con definitions 2010

8/8/2019 Con Definitions 2010

1/46

Transmission Systems

Dr. Hoda Boghdady

Lecture 1Concepts and Definitions


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Transmission System

Lecturer: Dr. Hoda Boghdady and Dr.

Abdallah Sharaf

Every two wednesday

[email protected]

3rd floor room 318

Tel: 22636082
mailto:[email protected]:[email protected]


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Definitions Absolute and reference signal level dB and Np

S/N, C/N, E/N Reflection and SWR

Noise Figure

G/T (high gain directive antennas) Passive vs. active devices

Linear and non-linear characteristics

Fourier Transform

Channel B.W. and devices B.W. High frequency characteristics

Types of information

Digital voice channel 64Kbps


4/46

Signal Level

Absolute signal level the exact value of

the signal is given

3V, 100 mW, 0.5 amp

Reference signal level signal level isunknown, it is normalized . To know theexact value a reference should be defined

Percentage: reflection is 2% of the incident

Ratio: output power is 3 times the input


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Reference Signal -Decibels (dB)

Decibels is defined as a power ratioandgiven by

1dB = 10 log(P2/P1)P1 the input and P2can the output

Or P1 can be a reference power level, i.e.

1watt, 1mwatt, thereforedBw, dBm

For antennas dBi is usually used to specify

power referred to isotropic radiated power


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Note that :

dBx represents an absolute value of power

While dB represents a relative power level

-20dBm

-10dBm

-10dB

dBx Abs. level

0dBm 1mwatt

10dBm 10mwatt

0dBw

1watt

30dBw 1000watt

-30dBw 0.001watt

10dB gain means 10 times

20 dB gain means 100 times

60 dB gain means million times


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Neper (Np)The Neper is the unit for voltage or current ratio

8.686dB1Npln

log20log10

networkmatchedfor

/

/log10log10

1

2

1

2

2

1

2

21

1

2

1

2

2

2

1

2

V

VNp

V

V

V

VdB

RR

RV

RV

P

PdB


8/46

S/N, C/N and Eb/N

At the receiver what matter is the ratio of the signalto the noise not the signal level

S/N is the amount by which the signal exceeds the

noise level (analog signal)C/N is the carrier level to the noise level

Eb/N is equivalent to S/N for digital signals, Eb isthe energy contained in one bit, N is the noise

power per 1Hz cycle, BER is specified at a givenreceiver threshold

All above ratios are measured in dB


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Threshold Level

In analog system signals variations is very

critical, signal envelop should always bepreserved

In digital system a threshold is defined todifferentiate between 1 and 0, and no

ISI should be allowed


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As the receiving signal is very critical intelecommunication systems, noisecontribution from the receiver is acritical issue.Noise contribution of the receiver itself

should be kept to a minimum.

Noise figure (NF) is always specified atthe receiver only. It is the ratio of the

S/N at the input to the S/N at the output.

Noise Figure


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Noise Figure cont.

in

devdB

in

dev

in

devin

in

out

out

in

out

in

GN

NNF

GN

N

N

NGN

GNF

N

N

S

S

NS

NSNF

1log10

11

)/

)/

0devN

Best case

NF = 0 dB

Worst case

NF = 3 dB

indev GNN


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Noise Figure cont.

For cascaded elements

1

321

21

3

1

21

1....If

......11

NFNF

GGGGG

NF

G

NFNFNF

NF1, G1 NF2, G2 NF3, G3


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Microwave vs. Low Frequency

Wavelength is the distance a wave travel

to have a 2 phase change (comes to thesame point assuming sinusoidal wave)

Phase Difference is very important

F=10Ghz, = 3cm (/2 antenna = 1.5cm)

F=60hz, =5x106 m = 5000Km ( /2 =2500km)


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Sinusoidal Wave

t2

d

F = 10 GHz = 3 cm

F = 50 Hz = 5000 Km


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Microwave Frequency MainCharacteristics

Wave length = speed of light / frequency The higher the frequency the smaller the wave length

(smaller dimensions, scattering, energy focusing,phase referenceetc.)

Lumped elements performance decreases asfrequency increases

Different types of transmission lines are used

Frequency dependent components Phase references is a critical issue


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Low vs. High Frequency

At low frequency V and I affect the entire

circuit at the same instant of time

At high frequency at a given instantvoltage and current waves have differentvalues at different locations on the circuit

When voltage or current waves enter atransmission line at high frequencies ittakes time to travel down the line


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Voltage and Current Wave

Voltage (current) on microwave T.L. arethe sum of a two voltage (current) wave

components: incident and reflected

ZjrefZjinc eVeVV

LoadVinc

Vref


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Line Parameters

)( ZjrefZjinc eVeV

Z

I 0

1

ZjrefZjinc eVeVV Total voltage andcurrent along the line

Z0 is the line

characteristic impedance

is the propagation

constant of the line

C

LZ

0

2


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Terminated Line

If the load terminating the line is not

matched (Zload Z0) then a reflection willexist on the line

0

0

ZZ

ZZ

load

load

The reflection coefficient has an amplitude andphase depending on the load impedance, Zo is

always real, it is used to calculate unknown

impedance


20/46

Reflection Coefficient

Short Circuit, ZL

= 0, =-1 (=1, =180)

Open circuit, ZL = , =1, ( =1, =0 or 2n)

Matched load, ZL = Z0, = 0, (Noreflection) perfect case

je


21/46

Standing Wave Ratio

Short circuit, = 1, SWR =

Open circuit, = 1, SWR = Matched load, = 0, SWR = 1 (best value) What is infinity means? In worst case half the

power will be reflected, = = 0.707, SWR=

5.8 SWR should vary from 1 up to 3 max. The

higher the SWR the higher the reflection on theline

1

1

min

max

V

VSWR Note: SWR is a real value it has no phase


22/46

SWR MeasurementApplications - Antenna

an SWR meter can be placed in the feedline

The max. and min voltages are recorded

SWR is calculated as the ratio of the max.to the min.

The SWR reading should be close to 1

and below 2.5 to insure good matchingNote: the measurement can be carried

anywhere on the feed line


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SWR MeasurementApplications Line Impedance

SWR reading can be used to calculate the

impedance terminating a T.L.

The magnitude of the reflection coefficientis directly obtained from the SWR reading

The position of first minima from the loadunder test should also be identified

The later is used to calculate the phase ofthe reflection coefficient then the loadimpedance can be calculated


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Impedance Measurement

To measure an unknown impedance, connectit to a line with characteristic impedance Z0,then measure the reflection coefficient

is measured by measuring max and min.voltage on the line

The phase by measuring the position of thefirst minima from the load ( = - zmin)

1

1load

Z1

1

0

0

load

load

load

load

Z

Z

ZZ

ZZ


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Input Impedance

The concept of input impedance is very

important as what we see at each port isdifferent and at any point on the line as

well

The input impedance depends on the linetermination and the operating wavelength

and the distance between observationpoint and the load


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Input Impedance cont.

tan1

tan

L

LinZj

jZZ

tanjZin

Short circuittermination

Open circuit

termination

Matched load

termination

cotjZin

1inZ


27/46

Lumped Element in Microwave

OC =capacitor

SC =inductor

tanjZin

cotjZin

4/True for


28/46

Impedance Matching

In high frequency system all components

should be matched to each other to insuremaximum power transfer

This is achieved if impedances of differentcomponents are equal

i.e. output impedance of one device

equals input impedance of the followingone


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T.L. Types

A transmission line should be able to

transmit power with minimum loss andradiation

2- wire for low frequency, can carry up to2Mb/sec over 200m

Coaxial DC up to 1GHz maximum

Waveguide Microstrip lines, not for transmission system

but for circuits


30/46

Coaxial

The dominant mode is the TEM,

This allows wide band operation starting at DC

The cut off of the coaxial cable gives the upper

frequency limit, dimensions will be chosen suchthat higher order mode are prohibited

TE11 mode is the lowest higher order mode

)( bac


31/46

Coaxialcont.

Coaxial cables are used at frequencies

below microwave

It can be used at high frequency withsmaller diameter

The larger the diameter the lower theattenuation and the greater its power

handling capability


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Coaxial - Power Handling

The power handling capability is limited

since the it is easily dissipated in the innerconductor

The power is conducted through thedielectric between inner and outerconductor, good for radiation loss

At lower frequency power loss is lower andcable can handle more power


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Waveguide

Waveguide TL are hollow conducting pipes thatcan take different shapes Rectangular, easier to manufacture and to excite, but

has edge discontinuities, larger BW than circular

Circular, easier to manufacture good for circularpolarization or dual polarizer, difficult power coupling

elliptical, difficult to manufacture, E and H plane notsymmetrical

Waveguide are used for transmission over few

hundreds of meters, because of unflexibility circular and elliptical can be designed rigid or

flexible


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Waveguide

Wave propagates in modes

As modes increase group delay increasescausing signal distortion

Only one mode is usually excited, thedominant mode (TE01 for rectangular)

The dimensions of the guide designate the

dominant mode cut off Cut off is the lowest frequency that a guide

can handle (High pass filter)


35/46

Modes in Rectangular WG

22, )()(

bm

anCf nmc

a

Cfc

2

01,

a is the smallest side of the rectangle

b is the largest side of the rectangle

n,m are the order of the mode

The dominant mode frequencyDimensions are standard b=2a

a

Cfc 10,< Band

con definitions 2010

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