, and resonance isolators. In all these kinds of devices, the observed non-reciprocity arises from the wave-material interaction which depends on the direction of propagation.
Resonance absorption[edit]
In this type the ferrite absorbs energy from the microwave signal travelling in one direction. A suitable rotating magnetic field is found in the dominant TE10 mode of rectangular waveguide. The rotating field exists away from the centre-line of the broad wall, over the full height of the guide. However, to allow heat from the absorbed power to be conducted away, the ferrite does not usually extend from one broad-wall to the other, but is limited to a shallow strip on each face. For a given bias field, resonance absorption occurs over a fairly narrow frequency band, but since in practice the bias field is not perfectly uniform throughout the ferrite, the isolator functions over a somewhat wider band.
Field displacement[edit]
This type is superficially very similar to a resonance absorption isolator, but the magnetic biasing differs, and the energy from the backward travelling signal is absorbed in a resistive film or card on one face of the ferrite block rather than within the ferrite itself.
The bias field is weaker than that necessary to cause resonance at the operating frequency, but is instead designed to give the ferrite near-zero permeability for one sense of rotation of the microwave signal field. The bias polarity is such that this special condition arises for the forward signal; the backward signal sees the ferrite as an ordinary dielectricmaterial (with little permeability, as the ferrite is already saturated by the bias field). Consequently, for the electromagnetic field of the forward signal, the ferrite has very low characteristic wave impedance, and the field tends to be excluded from the ferrite. This results in a null of the electric field of the forward signal on the surface of the ferrite where the resistive film is placed. Conversely for the backward signal, the electric field is strong over this surface and so its energy is dissipated in driving current through the film.
In rectangular waveguide the ferrite block will typically occupy the full height from one broad-wall to the other, with the resistive film on the side facing the centre-line of the guide.
Terminated circulator[edit]
A circulator is a non-reciprocal three- or four-port device, in which power entering any port is transmitted to the next port in rotation (only). So to within a phase-factor, the scattering matrix for a three-port circulator is
A two-port isolator is obtained simply by terminating one of the three ports with a matched load, which absorbs all the power entering it. The biased ferrite is part of the circulator and causes a differential phase-shift for signals travelling in different directions. The bias field is lower than that needed for resonance absorption, and so this type of isolator does not require such a heavy permanent magnet. Because the power is absorbed in an external load, cooling is less of a problem than with a resonance absorption isolator.
Faraday rotation isolator[edit]
A last physical principle useful to design isolators is the Faraday rotation. When a linearly polarized wave propagates through ferrite having a magnetization aligned with the direction of propagation of the wave, the polarization plane will rotate along the propagation axis. This rotation may be used to create microwave devices as isolators, circulators, gyrators, etc. In rectangular waveguide topology, it also requires the implementation of circular waveguide sections which come out of the device plane.
See also[edit]
https://en.wikipedia.org/wiki/Isolator_(microwave)
A gyrator is a passive, linear, lossless, two-port electrical network element proposed in 1948 by Bernard D. H. Tellegen as a hypothetical fifth linear element after the resistor, capacitor, inductor and ideal transformer.[1] Unlike the four conventional elements, the gyrator is non-reciprocal. Gyrators permit network realizationsof two-(or-more)-port devices which cannot be realized with just the conventional four elements. In particular, gyrators make possible network realizations of isolators and circulators.[2] Gyrators do not however change the range of one-port devices that can be realized. Although the gyrator was conceived as a fifth linear element, its adoption makes both the ideal transformer and either the capacitor or inductor redundant. Thus the number of necessary linear elements is in fact reduced to three. Circuits that function as gyrators can be built with transistors and op-amps using feedback.
Tellegen invented a circuit symbol for the gyrator and suggested a number of ways in which a practical gyrator might be built.
An important property of a gyrator is that it inverts the current–voltage characteristic of an electrical component or network. In the case of linear elements, the impedance is also inverted. In other words, a gyrator can make a capacitivecircuit behave inductively, a series LC circuit behave like a parallel LC circuit, and so on. It is primarily used in active filterdesign and miniaturization.
https://en.wikipedia.org/wiki/Gyrator
In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.[4][5]
This is in contrast to an ordinary resistor in which an increase of applied voltage causes a proportional increase in current due to Ohm's law, resulting in a positive resistance.[6] While a positive resistance consumes power from current passing through it, a negative resistance produces power.[7][8] Under certain conditions it can increase the power of an electrical signal, amplifying it.[3][9][10]
Negative resistance is an uncommon property which occurs in a few nonlinear electronic components. In a nonlinear device, two types of resistance can be defined: 'static' or 'absolute resistance', the ratio of voltage to current , and differential resistance, the ratio of a change in voltage to the resulting change in current . The term negative resistance means negative differential resistance (NDR), . In general, a negative differential resistance is a two-terminal component which can amplify,[3][11] converting DCpower applied to its terminals to AC output power to amplify an AC signal applied to the same terminals.[7][12]They are used in electronic oscillators and amplifiers,[13] particularly at microwave frequencies. Most microwave energy is produced with negative differential resistance devices.[14] They can also have hysteresis[15] and be bistable, and so are used in switching and memory circuits.[16] Examples of devices with negative differential resistance are tunnel diodes, Gunn diodes, and gas discharge tubes such as neon lamps, and fluorescent lights. In addition, circuits containing amplifying devices such as transistors and op amps with positive feedback can have negative differential resistance. These are used in oscillators and active filters.
Because they are nonlinear, negative resistance devices have a more complicated behavior than the positive "ohmic" resistances usually encountered in electric circuits. Unlike most positive resistances, negative resistance varies depending on the voltage or current applied to the device, and negative resistance devices can only have negative resistance over a limited portion of their voltage or current range.[10][17] Therefore, there is no real "negative resistor" analogous to a positive resistor, which has a constant negative resistance over an arbitrarily wide range of current.
Reflection amplifier[edit]
One widely used circuit is the reflection amplifier in which the separation is accomplished by a circulator.[86][138][139][140] A circulator is a nonreciprocal solid-state component with three ports (connectors) which transfers a signal applied to one port to the next in only one direction, port 1 to port 2, 2 to 3, and 3 to 1. In the reflection amplifier diagram the input signal is applied to port 1, a biased VCNR negative resistance diode N is attached through a filter F to port 2, and the output circuit is attached to port 3. The input signal is passed from port 1 to the diode at port 2, but the outgoing "reflected" amplified signal from the diode is routed to port 3, so there is little coupling from output to input. The characteristic impedance of the input and output transmission lines, usually 50Ω, is matched to the port impedance of the circulator. The purpose of the filter F is to present the correct impedance to the diode to set the gain. At radio frequencies NR diodes are not pure resistive loads and have reactance, so a second purpose of the filter is to cancel the diode reactance with a conjugate reactance to prevent standing waves.[140][141]
The filter has only reactive components and so does not absorb any power itself, so power is passed between the diode and the ports without loss. The input signal power to the diode is
The output power from the diode is
So the power gain of the amplifier is the square of the reflection coefficient[138][140][141]
is the negative resistance of the diode −r. Assuming the filter is matched to the diode so [140] then the gain is
The VCNR reflection amplifier above is stable for .[140] while a CCNR amplifier is stable for . It can be seen that the reflection amplifier can have unlimited gain, approaching infinity as approaches the point of oscillation at .[140] This is a characteristic of all NR amplifiers,[139] contrasting with the behavior of two-port amplifiers, which generally have limited gain but are often unconditionally stable. In practice the gain is limited by the backward "leakage" coupling between circulator ports.
Masers and parametric amplifiers are extremely low noise NR amplifiers that are also implemented as reflection amplifiers; they are used in applications like radio telescopes.[141]
Switching circuits[edit]
Negative differential resistance devices are also used in switching circuits in which the device operates nonlinearly, changing abruptly from one state to another, with hysteresis.[15] The advantage of using a negative resistance device is that a relaxation oscillator, flip-flop or memory cell can be built with a single active device,[81] whereas the standard logic circuit for these functions, the Eccles-Jordan multivibrator, requires two active devices (transistors). Three switching circuits built with negative resistances are
- Astable multivibrator – a circuit with two unstable states, in which the output periodically switches back and forth between the states. The time it remains in each state is determined by the time constant of an RC circuit. Therefore, it is a relaxation oscillator, and can produce square waves or triangle waves.
- Monostable multivibrator – is a circuit with one unstable state and one stable state. When in its stable state a pulse is applied to the input, the output switches to its other state and remains in it for a period of time dependent on the time constant of the RC circuit, then switches back to the stable state. Thus the monostable can be used as a timer or delay element.
- Bistable multivibrator or flip flop – is a circuit with two stable states. A pulse at the input switches the circuit to its other state. Therefore, bistables can be used as memory circuits, and digital counters.
Other applications[edit]
Neuronal models[edit]
Some instances of neurons display regions of negative slope conductances (RNSC) in voltage-clamp experiments.[142] The negative resistance here is implied were one to consider the neuron a typical Hodgkin–Huxley style circuit model.
History[edit]
Negative resistance was first recognized during investigations of electric arcs, which were used for lighting during the 19th century.[143] In 1881 Alfred Niaudet[144]had observed that the voltage across arc electrodes decreased temporarily as the arc current increased, but many researchers thought this was a secondary effect due to temperature.[145] The term "negative resistance" was applied by some to this effect, but the term was controversial because it was known that the resistance of a passive device could not be negative.[68][145][146] Beginning in 1895 Hertha Ayrton, extending her husband William's research with a series of meticulous experiments measuring the I–V curve of arcs, established that the curve had regions of negative slope, igniting controversy.[65][145][147] Frith and Rodgers in 1896[145][148] with the support of the Ayrtons[65] introduced the concept of differential resistance, dv/di, and it was slowly accepted that arcs had negative differential resistance. In recognition of her research, Hertha Ayrton became the first woman voted for induction into the Institute of Electrical Engineers.[147]
Arc transmitters[edit]
George Francis FitzGerald first realized in 1892 that if the damping resistance in a resonant circuit could be made zero or negative, it would produce continuous oscillations.[143][149] In the same year Elihu Thomson built a negative resistance oscillator by connecting an LC circuit to the electrodes of an arc,[105][150] perhaps the first example of an electronic oscillator. William Duddell, a student of Ayrton at London Central Technical College, brought Thomson's arc oscillator to public attention.[105][143][147] Due to its negative resistance, the current through an arc was unstable, and arc lights would often produce hissing, humming, or even howling noises. In 1899, investigating this effect, Duddell connected an LC circuit across an arc and the negative resistance excited oscillations in the tuned circuit, producing a musical tone from the arc.[105][143][147] To demonstrate his invention Duddell wired several tuned circuits to an arc and played a tune on it.[143][147] Duddell's "singing arc" oscillator was limited to audio frequencies.[105] However, in 1903 Danish engineers Valdemar Poulsen and P. O. Pederson increased the frequency into the radio range by operating the arc in a hydrogen atmosphere in a magnetic field,[151] inventing the Poulsen arc radio transmitter, which was widely used until the 1920s.[105][143]
Vacuum tubes[edit]
By the early 20th century, although the physical causes of negative resistance were not understood, engineers knew it could generate oscillations and had begun to apply it.[143] Heinrich Barkhausen in 1907 showed that oscillators must have negative resistance.[84] Ernst Ruhmer and Adolf Pieper discovered that mercury vapor lamps could produce oscillations, and by 1912 AT&T had used them to build amplifying repeaters for telephone lines.[143]
In 1918 Albert Hull at GE discovered that vacuum tubes could have negative resistance in parts of their operating ranges, due to a phenomenon called secondary emission.[9][36][152] In a vacuum tube when electrons strike the plate electrode they can knock additional electrons out of the surface into the tube. This represents a current away from the plate, reducing the plate current.[9] Under certain conditions increasing the plate voltage causes a decrease in plate current. By connecting an LC circuit to the tube Hull created an oscillator, the dynatron oscillator. Other negative resistance tube oscillators followed, such as the magnetron invented by Hull in 1920.[60]
The negative impedance converter originated from work by Marius Latour around 1920.[153][154] He was also one of the first to report negative capacitance and inductance.[153] A decade later, vacuum tube NICs were developed as telephone line repeaters at Bell Labs by George Crisson and others,[26][127] which made transcontinental telephone service possible.[127] Transistor NICs, pioneered by Linvill in 1953, initiated a great increase in interest in NICs and many new circuits and applications developed.[125][127]
Solid state devices[edit]
Negative differential resistance in semiconductors was observed around 1909 in the first point-contact junction diodes, called cat's whisker detectors, by researchers such as William Henry Eccles[155][156] and G. W. Pickard.[156][157] They noticed that when junctions were biased with a DC voltage to improve their sensitivity as radio detectors, they would sometimes break into spontaneous oscillations.[157] However the effect was not pursued.
The first person to exploit negative resistance diodes practically was Russian radio researcher Oleg Losev, who in 1922 discovered negative differential resistance in biased zincite (zinc oxide) point contact junctions.[157][158][159][160][161] He used these to build solid-state amplifiers, oscillators, and amplifying and regenerative radio receivers, 25 years before the invention of the transistor.[155][159][161][162] Later he even built a superheterodyne receiver.[161] However his achievements were overlooked because of the success of vacuum tube technology. After ten years he abandoned research into this technology (dubbed "Crystodyne" by Hugo Gernsback),[162] and it was forgotten.[161]
The first widely used solid-state negative resistance device was the tunnel diode, invented in 1957 by Japanese physicist Leo Esaki.[67][163] Because they have lower parasitic capacitance than vacuum tubes due to their small junction size, diodes can function at higher frequencies, and tunnel diode oscillators proved able to produce power at microwave frequencies, above the range of ordinary vacuum tube oscillators. Its invention set off a search for other negative resistance semiconductor devices for use as microwave oscillators,[164] resulting in the discovery of the IMPATT diode, Gunn diode, TRAPATT diode, and others. In 1969 Kurokawa derived conditions for stability in negative resistance circuits.[136] Currently negative differential resistance diode oscillators are the most widely used sources of microwave energy,[80] and many new negative resistance devices have been discovered in recent decades.[67]
Notes[edit]
- ^ Some microwave texts use this term in a more specialized sense: a voltage controlled negative resistance device (VCNR) such as a tunnel diode is called a "negative conductance" while a current controlled negative resistance device (CCNR) such as an IMPATT diode is called a "negative resistance". See the Stability conditions section
- ^ a b c d The terms "open-circuit stable" and "short-circuit stable" have become somewhat confused over the years, and are used in the opposite sense by some authors. The reason is that in linear circuits if the load line crosses the I-V curve of the NR device at one point, the circuit is stable, while in nonlinear switching circuits that operate by hysteresis the same condition causes the circuit to become unstable and oscillate as an astable multivibrator, and the bistable region is considered the "stable" one. This article uses the former "linear" definition, the earliest one, which is found in the Abraham, Bangert, Dorf, Golio, and Tellegen sources. The latter "switching circuit" definition is found in the Kumar and Taub sources.
https://en.wikipedia.org/wiki/Negative_resistance#Reflection_amplifier
A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suzuki when they were working at Tokyo Tsushin Kogyo, now known as Sony.[1][2][3][4] In 1973, Esaki received the Nobel Prize in Physics, jointly with Brian Josephson, for discovering the electron tunneling effect used in these diodes. Robert Noyce independently devised the idea of a tunnel diode while working for William Shockley, but was discouraged from pursuing it.[5] Tunnel diodes were first manufactured by Sony in 1957,[6] followed by General Electric and other companies from about 1960, and are still made in low volume today.[7]
Tunnel diodes have a heavily doped positive-to-negative (P-N) junction that is about 10 nm (100 Å) wide. The heavy doping results in a broken band gap, where conduction band electron states on the N-side are more or less aligned with valence band hole states on the P-side. They are usually made from germanium, but can also be made from gallium arsenide and silicon materials.
Uses[edit]
Their "negative" differential resistance in part of their operating range allows them to function as oscillators and amplifiers, and in switching circuits using hysteresis. They are also used as frequency converters and detectors.[8]: 7–35 Their low capacitance allows them to function at microwave frequencies, far above the range of ordinary diodes and transistors.
Due to their low output power, tunnel diodes are not widely used: Their RF output is limited to a few hundred milliwatts due to their small voltage swing. In recent years, however, new devices that use the tunneling mechanism have been developed. The resonant-tunneling diode (RTD) has achieved some of the highest frequencies of any solid-state oscillator.[9]
Another type of tunnel diode is a metal–insulator–insulator–metal (MIIM) diode, where an additional insulator layer allows "step tunneling" for more precise control of the diode.[10] There is also a metal–insulator–metal (MIM) diode, but due to inherent sensitivities, its present application appears to be limited to research environments.[11]
Forward bias operation[edit]
Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow P-N junction barrier and fill electron states in the conduction band on the N-side which become aligned with empty valence band hole states on the P-side of the P-N junction. As voltage increases further, these states become increasingly misaligned, and the current drops. This is called negative differential resistance because current decreases with increasing voltage. As voltage increases beyond a fixed transition point, the diode begins to operate as a normal diode, where electrons travel by conduction across the P-N junction, and no longer by tunneling through the P–N junction barrier. The most important operating region for a tunnel diode is the "negative resistance" region. Its graph is different from normal P-N junction diode.
Reverse bias operation[edit]
When used in the reverse direction, tunnel diodes are called back diodes (or backward diodes) and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction). Under reverse bias, filled states on the P-side become increasingly aligned with empty states on the N-side, and electrons now tunnel through the P-N junction barrier in reverse direction.
Technical comparisons[edit]
In a conventional semiconductor diode, conduction takes place while the P-N junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the "reverse breakdown voltage" at which point conduction begins (often accompanied by destruction of the device). In the tunnel diode, the dopant concentrations in the P and N layers are increased to a level such that the reverse breakdown voltage becomes zeroand the diode conducts in the reverse direction. However, when forward-biased, an effect occurs called quantum mechanical tunneling which gives rise to a region in its voltage vs. current behavior where an increase in forward voltage is accompanied by a decrease in forward current. This "negative resistance" region can be exploited in a solid state version of the dynatron oscillator which normally uses a tetrode thermionic valve (vacuum tube).
See also[edit]
Electronic components
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https://en.wikipedia.org/wiki/Tunnel_diode
An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine waveor a square wave or a triangle wave.[1][2][3] Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. They are widely used in many electronic devices ranging from simplest clock generators to digital instruments (like calculators) and complex computers and peripherals etc.[3] Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.[1]
Oscillators are often characterized by the frequency of their output signal:
- A low-frequency oscillator (LFO) is an electronic oscillator that generates a frequency below approximately 20 Hz. This term is typically used in the field of audio synthesizers, to distinguish it from an audio frequency oscillator.
- An audio oscillator produces frequencies in the audio range, about 16 Hz to 20 kHz.[2]
- An RF oscillator produces signals in the radio frequency (RF) range of about 100 kHz to 100 GHz.[2]
In AC power supplies, an oscillator that produces AC power from a DC supply is usually called an inverter. Before the advent of diode-based rectifiers, an electromechanical device that similarly converted AC power to DC was called a converter,[4] though the term is now used more commonly to refer to DC-DC buck converters.
There are two main types of electronic oscillator – the linear or harmonic oscillator and the nonlinear or relaxation oscillator.[2][5]
Crystal oscillators are ubiquitous in modern electronics and produce frequencies from 32 kHz to over 150 MHz, with 32 kHz crystals commonplace in time keeping and the higher frequencies commonplace in clock generation and RF applications.
https://en.wikipedia.org/wiki/Electronic_oscillator
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively.[1][2][3][4][5] Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF (millimeter wave) bands. A more common definition in radio-frequency engineering is the range between 1 and 100 GHz (wavelengths between 0.3 m and 3 mm).[2] In all cases, microwaves include the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.
The prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range. Rather, it indicates that microwaves are "small" (having shorter wavelengths), compared to the radio waves used prior to microwave technology. The boundaries between far infrared, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study.
Microwaves travel by line-of-sight; unlike lower frequency radio waves they do not diffract around hills, follow the earth's surface as ground waves, or reflect from the ionosphere, so terrestrial microwave communication links are limited by the visual horizon to about 40 miles (64 km). At the high end of the band, they are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer. Microwaves are widely used in modern technology, for example in point-to-point communication links, wireless networks, microwave radio relay networks, radar, satellite and spacecraft communication, medical diathermy and cancer treatment, remote sensing, radio astronomy, particle accelerators, spectroscopy, industrial heating, collision avoidance systems, garage door openers and keyless entry systems, and for cooking food in microwave ovens.
https://en.wikipedia.org/wiki/Microwave
Coaxial cable, or coax (pronounced /ˈkoʊ.æks/) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric (insulating material); many coaxial cables also have a protective outer sheath or jacket. The term "coaxial" refers to the inner conductor and the outer shield sharing a geometric axis.
Coaxial cable is a type of transmission line, used to carry high-frequency electrical signalswith low losses. It is used in such applications as telephone trunklines, broadband internetnetworking cables, high-speed computer data busses, cable television signals, and connecting radio transmitters and receivers to their antennas. It differs from other shielded cables because the dimensions of the cable and connectors are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a transmission line.
Coaxial cable was used in the first (1858) and following transatlantic cable installations, but its theory wasn't described until 1880 by English physicist, engineer, and mathematician Oliver Heaviside, who patented the design in that year (British patent No. 1,407).[1]
https://en.wikipedia.org/wiki/Coaxial_cable
Microstrip is a type of electrical transmission line which can be fabricated with any technology where a conductor is separated from a ground plane by a dielectric layer known as the substrate. Microstriplines are used to convey microwave-frequency signals.
Typical realisation technologies are printed circuit board, alumina coated with a dielectric layer or sometimes silicon or some other similar technologies. Microwave components such as antennas, couplers, filters, power dividers etc. can be formed from microstrip, with the entire device existing as the pattern of metallization on the substrate. Microstrip is thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact. Microstrip was developed by ITT laboratories as a competitor to stripline (first published by Grieg and Engelmann in the December 1952 IRE proceedings[1]).
The disadvantages of microstrip compared with waveguide are the generally lower power handling capacity, and higher losses. Also, unlike waveguide, microstrip is typically not enclosed, and is therefore susceptible to cross-talk and unintentional radiation.
For lowest cost, microstrip devices may be built on an ordinary FR-4 (standard PCB) substrate. However it is often found that the dielectric losses in FR4 are too high at microwave frequencies, and that the dielectric constant is not sufficiently tightly controlled. For these reasons, an alumina substrate is commonly used. From monolithic integration perspective microtrips with integrated circuit/monolithic microwave integrated circuit technologies might be feasible however their performance might be limited by the dielectric layer(s) and conductor thickness available.
Microstrip lines are also used in high-speed digital PCB designs, where signals need to be routed from one part of the assembly to another with minimal distortion, and avoiding high cross-talk and radiation.
Microstrip is one of many forms of planar transmission line, others include stripline and coplanar waveguide, and it is possible to integrate all of these on the same substrate.
A differential microstrip—a balanced signal pair of microstrip lines—is often used for high-speed signals such as DDR2 SDRAM clocks, USB Hi-Speed data lines, PCI Express data lines, LVDS data lines, etc., often all on the same PCB.[2][3][4] Most PCB design tools support such differential pairs.[5][6]
https://en.wikipedia.org/wiki/Microstrip
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmission must be taken into account. This applies especially to radio-frequency engineering because the short wavelengths mean that wave phenomena arise over very short distances (this can be as short as millimetres depending on frequency). However, the theory of transmission lines was historically developed to explain phenomena on very long telegraph lines, especially submarine telegraph cables.
Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas (they are then called feed lines or feeders), distributing cable television signals, trunklines routing calls between telephone switching centres, computer network connections and high speed computer data buses. RF engineers commonly use short pieces of transmission line, usually in the form of printed planar transmission lines, arranged in certain patterns to build circuits such as filters. These circuits, known as distributed-element circuits, are an alternative to traditional circuits using discrete capacitors and inductors.
Ordinary electrical cables suffice to carry low frequency alternating current (AC) and audio signals. However, they cannot be used to carry currents in the radio frequency range above about 30 kHz, because the energy tends to radiate off the cable as radio waves, causing power losses. RF currents also tend to reflect from discontinuities in the cable such as connectors and joints, and travel back down the cable toward the source. These reflections act as bottlenecks, preventing the signal power from reaching the destination. Transmission lines use specialized construction, and impedance matching, to carry electromagnetic signals with minimal reflections and power losses. The distinguishing feature of most transmission lines is that they have uniform cross sectional dimensions along their length, giving them a uniform impedance, called the characteristic impedance, to prevent reflections. The higher the frequency of electromagnetic waves moving through a given cable or medium, the shorter the wavelength of the waves. Transmission lines become necessary when the transmitted frequency's wavelength is sufficiently short that the length of the cable becomes a significant part of a wavelength.
At microwave frequencies and above, power losses in transmission lines become excessive, and waveguides are used instead which function as "pipes" to confine and guide the electromagnetic waves. At even higher frequencies, in the terahertz, infrared and visible ranges, waveguides in turn become lossy, and optical methods, (such as lenses and mirrors), are used to guide electromagnetic waves.
https://en.wikipedia.org/wiki/Transmission_line
In radio-frequency engineering and communications engineering, waveguide is a hollow metal pipe used to carry radio waves.[1] This type of waveguide is used as a transmission line mostly at microwave frequencies, for such purposes as connecting microwave transmitters and receivers to their antennas, in equipment such as microwave ovens, radar sets, satellite communications, and microwave radio links.
The electromagnetic waves in a (metal-pipe) waveguide may be imagined as travelling down the guide in a zig-zag path, being repeatedly reflected between opposite walls of the guide. For the particular case of rectangular waveguide, it is possible to base an exact analysis on this view. Propagation in a dielectric waveguide may be viewed in the same way, with the waves confined to the dielectric by total internal reflection at its surface. Some structures, such as non-radiative dielectric waveguides and the Goubau line, use both metal walls and dielectric surfaces to confine the wave.
https://en.wikipedia.org/wiki/Waveguide_(radio_frequency)
In electronics/spintronics, a tunnel junction is a barrier, such as a thin insulating layer or electric potential, between two electrically conducting materials. Electrons (or quasiparticles) pass through the barrier by the process of quantum tunnelling. Classically, the electron has zero probability of passing through the barrier. However, according to quantum mechanics, the electron has a non-zero wave amplitude in the barrier, and hence it has some probability of passing through the barrier. Tunnel junctions serve a variety of different purposes.
https://en.wikipedia.org/wiki/Tunnel_junction
In electronics, the dynatron oscillator, invented in 1918 by Albert Hull[1][2] at General Electric, is an obsolete vacuum tube electronic oscillator circuit which uses a negative resistance characteristic in early tetrode vacuum tubes, caused by a process called secondary emission.[3][4][5][6] It was the first negative resistance vacuum tube oscillator.[7] The dynatron oscillator circuit was used to a limited extent as beat frequency oscillators (BFOs), and local oscillators in vacuum tube radio receivers as well as in scientific and test equipment from the 1920s to the 1940s but became obsolete around World War 2 due to the variability of secondary emission in tubes.[8][9][10][11]
Negative transconductance oscillators,[8] such as the transitron oscillator invented by Cleto Brunetti in 1939,[12][13] are similar negative resistance vacuum tube oscillator circuits which are based on negative transconductance (a fall in current through one grid electrode caused by an increase in voltage on a second grid) in a pentode or other multigrid vacuum tube.[5][14] These replaced the dynatron circuit[14] and were employed in vacuum tube electronic equipment through the 1970s.[8][10][11]
https://en.wikipedia.org/wiki/Dynatron_oscillator
In semiconductor devices, a backward diode (also called back diode[2]) is a variation on a Zener diode or tunnel diode having a better conduction for small reverse biases (for example –0.1 to –0.6 V) than for forward bias voltages.
The reverse current in such a diode is by tunneling, which is also known as the tunnel effect.[3][4][5]
Current–voltage characteristics of backward diode[edit]
The forward I–V characteristic is the same as that of an ordinary P–N diode. The breakdown starts when reverse voltage is applied. In the case of Zener breakdown, it starts at a particular voltage. In this diode the voltage remains relatively constant (independent of current) when it is connected in reverse bias. The backward diode is a special form of tunnel diode in which the tunneling phenomenon is only incipient, and the negative resistance region virtually disappears. The forward current is very small and becomes equivalent to the reverse current of a conventional diode.
Applications of backward diodes[edit]
- Detector
- Since it has low capacitance and no charge storage effect,[4] and a strongly nonlinear small-signal characteristic, the backward diode can be used as a detector up to 40 GHz.
- Rectifier
- A backward diode can be used for rectifying weak signals with peak amplitudes of 0.1 to 0.7 V.
- Switch
- A backward diode can be used in high speed switching applications.
References[edit]
- ^ Stanley William Amos, Roger S. Amos (1999). Newnes Dictionary of Electronics. Newnes. ISBN 0-7506-4331-5.
- ^ Paul Horowitz, Winfield Hill (1989). The Art of Electronics, 2nd edition. p. 891.
- ^ Anwar A. Khan and Kanchan K. Dey (2006). A First Course in Electronics. Prentice Hall of India. ISBN 81-203-2776-4.
- ^ a b S.L. Kakani (2004). Electronics Theory and Applications. New Age Intl. Ltd. ISBN 81-224-1536-9.
- ^ Karlheinz Seeger (2004). Semiconductor Physics: An Introduction. Springer. ISBN 3-540-21957-9.
https://en.wikipedia.org/wiki/Backward_diode
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