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Microwave components and devices

The electromagnetic spectrum is a series of frequencies ranging from radio waves to microwaves, visible light, X-rays, and gamma rays. As the wavelength of the electromagnetic radiation shortens, the waves have a higher frequency—how quickly electromagnetic waves follow each other—and therefore more energy.

Radiofrequency is a type of electromagnetic wave that originates in the lower range of the electromagnetic spectrum whereas microwave is one of the higher radiofrequency divisions. RF refers to radio frequencies lower than 1 gigahertz, and microwave refers to radio frequencies from 1 gigahertz to 300 gigahertz.

RF and microwave engineering has innumerable applications, from radar (e.g. for air traffic control and meteorology) through electro-heat applications (e.g. in paper manufacture and domestic microwave ovens), to radiometric remote sensing of the environment, continuous process measurements and non-destructive testing


Common Microwave components

But, whether designing wireless connectivity devices, producing global positioning systems, or building microwave ovens, many manufacturers rely on a specific set of components commonly found in most microwave assemblies. These electronic parts influence a host of variables, such as bandwidth and frequency rate, and are necessary for the proper use and development of microwave products.


Transmission Lines: A transmission line is a connector that transmits energy from one point to another. The study of transmission line theory is helpful in the practical usage of power and equipment. There are basically four types of transmission lines −Two-wire parallel transmission lines; Coaxial lines; Strip type substrate transmission lines; and Waveguides

Waveguides: Microwaves propagate through microwave circuits, components and devices, which act as a part of Microwave transmission lines, broadly called Waveguides. A hollow metallic tube of the uniform cross-section for transmitting electromagnetic waves by successive reflections from the tube’s inner walls is called a Waveguide.

They are usually constructed with electrically conductive materials, such as copper or silver plating, but can also be formed of dielectric insulators if the interior walls are properly coated with conductors. A waveguide customarily consists of a circulator that provides a one-way channel for the signal, an attenuator that regulates signal strength, and an amplifier to offset transmission loss, along with numerous secondary components.

A few advantages of Waveguides are: Waveguides are easy to manufacture; They can handle very large power in kilowatts; Power loss is very negligible in waveguides. There are five types of waveguides: Rectangular waveguide, Circular waveguide, Elliptical waveguide, Single-ridged waveguide, and Double-ridged waveguide

Connectors: Electrical connectors constitute a large portion of the microwave technology manufacturing market. They are conductive devices used to bridge electrical circuits and can serve as permanent joints between microwave components. Microwave connectors often form termination units in coaxial connections and provide housing and circuit board support. Most microwave assemblies use them in 50 or 70 ohm settings.

Some common connector types include:

SMA: The Sub-Miniature Version A (SMA) connector is a coaxial cable device that functions on the multi-megahertz or gigahertz frequency range. The base design has a 4.2-millimeter diameter and can withstand maximum frequencies from 18 to 26 gigahertz. SMA connectors can also be coated with gold or stainless steel plated heads. SMB and SMC connectors are smaller versions of the SMA.
Type N: This is one of the oldest types of coaxial cable connectors suitable for microwave transmissions. N connectors can usually handle between 10 and 12 gigahertz maximum frequency and are most often found in communication and cable television systems.
GPO: A GPO is a type of push-on connector that is typically used when threaded connections are impractical. It has a non-threaded lock design, often coupled with a spring mechanism to improve alignment. GPO connectors are generally easier to apply and remove than other connectors, as they require no additional tools for installation.

Absorbers: Microwave absorbers help to convert electromagnetic wave discharges into heat units and, rather than reflecting waves, absorb unwanted energy in order to dissipate it. They are usually produced with carbon-based foam, die-cut elastomers, or thermoplastic materials. Absorbers are often used to offset design or production errors in microwave systems, but they can also be a valuable addition because they provide improved signal functions by addressing antenna pattern problems and frequency interference.

Absorbers are commonly measured by their capacity for attenuation, which refers to the signal strength reduction in decibels. They tend to be lightweight and most can handle frequencies up to 40 gigahertz. Absorbers can also be specified for broadband or narrowband performance, corrosion resistance, die-cut or molded fabrication, and a variety of different frequency ranges. Absorbers have numerous applications in the military industry, particularly radar systems, as well as telecommunications, aerospace, medical equipment, and automobile manufacturing.

Varactor Diode: A voltage variable capacitance of a reverse biased junction can be termed as a Varactor diode. Varactor diode is a semi-conductor device in which the junction capacitance can be varied as a function of the reverse bias of the diode. Varactor diodes are used in the  applications such as Up conversion; Parametric amplifier; Pulse generation; Pulse shaping; Switching circuits; and Modulation of microwave signals. Varactor diodes may be used as phase shifters by taking advantage of their variable reactance. The non-linear response of varactor capacitance may be used for harmonic generation

Schottky Barrier Diode: The Schottky junction is a metal-semiconductor hetero-junction. This is a simple diode that exhibits non-linear impedance. These diodes are mostly used for microwave detection and mixing. The application of Schottky diodes as varactors is similar to that of P-N junctions although, due to the smaller breakdown voltages of the former, smaller capacitance variation ranges are generally obtained. Other applications of the Schottky diode stem from the non-linear characteristic of its equivalent current source. Among them, applications as detectors and frequency mixers are the most usual.

Detectors: A detector can be used to demodulate a microwave signal or to measure its power and microwave detectors are often based on the use of a Schottky diode.

PIN Diodes: A PIN diode is obtained by connecting a highly doped P+layer of semiconductor, a long intrinsic (I) layer and a highly doped N+ layer. The presence of a long intrinsic section increases the breakdown voltage of the device thus allowing high reverse voltages. This is advantageous when handling high input power. ‘PINs’ being often used as variable resistances or variable attenuators. For a relatively high value of forward bias, the resistance of the intrinsic section is noticeably reduced. Switching applications, between reverse biased (no conduction or high resistance) state and forward biased (good conduction or low resistance) state, are also possible.

Gunn Diodes: Gunn diodes are manufactured using III–V compound semiconductors, such as GaAs or PIn (Phosphorus-Indium). They exhibit negative resistance, related to the multi-valley nature of their conduction bands. Gunn devices are used, principally, in the design of negative resistance amplifiers and oscillators. The main attractions of Gunn-based oscillators are their capability to operate over a large frequency band, their low noise performance and their high output power. Gunn oscillations have been obtained up to frequencies of about 150 GHz and output powers of 15 mW at 100 GHz can be realised.

IMPATT Diodes: IMPATT (Impact Avalanche and Transit Time) diodes are very powerful microwave sources, providing the highest (solid state device) output power in the millimetre-wave frequency range. They exhibit a dynamic negative resistance based on transit time effects and they are often used in the design of oscillators and amplifiers when high output power is required. They are manufactured in Si, GaAs and InP.

Microwave Transistors: For the microwave circuit designer transistors are the key active elements most often used to achieve signal generation (oscillators), signal amplification and a wide range of other, more complex, switching and signal processing functions.There is a need to develop special transistors to tolerate the microwave frequencies. Hence for microwave applications, silicon n-p-n transistors that can provide adequate powers at microwave frequencies have been developed. They are with typically 5 watts at a frequency of 3GHz with a gain of 5dB. GaAs MESFETs : The GaAs MESFET (metal semiconductor field effect transistor) is widely used at microwave frequencies in the realisation of oscillators, amplifiers, mixers, etc.

HBTs: The heterojunction bipolar transistor (HBT) has high transconductance and output resistance, high power handling capability, and high breakdown voltage. It is possible to obtain low base resistance combined with wide emitter terminal dimensions and, as a consequence, very high operating frequency can be achieved. This makes these devices attractive for high power microwave and millimeter-wave applications.

References and Resources also include:[Ian_A._Glover,_Steve_Pennock,_Peter_Shepherd]_Mic.pdf


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