Active Rf and Microwave Devices

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    01-Mar-2016

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ACTIVE RF AND MICROWAVE DEVICES

** What is a Schottky barrier diode (surface-barrier or hot-carrier diode)? -relies on a semiconductor-metal junction -operates at higher frequencies -generally use n-type GaAs material -often biased with a small DC forward current, but can be used without bias -primary application: frequency conversion of an input signal; switching power supplies** Basic Frequency Conversion Operations(1) Rectification

(2) Detection

(3) Mixing

**What are PIN Diodes and Control Circuits? current-controlled resistor at radio and microwave frequencies should ideally control the RF signal level without introducing distortion important feature: its ability to control large RF signals while using much smaller levels of dc excitation** What are Varactor Diodes? produces a junction capacitance that varies smoothly with bias voltage common application: to provide electronic frequency tuning of the local oscillator in a multichannel receiver generally made from Si for RF applications , and GaAs for microwave applications**What are Gunn Diodes? its operation is based on the transferred-electron mechanism has an I-V characteristic that exhibits a negative differential resistance ** What are IMPATT Diodes? impact avalanche and transit time operated with a relatively high voltage can be used to directly convert DC to RF power compared to Gunn diodes: generally noisier, have better temperature stability** What are Tunnel Diodes? a pn junction with a doping profile that allows electron tunneling**What are BARITT Diodes? barrier injection transit time diodes a transit time device generally has a lower power capability but of lower AM noise** What are BJTs? typically have very low 1/f noise characteristics preferred over FETs at frequencies below 2-4GHz because of higher gain, lower cost, and the possibility of biasing with a single supply upper frequency limit of the BJT is controlled primarily by the base length ** What are Heterojunction BJTs? has a base-emitter junction made from a compound semiconductor material can operate at frequencies exceeding 100GHz**What are FETs? monopolar can take many forms: MESFET, MOSFET, HEMT, PHEMT** What are MOSFETs? GaAs MESFETs can be used at frequencies well into the millimeter wave with high gain and low noise figure. gate junction is formed as Schottky barrier maximum frequency of operation is limited by the gate length can be used at UHF range and can provide powers of several hundred watts when devices are packaged in parallel** What is a high electron mobility transistor?

a heterojunction FET can operate at frequencies above 100 GHz also known as a MODFET (modulation-doped FET), a TEGFET (two-dimensional electron gas FET), and an SDFET (selectivity doped FET)

** What is a Pseudomorphic HEMT? uses a lattice mismatch between the layers can operate at frequencies above 100 GHz

MICROWAVE ACTIVE COMPONENTS

MICROWAVE OSCILLATOR**An oscillator is a nonlinear circuit that converts DC power to an AC waveform.** RF and microwave oscillators are found in all modern wireless communications, radar,and remote sensing systems to provide signal sources for frequency conversion and carriergeneration. ** A solid-state oscillator uses an active nonlinear device, such as a diode or transistor, in conjunction with a passive circuit to convert DC to a sinusoidal steady-state RF signal. ** Basic transistor oscillator circuits can generally be used at low frequencies, often with crystal resonators to provide improved frequency stability and low noise performance. **At higher frequencies, diodes or transistors biased to a negative resistance operating point canbe used with cavity, transmission line, or dielectric resonators to produce fundamental frequencyoscillations up to 100 GHz.** Alternatively, frequency multipliers, in conjunction with a lower frequency source, can be used to produce power at millimeter wave frequencies.MICROWAVE MIXER**A mixer is a three-port device that uses a nonlinear or time-varying element to achieve frequency conversion. ** They are designed to yield both, a sum and a difference frequency at a single output port when two distinct input frequencies are inserted into the other two ports. **Mixers are used for frequency conversion and are critical components in modern radio frequency (RF) systems. A mixer converts RF power at one frequency into power at another frequency to make signal processing easier and also inexpensive. A fundamental reason for frequency conversion is to allow amplification of the received signal at a frequency other than the RF, or the audio, frequency.

Superposition of a strong local oscillator (LO) signal with a weaker radio frequency (RF) signal on nonlinear device generates an intermediate frequency IF = LO RF.

MICROWAVE TUBES- also known as millimeter wave tubes.- widely used for generation and amplification of RF signals.

Radar systems generally require a relatively high-power source, sometimes as high as 1-10kW. Electronic warfare systems use sources with powers in the range of 100W to 1kW with the additional requirement for tenability over a wide bandwidth. Microwave systems requires single frequency, high power source in the range of 700W.

KLYSTRON- a specializedlinear-beamvacuum tube that can be used as either an amplifier or an oscillator.- invented in 1937 by American electrical engineersRussel and Sigurd Varian.

** The REFLEX KLYSTRON is mostly used as a microwave oscillator; it works as a local oscillator in the microwave receiver circuits. It also works as the R.F generator in the microwave transmitter circuits.

** A TRAVELING TUBE (TWT)is a specializedvacuum tubethat is used inelectronicsto amplifyradio frequency(RF) signals in the microwaverange.The TWT belongs to a category of "linear beam" tubes, such as theklystron, in which the radio wave is amplified by absorbing power from a beam ofelectronsas it passes down the tube.The TWT was invented by Rudolf Kompfner in 1942 in England.The classic tube uses a helix, but coupled cavity are also used. ** A Magnetronis a high-poweredvacuum tubethat generatesmicrowavesusing the interaction of a stream ofelectronswith a magnetic fieldwhile moving past a series of open metal cavities (cavity resonators). Bunches of electrons passing by the openings to the cavities excite radio wave oscillations in the cavity, much as a guitar's strings excite sound in its sound box. ATTENUATOR_ An electronic device that reduces the power of a signal without distorting its waveform Passive resistive elements that do the opposite of amplifiers, they kill gain An attenuator introduces a known amount of loss when functioning between two resistive impedances: Input (Load) Impedance Zin and Output (Source) Impedance Zout. This intentional loss in the circuit could be in order to protect delicate electronics, or as a trade-off to maximize power transferred by the circuit 1. Use in reducing voltage and in dissipating power A measuring device will employ an attenuator when measuring signals to lower the amplitude of a signal. This protects the device from possibly damaging signal levels. 2. Use in improving impedance matching When the impedance of the load (Input Impedance Zin) is set equal to the impedance of the source (Output Impedance Zout), the power transfer in the circuit will be maximized, and reflections from the load are minimized. This is also called also calledreflectionless matchingorbroadband matching. 3. Temperature compensation Ideally, a circuit's performance should not be temperature dependent. Amplifiers may exhibit reduced gain as their temperature rises. A temperature compensating attenuator can dissipate power in response to temperature changewith minimal attenuation at high temperatures, and maximal attenuation at low temperatures--to set off the amplifier's temperature dependence. The resulting circuit will thereby behave more uniformly under a wider range of temperatures. 4. Measuring gain/loss of a device An attenuator can be used to measure the insertion loss.RESONATORS is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical (including acoustic). are used to either generate waves of specific frequencies from a signal. Musical instruments use acoustic resonators that produce sound waves of specific tunes.

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