Microwave Devices

IntroductionMicrowaves have frequencies > 1 GHz approx.Stray reactances are more important as frequency increasesTransmission line techniques must be applied to short conductors like circuit board tracesDevice capacitance and transit time are importantCable losses increase: waveguides often used instead

WaveguidesPipe through which waves propagateCan have various cross sectionsRectangularCircularEllipticalCan be rigid or flexibleWaveguides have very low loss

ModesWaves can propagate in various waysTime taken to move down the guide varies with the modeEach mode has a cutoff frequency below which it wont propagateMode with lowest cutoff frequency is dominant mode

Mode DesignationsTE: transverse electricElectric field is at right angles to direction of travelTM: transverse magneticMagnetic field is at right angles to direction of travelTEM: transverse electromagneticWaves in free space are TEM

Rectangular WaveguidesDominant mode is TE101 half cycle along long dimension (a)No half cycles along short dimension (b)Cutoff for a = c/2Modes with next higher cutoff frequency are TE01 and TE20Both have cutoff frequency twice that for TE10

Cutoff FrequencyFor TE10 mmode in rectangular waveguide with a = 2 b

Usable Frequency RangeSingle mode propagation is highly desirable to reduce dispersion This occurs between cutoff frequency for TE10 mode and twice that frequencyIts not good to use guide at the extremes of this range

Example WaveguideRG-52/UInternal dimensions 22.9 by 10.2 mmCutoff at 6.56 GHzUse from 8.2-12.5 GHz

Group VelocityWaves propagate at speed of light c in guideWaves dont travel straight down guideSpeed at which signal moves down guide is the group velocity and is always less than c

Phase VelocityNot a real velocity (>c)Apparent velocity of wave along wall Used for calculating wavelength in guideFor impedance matching etc.

Characteristic ImpedanceZ0 varies with frequency

Guide WavelengthLonger than free-space wavelength at same frequency

Impedance MatchingSame techniques as for coax can be usedTuning screw can add capacitance or inductance

Coupling Power to Guides3 common methodsProbe: at an E-field maximumLoop: at an H-field maximumHole: at an E-field maximum

Directional CouplerLaunches or receives power in only 1 directionUsed to split some of power into a second guideCan use probes or holes

Passive CompenentsBendsCalled E-plane or H-Plane bends depending on the direction of bendingTeesAlso have E and H-plane varietiesHybrid or magic tee combines both and can be used for isolation

Resonant CavityUse instead of a tuned circuitVery high Q

Attenuators and LoadsAttenuator works by putting carbon vane or flap into the waveguideCurrents induced in the carbon cause lossLoad is similar but at end of guide

Circulator and IsolatorBoth use the unique properties of ferrites in a magnetic fieldIsolator passes signals in one direction, attenuates in the otherCirculator passes input from each port to the next around the circle, not to any other port

Microwave Solid-State Devices

Microwave TransistorsDesigned to minimize capacitances and transit timeNPN bipolar and N channel FETs preferred because free electrons move faster than holesGallium Arsenide has greater electron mobility than silicon

Gunn DeviceSlab of N-type GaAs (gallium arsenide)Sometimes called Gunn diode but has no junctionsHas a negative-resistance region where drift velocity decreases with increased voltageThis causes a concentration of free electrons called a domain

Transit-time ModeDomains move through the GaAs till they reach the positive terminalWhen domain reaches positive terminal it disappears and a new domain formsPulse of current flows when domain disappearsPeriod of pulses = transit time in device

Gunn Oscillator FrequencyT=d/vT = period of oscillationd = thickness of devicev = drift velocity, about 1 105 m/sf = 1/T

IMPATT DiodeIMPATT stands for Impact Avalanche And Transit TimeOperates in reverse-breakdown (avalanche) regionApplied voltage causes momentary breakdown once per cycleThis starts a pulse of current moving through the deviceFrequency depends on device thickness

PIN DiodeP-type --- Intrinsic --- N-typeUsed as switch and attenuatorReverse biased - offForward biased - partly on to on depending on the bias

Varactor DiodeLower frequencies: used as voltage-variable capacitorMicrowaves: used as frequency multiplierthis takes advantage of the nonlinear V-I curve of diodes

YIG DevicesYIG stands for Yttrium - Iron - GarnetYIG is a ferriteYIG sphere in a dc magnetic field is used as resonant cavityChanging the magnetic field strength changes the resonant frequency

Dielectric Resonatorresonant cavity made from a slab of a dielectric such as aluminaMakes a good low-cost fixed-frequency resonant circuit

Microwave TubesUsed for high power/high frequency combinationTubes generate and amplify high levels of microwave power more cheaply than solid state devicesConventional tubes can be modified for low capacitance but specialized microwave tubes are also used

MagnetronHigh-power oscillatorCommon in radar and microwave ovensCathode in center, anode around outsideStrong dc magnetic field around tube causes electrons from cathode to spiral as they move toward anodeCurrent of electrons generates microwaves in cavities around outside

Slow-Wave StructureMagnetron has cavities all around the outsideWave circulates from one cavity to the next around the outsideEach cavity represents one-half periodWave moves around tube at a velocity much less than that of lightWave velocity approximately equals electron velocity

Duty CycleImportant for pulsed tubes like radar transmittersPeak power can be much greater than average power

Crossed-Field and Linear-Beam TubesMagnetron is one of a number of crossed-field tubesMagnetic and electric fields are at right anglesKlystrons and Traveling-Wave tubes are examples of linear-beam tubesThese have a focused electron beam (as in a CRT)

KlystronUsed in high-power amplifiersElectron beam moves down tube past several cavities.Input cavity is the buncher, output cavity is the catcher.Buncher modulates the velocity of the electron beam

Velocity ModulationElectric field from microwaves at buncher alternately speeds and slows electron beamThis causes electrons to bunch upElectron bunches at catcher induce microwaves with more energy The cavities form a slow-wave structure

Traveling-Wave Tube (TWT)Uses a helix as a slow-wave structureMicrowaves input at cathode end of helix, output at anode endEnergy is transferred from electron beam to microwaves

Microwave AntennasConventional antennas can be adapted to microwave useThe small wavelength of microwaves allows for additional antenna typesThe parabolic dish already studied is a reflector not an antenna but we saw that it is most practical for microwaves

Horn AntennasNot practical at low frequencies because of sizeCan be E-plane, H-plane, pyramidal or conical Moderate gain, about 20 dBiCommon as feed antennas for dishes

Slot AntennaSlot in the wall of a waveguide acts as an antennaSlot should have length g/2Slots and other basic antennas can be combined into phased arrays with many elements that can be electrically steered

Fresnel LensLenses can be used for radio waves just as for lightEffective lenses become small enough to be practical in the microwave regionFresnel lens reduces size by using a stepped configuration

RadarRadar stands for Radio Dedtection And Ranging Two main typesPulse radar locates targets by measuring time for a pulse to reflect from target and returnDoppler radar measures target speed by frequency shift of returned signalIt is possible to combine these 2 types

Radar Cross SectionIndicates strength of returned signal from a targetEquals the area of a flat conducting plate facing the source that reflects the same amount of energy to the source

Radar EquationExpression for received power from a target

Pulse RadarDirection to target found with directional antennaDistance to target found from time taken for signal to return from targetR = ct/2

Maximum RangeLimited by pulse periodIf reflection does not return before next pulse is transmitted the distance to the target is ambiguousRmax = cT/2

Minimum RangeIf pulse returns before end of transmitted pulse, it will not be detectedRmin = cTP/2A similar distance between targets is necessary to separate them

Doppler RadarMotion along line from radar to target changes frequency of reflectionMotion toward radar raises frequencyMotion away from radar lowers frequency

Doppler Effect

Limitations of Doppler RadarOnly motion towards or away from radar is measured accuratelyIf motion is diagonal, only the component along a line between radar and target is measured

StealthUsed mainly by military planes, etc to avoid detectionAvoid reflections by making the aircraft skin absorb radiationScatter reflections using sharp angles

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