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Microwave Devices

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Introduction

Microwaves have frequencies > 1 GHz approx.

Stray reactances are more important as frequency

increases Transmission line techniques must be applied to

short conductors like circuit board traces

Device capacitance and transit time are important Cable losses increase: waveguides often used

instead

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Waveguides

Pipe through which waves propagate

Can have various cross sections

Rectangular

Circular

Elliptical

Can be rigid or flexible

Waveguides have very low loss

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Modes

Waves can propagate in various ways

Time taken to move down the guide varies

with the mode

Each mode has a cutoff frequency below

which it wont propagate

Mode with lowest cutoff frequency is

dominant mode

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Mode Designations

TE: transverse electric

Electric field is at right angles to direction of

travel TM: transverse magnetic

Magnetic field is at right angles to direction oftravel

TEM: transverse electromagnetic

Waves in free space are TEM

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Rectangular Waveguides

Dominant mode is TE101 half cycle along long dimension (a)

No half cycles along short dimension (b)

Cutoff fora = c/2

Modes with next higher cutoff frequency

are TE01 and TE20Both have cutoff frequency twice that for TE10

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Cutoff Frequency

For TE10 mmode in rectangular waveguide

with a = 2 b

a

cfc 2

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Usable Frequency Range

Single mode propagation is highly desirable

to reduce dispersion

This occurs between cutoff frequency for

TE10 mode and twice that frequency

Its not good to use guide at the extremes of

this range

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Example Waveguide

RG-52/U

Internal dimensions 22.9 by 10.2 mm

Cutoff at 6.56 GHz

Use from 8.2-12.5 GHz

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Group Velocity

Waves propagate at speed of light c in guide

Waves dont travel straight down guide

Speed at which signal moves down guide is

the group velocity and is always less than c

2

1

f

fcv cg

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Phase Velocity

Not a real velocity (>c)

Apparent velocity of wave along wall

Used for calculating wavelength in guide

For impedance matching etc.

2

1

f

fcv

c

p

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Characteristic Impedance

Z0 varies with frequency

2

0

1

377

ff

Z

c

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Guide Wavelength

Longer than free-space wavelength at same

frequency

2

1

ffc

g

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Impedance Matching

Same techniques as for coax can be used

Tuning screw can add capacitance or

inductance

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Coupling Power to Guides

3 common methods

Probe: at an E-field maximum

Loop: at an H-field maximum

Hole: at an E-field maximum

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Directional Coupler

Launches or receives power in only 1

direction

Used to split some of power into a secondguide

Can use probes or holes

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Passive Compenents

Bends

Called E-plane or H-Plane bends depending on

the direction of bending

Tees

Also have E and H-plane varieties

Hybrid or magic tee combines both and can beused for isolation

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Resonant Cavity

Use instead of a tuned circuit

Very high Q

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Attenuators and Loads

Attenuator works by putting carbon vane or

flap into the waveguide

Currents induced in the carbon cause loss

Load is similar but at end of guide

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Circulator and Isolator

Both use the unique properties of ferrites in

a magnetic field

Isolator passes signals in one direction,attenuates in the other

Circulator passes input from each port to the

next around the circle, not to any other port

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Microwave Solid-State Devices

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Microwave Transistors

Designed to minimize capacitances and

transit time

NPN bipolar and N channel FETs preferredbecause free electrons move faster than

holes

Gallium Arsenide has greater electronmobility than silicon

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Gunn Device

Slab of N-type GaAs (gallium arsenide)

Sometimes called Gunn diode but has no

junctions

Has a negative-resistance region where drift

velocity decreases with increased voltage

This causes a concentration of free electrons

called a domain

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Transit-time Mode

Domains move through the GaAs till they

reach the positive terminal

When domain reaches positive terminal itdisappears and a new domain forms

Pulse of current flows when domain

disappears

Period of pulses = transit time in device

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Gunn Oscillator Frequency

T=d/v

T = period of oscillation

d = thickness of device

v = drift velocity, about 1 105 m/s

f = 1/T

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IMPATT Diode

IMPATT stands for Impact Avalanche And

Transit Time

Operates in reverse-breakdown (avalanche) region Applied voltage causes momentary breakdown

once per cycle

This starts a pulse of current moving through the

device

Frequency depends on device thickness

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PIN Diode

P-type --- Intrinsic --- N-type

Used as switch and attenuator

Reverse biased - off

Forward biased - partly on to on depending

on the bias

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Varactor Diode

Lower frequencies: used as voltage-variable

capacitor

Microwaves: used as frequency multiplierthis takes advantage of the nonlinear V-I curve

of diodes

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YIG Devices

YIG stands for Yttrium - Iron - Garnet

YIG is a ferrite

YIG sphere in a dc magnetic field is used asresonant cavity

Changing the magnetic field strength

changes the resonant frequency

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Microwave Tubes

Used for high power/high frequencycombination

Tubes generate and amplify high levels ofmicrowave power more cheaply than solidstate devices

Conventional tubes can be modified for lowcapacitance but specialized microwavetubes are also used

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Magnetron

High-power oscillator

Common in radar and microwave ovens

Cathode in center, anode around outside Strong dc magnetic field around tube causes

electrons from cathode to spiral as they

move toward anode Current of electrons generates microwaves

in cavities around outside

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Slow-Wave Structure

Magnetron has cavities all around the outside

Wave circulates from one cavity to the next

around the outside Each cavity represents one-half period

Wave moves around tube at a velocity much less

than that of light

Wave velocity approximately equals electron

velocity

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Duty Cycle

Important for pulsed tubes like radar

transmitters

Peak power can be much greater thanaverage power

DPP

T

TD

Pavg

T

on

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Crossed-Field and Linear-Beam

Tubes Magnetron is one of a number of crossed-

field tubes

Magnetic and electric fields are at right angles

Klystrons and Traveling-Wave tubes are

examples of linear-beam tubes

These have a focused electron beam (as in aCRT)

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Klystron

Used in high-power amplifiers

Electron beam moves down tube past

several cavities.

Input cavity is the buncher, output cavity is

the catcher.

Bunchermodulates the velocity of theelectron beam

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Velocity Modulation

Electric field from microwaves at buncher

alternately speeds and slows electron beam

This causes electrons to bunch up

Electron bunches at catcher induce

microwaves with more energy

The cavities form a slow-wave structure

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Traveling-Wave Tube (TWT)

Uses a helix as a slow-wave structure

Microwaves input at cathode end of helix,

output at anode end

Energy is transferred from electron beam to

microwaves

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Microwave Antennas

Conventional antennas can be adapted to

microwave use

The small wavelength of microwavesallows for additional antenna types

The parabolic dish already studied is a

reflector not an antenna but we saw that it ismost practical for microwaves

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Horn Antennas

Not practical at low frequencies because of

size

Can be E-plane, H-plane, pyramidal orconical

Moderate gain, about 20 dBi

Common as feed antennas for dishes

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Slot Antenna

Slot in the wall of a waveguide acts as an

antenna

Slot should have length g/2 Slots and other basic antennas can be

combined into phased arrays with many

elements that can be electrically steered

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Fresnel Lens

Lenses can be used for radio waves just as

for light

Effective lenses become small enough to bepractical in the microwave region

Fresnel lens reduces size by using a stepped

configuration

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Radar

Radar stands for Radio Dedtection And

Ranging

Two main typesPulse radar locates targets by measuring time

for a pulse to reflect from target and return

Doppler radar measures target speed byfrequency shift of returned signal

It is possible to combine these 2 types

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Radar Cross Section

Indicates strength of returned signal from a

target

Equals the area of a flat conducting platefacing the source that reflects the same

amount of energy to the source

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Radar Equation

Expression for received power from a target

43

22

4 r

GPP TR

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Pulse Radar

Direction to target found with directional

antenna

Distance to target found from time taken forsignal to return from target

R = ct/2

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Maximum Range

Limited by pulse period

If reflection does not return before next

pulse is transmitted the distance to the targetis ambiguous

Rmax = cT/2

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Minimum Range

If pulse returns before end of transmitted

pulse, it will not be detected

Rmin = cTP/2 A similar distance between targets is

necessary to separate them

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Doppler Radar

Motion along line from radar to target

changes frequency of reflection

Motion toward radar raises frequency Motion away from radar lowers frequency

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Doppler Effect

cfvf ir

D

2

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Limitations of Doppler Radar

Only motion towards or away from radar is

measured accurately

If motion is diagonal, only the componentalong a line between radar and target is

measured

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Stealth

Used mainly by military planes, etc to avoid

detection

Avoid reflections by making the aircraftskin absorb radiation

Scatter reflections using sharp angles