transmission lines data communication systems

7/29/2019 Transmission Lines DATA COMMUNICATION SYSTEMS

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Metallic Cable Transmission Media

Introduction

Transmission media-It is included in the lowest layer of the OSI protocol hierarchy the physical layer.

-Transmission medium is simply the path between a transmitter and a receiver in a communications system.Guided Transmission media-some form of conductor that provide conduit in which signals are contained-the conductor directs the signalexamples: copper wire, optical fiberUnguided Transmission media-wireless systems without physical conductor-signals are radiated through air or vacuum-direction depends on which direction the signal is emittedexamples: air, free spaceCable transmission media

-guided transmission medium and can be any physical facility used to propagate EM signals between twlocationse.g.: metallic cables (open wire, twisted pair), optical cables (plastic, glass core)Incident and Reflected wave-Incident voltage is the voltage that propagates from sources towards the load-Reflected wave is the voltage that propagates from the load towards the source.

Transmission line classifications

1) Balanced Transmission line- two wire balanced line.- both conductors carry current. But only one conductor carries signals.

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2) Unbalanced Transmission line- One wire is at ground potential and the other wire is at signal potential- advantage only one wire for each signal- disadvantage reduced immunity to noises

BalunsBalanced transmission lines connected to unbalanced transmission linese.g.: coaxial cable to be connected to antenna

Metallic Transmission Line Types

1)Parallel conductors

2)Coaxial cable

1)Parallel conductors consists of two or more metallic conductors(copper) separated by insulatorair,rubbetc.Most common -- Open Wire

Twin leadTwisted Pair (UTP & STP)

Open Wire- two-wire parallel conductors-Closely spaces by air-Non conductive spaces support and constant distance between conductors (2-6 inches)-Adv simple construction-Disadv no shielding, high radiation loss, crosstalkapplication standard voice grade telephone

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Twin lead-spacers between the two conductor are replaced with continuous dielectric uniform spacing-application to connect TV to rooftop antennas-material used for dielectric Teflon, polyethylene

Twisted pair--(UTP & STP)-formed by twisting two insulated conductors around each other-Neighboring pairs is twisted each other to reduce EMI and RFI from external sources-reduce crosstalk between cable pairs

----Unshielded Twisted Pair-two copper wire encapsulated in PVC-twisted to reduce crosstalk and interference-improve the bandwidth significantly-Used for telephone systems and local area network

Level 1 (Category 1) -- ordinary thin cables--for voice grade telephone and low speed data

Level 2 (Category 2) -- Better than category 1--For token ring LAN at txn. rate of 4 MbpsCategory 3 -- more stringent requirement than level 1 and 2

-- more immunity than crosstalk-- for token ring (16Mbps), 10Base T Ethernet (10Mbps)

Category 4 -- upgrade version of category 3-- tighter constraints for attenuation and crosstalk-- up to 100 Mbps

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Category 5 -- better attenuation and crosstalk characteristics-- used in modern LAN. Data up to 100Mbps

Category 5e -- enhanced category 5-- data speed up to 350 Mbps

Category 6 -- data speed up to 550 Mbps

-- fabricated with closer tolerances and use more advance connectorsShielded Twisted Pair (STP)-- wires and dielectric are enclosed in a conductive metal sleeve called foil or mesh called braid-- the sleeve connected to ground acts as shield prevent the signal radiating beyond the boundaries

STP CategoryCategory 5e -- Feature individually shielded pairs of twisted wireCategory 7 -- 4 pairs

-- surrounded by common metallic foil shield and shielded foil twisted pair-- 1Gbps

Foil twisted pair -- Four pairs of 24-AWG copper wires encapsulated in a common metallic-foil shield withPVC outer sheath

-- to minimize EMI susceptibility while maximizing EMI immunity

-- > 1GbpsShielded-foil twisted pair-- Four pairs of 24-AWG copper wires surrounded by a common metallic-foil shield encapsulated in a braid

metallic shield-- offer superior EMI protection-- > 1Gbps2) Coaxial cable -- used for high data transmission-- coaxial reduce losses and isolate transmission path-- center conductor surrounded by insulation-- shielded by foil or braid

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Rigid air filled solid flexible

BNC Connectors

-- To connect coaxial cable to devices, it is necessary to use coaxial connectors.-- The most common type of connector is the Bayone-Neill-Concelman, or BNC, connectors.-- Types: BNC connector, BNC barrel, BNC T, Type-N, Type-N barrel.-- Applications include cable TV networks, and some traditional Ethernet LANs like 10Base-2, or 10-Base5.

Two-wire parallel transmission line electrical equivalent circuit

Characteristic Impedance of a Line- A terminated transmission line that is matched in its characteristic impedance is called a matched line- The characteristic impedance depends upon the electrical properties of the line, according to the formula:- The characteristic impedance can be calculated by using Ohms Law:

Zo = Eo / Io where Eo is source voltageIo is transmission line current

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The characteristic impedance for any type of transmission line can be calculated by calculating the inductan

and impedance per unit lengthFor a parallel line with an air the dielectric impedance is:

Zo = the characteristic impedance (ohms)D = the distance between the centersr = the radius of the conductorFor a Coaxial cable with an air the dielectric impedance is:

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r

DZ log2760

0

138log

r

DZ

d=

0r =

0

1

o

c

=


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Z0 = the characteristic impedance (ohms)D = the diameter of the outer conductord = the diameter of the inner conductor= the permittivity of the material

r = the relative permittivity or dielectric constant of the medium0 = the permeability of free space

For extremely high frequencies, characteristic impedance can be given byZo =

Wave propagation on Metallic transmission lines

Velocity factor and Dielectric constant--The ratio of the actual velocity of propagation of EM wave through a given medium to the velocity propagation through vacuum

Vf = velocity factorVp = actual velocity of propagationc = velocity of propagation in vacuum

rearranged equation

the velocity via txn. line depends on the dielectric constant of insulating material

r = dielectric constantThe velocity along txn. line varies with inductance and capacitance of the cable as

velocity x time = distance

Therefore,

normalized distance to 1 meter, Vp = velocity of propagation

LC = secondsL = inductanceC = capacitance

Q) For a coaxial cable with distributed capacitance C = 96.6 pf/H, Distributed inductance L = 241.56 nH/m,Relative dielectric constant.r = 2.3, determine the velocity of propagation and the velocity factor.

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CL /

p

f

VV

c=

f pV c V =

1p

rV =

T LC=distance

timepD

VT

= =

p

DV

LC=

1 meters

second

pV

LC

=


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Losses

Conductor Losses-- conductor heating loss - I2R power loss-- the loss varies depends on the length of the tx. LineDielectric Heating Losses

-- difference of potential between two conductors of a metallic txn line-- Negligible for air dielectric and increases with frequency for solid core tx line

Radiation Losses

-- the energy of electrostatic and EM field radiated from the wire and transfer to the nearby conductive materia--Reduced by shielding the cableCoupling Losses-- whenever connection is made between two tx line-- discontinuities due to mechanical connection where dissimilar material meets- tend to heat up, radiate energy and dissipate powerCorona-- luminous discharge that occurs between two conductors of transmission line-- when the difference of potential between lines exceeds the breakdown voltage of dielectric insulator

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Optical Fiber Communications

Optical fiber communications system is one that uses light as the carrier of informatioThe information carrying capacity of any electronic communications system is directly proportional bandwidth. Optical fibers have infinite bandwidth. So they have the capacity to carry much more informati

than metallic cables.

Advantages

1) Wider bandwidth and greater information capacity--Better than metallic cables, up to several thousand GHz--Speed up to several Gbps2)Immunity to crosstalk--glass fiber/plastic are non-conductor to electrical current--immune to adjacent cables3)Immunity to static interference--immune to static noise EMI, lightning etc.

4)Environmental Immunity--more resistant to environment, weather variations--wider temperature range operation--less affected by corrosive liquids and gases5)Safety and convenience--safer and easier to install and maintain--no current and voltage associated--no worry about explosion and fire caused--lighter and compact, flexible, lesser space required6)Lower transmission loss--lesser loss compared to metallic cables

--0.19 dB/km loss @ 1550 nm--amplifiers can be spaced more farther apart7)Security--virtually impossible to tap into a fiber cable8)Durability and reliability--last longer, higher tolerance to changes in environment and immune to corrosion9)Economics--Approximately the same cost as metallic cables--less loss between repeaters. Lower installation and overall systems costDisadvantages

1)Interfacing cost

--Optical cable transmission medium--Needs to be connected to standards electronics facilities often to be expensive2)Strength--lower tensile strength--can be improved with kevlar and protective jacket--glass fragile less required for portability3)Remote electrical power--need to be include electrical line within fiber cable for interfacing and signal regeneration

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4)Loss due to bending--bending causes irregularities in cable dimension the light escapes from fiber core loss of signal powerprone to manufacturing defect5)Specialized tools, equipment and training--tools to splice, repair cable

--test equipment for measurements--skilled technicians

Electromagnetic spectrum

Electromagnetic wavelength spectrum

Electromagnetic frequency spectrum

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1)Infrared: The band of light frequencies that is too high to be seen by the human eye with wavelengthranging from 770 nm to 106 nm. Optical fiber systems generally operate in infrared band.

2)Visible: The band of light frequencies to which the human eye will respond with wavelengths ranging fro390 nm to 770nm.3)Ultraviolet: The band of light frequencies that are too low to be seen by the human eye with wavelengt

ranging from 10 nm to 390

nm.

= wavelength (meters)Wavelength, c = velocity of light (300.000.000 meters per second)

f = frequency (hertz)

Optical Communication systems

Optical fiber communications link

In transmitter, the light source can be modulated by a digital or analog signal.The voltage-to-current converter serves as an electrical interface between the input circuitry and the ligsource.

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c

f=


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The light source is LED or ILD.The amount of light emitted is proportional to amount of drive current.The source-to-fiber coupler is a mechanical interface.In receiver, fiber-to-light detector coupling device is used to couple as much lightLight detector is generally a PIN (p-type intrinsic n-type) diode, an APD (avalanche photodiode). Detecto

convert light energy to current.A current-to-voltage converter is required to produce an output voltage proportional to the original sourinformation.Optical fiber construction

Optical fiber cable construction--Protective coating--special lacquer, silicone, or acrylate coating outside of cladding to seal and preserthe fibers strength, protects from moisture-- Buffer jacket additional cable strength against shocks--Strength members increase a tensile strength--Outer polyurethane jacketFiber cables either glass, plastic or both1) Plastic core and cladding (PCP)2) Glass core plastic cladding (PCS)3) Glass core glass cladding (SCS)--Plastic core more flexible - easier to install but higher attenuation than glass fiber not as good as glass--Glass core lesser attenuation best propagation characteristics but least ruggedSelection of fiber depends on its application trade off between economics and logistics of particulapplication.

The Physics of light

Einstein and Planck light behaves like EM wave and particles photon posses energy proportional to ifrequency.Plancks law:

When visible light or high-frequency electromagnetic radiation illuminates a metallic surface, electrons aemitted. The emitted electrons produce an electric current. Plancks law is expressed mathematically as

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--the lowest energy state grounds state--energy level above ground state excited state--if energy level decays to a lower level loss of energy is emitted as a photons of light--The process of decaying from one level to another spontaneous decay or spontaneous emission--Atoms can absorbs light energy and change its level to higher level absorption

Ep is the energy of photon measured in joules

Optical power

--flow of light energy past a given point in a specified time--also called radiant flux equal to joules per second

generally stated in decibel to define power level (dBm)

Question10 mW in dBm?

Velocity of Propagation

--in vacuum 3 x 108 m/s--but slower in a more dense material than free space--when it passes through different medium such from one medium to another denser material the ray chanits direction due to the change of speed--from less dense to more denser material the ray refracted closer to the normal--from more denser material to less denser material the ray refracted away from the normal

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energy of the photons

Planck constant

light frequency

pE

h

f

E hfp

=

=

=

=

p

p

E hf

hcE

=

=

2pE E=

= optical power

= instanteneous charge

= instanteneous change in time

(energy)(time)

P

dQ

dt

dPt

dQdt

=

=

10log1

PdBm

mW

=


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Refraction

--Occurs when the light travels between two different material density and changes its speed based on the ligfrequency

Refractive Index-- the ratio of the velocity of propagation of a light ray in free space to the velocity of propagation of a light rin a given material

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n = refractive index

c = speed of light

v = speed of light in a given material

cnv

=


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Snells Law

--how a light ray reacts when it meets the interface of two transmissive materials that have different indexes refraction

Refractive model for Snells lawAngle of incidence is the angle at which the propagating ray strike the interface with respect to the normalAngle of refraction is the angle formed between the propagating ray and the normal after the ray has enterthe 2nd medium

Questionmedium 1 glass = 1.5, medium 2 ethyl alcohol = 1.36, angle of incidence 30o

determine the angle of refraction?

Critical Angle

--the angle of incident ray in which the refracted ray is 90o and refracted along the interface--the minimum angle of incidence at which the refracted angle is 90o or greater--the light must travel from higher refractive index to a lesser refractive index material

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1

2

1

2

1 1 2 2

n = refractive index material 1

n = refractive index material 2

= angle of incidence

= angle of refraction

sin sinn n

=

21 2

1

2

2

1

1 2

1

sin sin

90

sin (1)

sin

c

c

nn

n

n

n

n

=

=

=

=


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Acceptance Angle

--the maximum angle in which external light rays may strike the air/glass interface and still propagate down tfiber

Numerical Aperture NA:

--to measure the magnitude of the acceptance angle--describe the light gathering or light-collecting ability of an optical fiber--the larger the magnitude of NA, the greater the amount of external light the fiber will accept

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(max)

0

1

2

= acceptance angle

= refractive index of air

= refractive index of fiber core

= refractive index of fiber cladding

2 21 21sin(max)

0

in

n

n

n

n n

in n

=

1 2 2sin 1 2(max) n nin=


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Propagation of light through an optical fiber cable

Modes of propagation

1) Single mode--only one path for light rays down the fiber2) Multimode--many higher order path rays down the fiber

Index Profile

--graphical presentation of the magnitude of the refractive index across the fiber--refractive index horizontal axis

--radial distance from core vertical axis1)step index single mode2)step index multimode3)graded index - multimode

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2 2

1 2

in

1

1

sin

= acceptance angle

NA = numerical aperture

n = refractive index fiber core

n = refractive index fiber cladding

1sin

inNA

NA n n

NAin

=

=

=


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Single Mode Step Index

--dominant widely used in telecommunication system and network--the core is significantly smaller in diameter than multimode fiberMultimode Mode Step Index

--similar to single mode step index fiber--but the core diameter is much larger--light enters the fiber follows many paths as it propagate down the fiber

--results in different time arrival for each of the pathMultimode Mode Graded Index--non-uniform refractive index decreases toward the outer edge--the light is guided back gradually to the center of the fiber

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Comparison

Single mode step index

(+) minimum dispersion same path propagation same time of arrival(+) wider bandwidth and higher information tx rate(-) small core hard to couple light into the fiber

(-) small line width of laser required(-) expensive difficult to manufactureMultimode step index

(+) relatively inexpensive, simple to manufacture(+) easier to couple light into the fiber(-) different path of rays different time arrival(-) less bandwidth and transfer rateMultimode graded index

-- intermediate characteristic between step index single and multimode

Losses in optical fiber

Attenuation--power loss reduction in the power of light wave as it travels down the cable--depends on signals wavelength--generally expressed as decibel loss per km, dB/km--effect on systems performance by reducing:1)systems bandwidth2)information transmission rate3)efficiency4)overall system capacity

Attenuation,

Optical power in watts measured at a given distance from a power source can be determined mathematically a

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(dB)

out

in

A = total reduction in power level

P = cable output power

P = cable input power

10log( )

Pout

A dB Pin

=

t

P = measured power level

P = transmitted power level

A = cable power loss

l = cable length

/1010 AlP Pt

=


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Fiber cable attenuationQuestion

Single-mode optical cable with input power 0.1 mW light source 0.25 dB/km cable loss, determine opticpower 100 km from the transmitter side

1)Absorption Loss

--absorption due to impurities absorb lights and convert it into heat--contributors:1)Ultraviolet ionized valence electron in the silica material.2)Infrared photons of light absorbed by glasss atom converted into random mechanical vibrations - heatin3)Ion resonance caused by OH- ion in the material. OH- trapped in the glass during manufacturing process

2)Material Rayleigh, Scattering Losses

--permanent submicroscopic irregularities during fiber drawing process

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--when the light propagates and strike one of the impurities, they are diffracted causes the light to disperse anspread out-some continues down the fiber, some escapes via cladding power loss

3)Chromatic Wavelength, Dispersion Loss

--many wavelengths being txn. from LED--each wavelength travels at different velocity--arrives at end of fiber at different time--resulting in chromatic distortion--solution: using monochromatic light source4)Radiation Losses

--loss due to small bends and kinks in the fiber--two types of bend:

1)microbend difference in the thermal contraction rates between core and cladding. Geometric imperfectialong the axis.2)constant radius bend excessive pressure and tension during handling and installation5)Modal Dispersion Losses

-- pulse spreading--difference in the propagation times of light rays that take different path-- occur only in multimode fiber-- solution: use graded index fiber or single mode step index fiber6)Coupling Losses

--imperfect physical connection--three types of optical junctions:

Light source to fiber connectionFiber to fiber connectionFiber to photo-detector connection

--Caused by:1) Lateral displacement 2) Gap displacement 3) Angular displacement 4) Imperfect surface finishLateral Displacement--axis displacement between 2 pieces of adjoining fiber cable--amount of loss couple tenth to several decibels

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Gap displacements miss alignment--end separation--the farther apart, the greater the light loss--if the two fiber is spliced, no gap between fiber--if the two fiber is joined with a connector, the ends should not touch each other

Angular displacement--less than 2o, the loss will typically less than 0.5 dBImperfect surface finish--end fiber should be polished and fit together squarely

Sources: Light source for optical communication system-- efficiently propagated by optical fiber-- sufficient power to allow light to propagate-- constructed so that their output can be efficiently coupled into and out of optical fiber

Tungsten lamp radiation and human eye response

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Optical sources

1)LED

--p-n junction diode--made from a semiconductor (AlGaAs)--emits light by spontaneous emission- light is emitted as a result of the recombination of electrons and holes.

Typical LED characteristics:

Output power versus forward current Output power versus temperature

Output power versus output wavelength

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2)ILD (Injection Laser Diode)--Above the threshold current, an ILD oscillates and lasing occurs--As current passes through a forward biased p-n junction diode, light is emitted by spontaneous emission afrequency determined by the energy gap of the semiconductor material.

Advantages: ILDs emit coherent light, used at higher bit rates than LEDs.Disadvantages: 10 times more expensive than LEDs shorter life time, more temperature dependent.

Detectors

PIN diodes

-- light doped material between two heavily doped n and p type semiconductor-- most common as light detectorAPD

-- avalanche photo diode-- more sensitive than PIN diode-- require less additional amplification

Characteristic of Light detectors

responsivity

-- a measure of conversion efficiency of photo-detector-- ratio of output current to the input optical powerdark current

--the leakage current that flows through photodiode when there is no light inputtransit time

-- time of light induced carrier to travel across the depletion region of semiconductorspectral response-- the range of wavelength values that a given photodiode will respondlight sensitivity

-- the minimum optical power a light detector can receive and still produce a usable electrical output signal

Lasers:

LASER-Light amplification stimulated by the emission of radiation--laser technology deals with the concentration of light into a very small, powerful beam--there are 4 types of lasers

1)Gas lasers: Helium and Neon enclosed in a glass tube laser, CO2 lasers--Output is continuous mono chromatic (one colour)2)Liquid lasers: organic dye enclosed in a glass tube for an active medium--A powerful pulse of light excites the organic dye3)Solid lasers: solid, cylindrical crystal such as ruby, for the active medium. Ruby is excited by a tungstelamp tied to an alternating-current power supply.--Output is continuous4)Semiconductor lasers: Made from semiconductor p-n junctions and are commonly called Injection lasdiodes (ILDs).-- a direct-current power supply controls the amount of current to the active medium

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Laser characteristics:

All lasers use1) an active materialconvert energy into laser light2) a pumping source

provide power or energy3) optics to direct the beathrough the active material to be amplified4) optics to direct the beainto a narrow powerful cone of divergence5) a feedback mechanismprovide continuous operation6) an output couplertransmit power out of the laser

transmission lines data communication systems

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