COMM 704: Communication Systems - GUC · Eng. Sara abd El Azim Grading: Assignments: 15% (3 x 5%)...

COMM 704:

Communication Systems

Lecture 1: Introduction

Dr. Mohamed Abd El Ghany,

Department of Electronics and Electrical Engineering

[email protected]

Course Objective

Give an introduction to the basic concepts of

electronic communication systems

Address the design of communication systems

building blocks: multipliers, Oscillators, Frequency

synthesizers and power amplifiers

Describe communications systems, such as

amplitude modulation (AM), frequency modulation

(FM), phase modulation (PM)

Discuss some significant systems, such as television

systems, satellite communications systems

2 Dr. Mohamed Abd el Ghany


COMM 704:Communication

Systems

Winter 2012

Text and Reference Books

Wayne Tomasi, “Electronic Communication

Systems,” Prentice Hall, ISBN: 0-13-049492-5

Frank R. Dungan, “Electronic communications

systems,” PWS Publishers, ISBN 0-534-07698-x

William schweber, “Electronic Communications

systems: Acomplete Course,” Prentice Hall, ISBN 0-

13-590092-1

Behzad Razavi, “RF Microelectronics,” Prentice Hall

PTR, ISBN 0-13-887571-5




Systems

Winter 2012

Prerequisites

Communication Microelectronics

(ELCT 508)

Signal and Systems (COMM 401)

Modulation I (COMM 601)

Modulation II (702)




Systems

Winter 2012

Administrative Rules

Course components:

Lecture: Thursday (third slot), 11:30-13:00 (H9)

Tutorial: 1 slot

Teaching assistant: Eng. Eman Azab

Eng. Sara abd El Azim

Grading:

Assignments: 15% (3 x 5%)

Quizzes: 15% (2x 7.5%)

Project : 10%

Mid term exam: 20%

Final exam: 40%




Systems

Winter 2012

Course Outline

Introduction

Multipliers

Filters

Oscillators

Power amplifiers

AM/FM modulation

Transceiver architectures

Television Systems

Satellite communications systems




Systems

Winter 2012

Radio Communication Services

Radio broadcasting

TV broadcasting

Satellite

communication

Mobile telephony

Internet

And more ….

8

Dr. Mohamed Abd el Ghany



Systems

Winter 2012

Block diagram of a Radio Communication system

A radio communication system consists of a transmitter, a channel, and a receiver In a transmitter:

The input sound signal is converted into equivalent electrical current/voltage by a transducer.

The transducer output is amplified by chain of amplifiers (so that it can travel longer distance)

The purpose of the transmit antenna is to efficiently transform the electrical signal into radiation energy

In a receiver: The receive antenna efficiently accepts the radiated energy and convert it to an

electrical signal as the signal suffered attenuation during travel it requires further amplification

The output transducer converts the electrical signal back into sound energy

9




Systems

Winter 2012

Types of communication

A B • Simplex- A can talk to B

Radio, T.V. broadcasting

Simplest type, requires one transmitter and one receiver

• Duplex- A and B both can talk to each other

simultaneously

Telephone, Telegraph

Complex, requires two transmitter and two receiver at both

ends

Needs two different channels for simultaneous transmission

• half-Duplex- A and B can both talk to each other but not

simultaneously

Fax,

Needs one single channel for transmission

Compromise between two, don’t require separate

transmitter and receiver

Same antenna and circuitry may be used for both

transmission and reception

A B

A B




Systems

Winter 2012

Antenna Design

Antenna dimension ~ λ/2

For voice signal (f~ 3KHz)

λ = c/f = (3x108)/(3x103)= 100 km

D = λ /2 = 50 km ! Impossible to realize




Systems

Winter 2012

Modulation

Modulation is the process of

superimposing a signal(of

relatively low frequency) on a

high frequency signal(carrier

wave), which is more suitable

to transmit.

Demodulation is the

opposite function of

modulation, performed

at receiver side




Systems

Winter 2012

Modulation

The modulation process consists of:

Firstly, a varying current is produced when sound waves strike a

microphone.

Secondly, the microphone output is then fed into the modulator

circuit where the audio and carrier waves are combined.




Systems

Winter 2012

Modulation Types

Modulation

Digital Analog

Amplitude Modulation (AM)

Frequency Modulation (FM)

Phase Modulation (PM)

Amplitude Shift Key (ASK)

Frequency Shift Key (FSK)

Phase Shift Key (PSK)




Systems

Winter 2012

AM/FM Modulation

In the FM process, the alternating

current from the microphone

modulates the carrier wave by

changing carrier wave’s

frequency.

In the AM process, the alternating

current from the microphone

modulates the carrier wave by

causing carrier wave’s amplitude

or strength to rise and fall.




Systems

Winter 2012

Carrier Frequency Bands

The carrier waves frequencies for radio

broadcasting are assigned by Federal

Communication Commission (FCC)

AM carrier frequency: 535 KHz to 1605

KHz

FM carrier frequency: 87.5 MHz to 108

MHz

16




Systems

Winter 2012

Carrier Frequency Bands Name Frequency

Range

Wave Length Application

ELF 300Hz to 3KHz 100Km to

1000Km

Navigation, long distance communication with ships

VLF 3KHz to 30KHz 10Km to

100Km

Navigation, long distance communication

LF 30KHz to

300KHz

1Km to 10Km Navigation, long distance communication with ships

MF 300KHz to 3MHz 100m to 1Km AM broadcasting, radio navigation

HF 3MHz to 30MHz 10m to 100m Radio broadcasting, fixed point to point (around the

world)

VHF 30MHz to

300MHz

1m to 10m Radio and TV broadcasting, mobile services

UHF 300MHz to 3GHz 10cm to 100cm Cellular telephony (GSM, NMT, AMPS). Digital TV,

fixed point-to-point, satellite, radar

SHF 3GHz to 30GHz 1cm to 10cm Broadband indoor systems, microwave links, satellite

communications

EHF 30 GHz to

300GHz

1mm to 10mm LOS communication (short distance or satellite)




Systems

Winter 2012

Managing Radio Spectrum

The frequency spectrum is common to all radio systems, so all radio

frequencies are regulated in order to avoid interference

International cooperation and regulations are required for an orderly

worldwide use of the radio spectrum

The international Telecommunication Union (ITU) is an agency part

of the united Nations that takes care of managing radio spectrum

worldwide.

With 184 membership countries, the ITU main activities are:

Frequency assignment

Standardization

Research

System compatibility issues

Coordination and planning of the international telecomm services




Systems

Winter 2012

AM Transmitter

AM signals vary in amplitude in response to AF signals from the

microphone.

The AM modulator actually produces an output that includes the

carrier and two sidebands. These sidebands are mirror images of

each other and contain the same information.

The carrier and both sidebands are amplified by RF amplifier and

transmitted




Systems

Winter 2012

FM Transmitter

Power

Amplifier AF Amplifier

Voltage

Controlled

Oscillator

The modulating signal, is a signal from the microphone. It is being

amplified in the AF amplifier and then led into the HF Voltage

Controlled Oscillator (VCO), where the carrier signal is being

created. The frequency of oscillator is changing in accordance with

the input voltage of oscillator. Therefore, the frequency modulation is

being obtained. The FM signal from the HF oscillator is being

proceeded to the power amplifier that provides the necessary output

power of the transmission signal.




Systems

Winter 2012

AM Superheterodyne Receiver

The carrier frequency of any radio signal is converted to

intermediate frequency using mixer and local oscillator components.

A typical value of IF for an AM communication receiver is 455KHz.

RF

Amplifier

Local

Oscillator

Detector AF

Amplifier Mixer

IF

Amplifier




Systems

Winter 2012

FM Superheterodyne Receiver

In FM receivers, a discriminator is a circuit designed to respond to frequency shift variations.

A discriminator is preceded by a limiter circuit, which limit all signals to the same amplitude level to minimize

noise interference.

The audio frequency component is then extracted by the discriminator, amplified in the AF amplifier, and used to

drive the speaker.

A typical value of IF for an AM communication receiver is 10.7MHz.

Local

Oscillator

Mixer IF

Amplifier

RF

Amplifier Limiter

Discrimi-

nator

AF

Amplifier




Systems

Winter 2012

Case Studies

Simplified architecture of Motorola’s FM receiver

The main components

of the Motorola’s FM

receiver are:

-Antenna

-LC matching network

-Mixer

-Bandpass Filter

-Voltage Controlled

Oscillator

-Crystal Oscillator

-Limiter




Systems

Winter 2012

Case Studies

Philips’DECT Transceiver




Systems

Winter 2012

Characteristics of Tuned LC Circuits

For any series or parallel LC circuit, the inductive reactance XL and

Capacitance Xc will be equal at some frequency. The frequency at which

XL=Xc is called the resonant frequency.

The resonant frequency can be calculated as:

The most common application of resonance is in radio-frequency (RF)

circuits where tuning is important.

Tuning refers to an LC circuit's ability to provide maximum voltage output

at resonant frequency compared with the voltage output at frequencies

either above or below resonance.

The use of tuned LC circuits is found in every television, video cassette

recorder (VCR), AM/FM receiver, and satellite.




Systems

Winter 2012

Series Resonant LC Circuit

When the generator frequency is

above or below the resonant

frequency, the net reactance X is

no longer zero and Zt increases.

Above the resonant frequency,

XL>Xc and the net reactance X is

inductive.

Below the resonant frequency,

Xc>XL and the net reactance X is

capacitive.




Systems

Winter 2012

Q of a series resonant circuit

The quality or figure of merit of a series resonant circuit is indicated by a factor known as Q

Q is a ratio of reactance to resistance at resonance.

The only way to in crease Q is to somehow increase the value of XL at fo

For example: if L is doubled and C is halved, then fo does not change, but XL and Xc each double in value. Assuming the series resistance remains the same, Q doubles.





Systems

Winter 2012

Bandwidth of a series resonant circuit

BW of a resonant circuit is defined as the gap between those frequencies for which the resonant effect is 70.7%.

The way to in crease the Q and thereby decrease the bandwidth is to increase the L/C ratio.





Systems

Winter 2012


Tuning an LC circuit

For any variable capacitor, the tuning range

(TR) is:

For any tuned LC circuit in which the

capacitance is varied, the following relationships

exist.




Systems

Winter 2012

Parallel Resonant LC Circuit

A parallel LC circuit is sometimes

called a tank circuit.

The inductive and capacitive branch

currents are equal at the resonant

frequency as a result of XL and Xc

being equal. Since the inductive

current IL and the capacitance current

Ic are 180° out of phase, the net line

current equals zero at resonant

frequency

With a total line current IT of zero, the

tank impedance Ztank approaches

infinity at the resonant frequency.




Systems

Winter 2012


Practical LC Tank Circuit

In a practical LC tank

circuit, a coil always

contains amount of internal

resistance.

IL is always slightly less

than IC at fo.

The net current IT is never

exactly zero, and as a

result the tank impedance

is never actually infinity.




Systems

Winter 2012


If Q ≥10, which is usually the

case, then the following

approximations can be made

Tank Impedance at the resonant frequency

Since Q = XL /rs, the equation can be

reduced to :

Since XC = XL at fo, Ztank is usually

stated as




Systems

Winter 2012


Q of a parallel resonant circuit

Bandwidth of a parallel resonant circuit

The bandwidth includes the frequencies

extending from f1 to f2 (the edge frequencies).

f1 and f2 are defined as those frequencies at

which Ztank is reduced to 70.7% of its maximum

value at fo.

Or




Systems

Winter 2012


RL

Adding external load

Decrease Q Increase BW

RL reduces the sharpness of the resonant

effect.




Systems

Winter 2012

Problem

For the tank circuit shown in

Fig.1, calculate the following

values at the resonant

frequency fo :

a) XL, XC, IL, IC, Q, Ztank and

IT

b) The bandwidth and edge

frequencies

c) Assume that a 100KΩ

load RL is placed in

parallel with the tank,

calculate Qoverall and BW

VA= 300 mV L= 200 µH

rs= 25 Ω

C= 75 pF

Fig.1




Systems

Winter 2012

Solution

a)




Systems

Winter 2012

Solution

b)




Systems

Winter 2012

Solution

c) Since Ztank < 10 RL




Systems

Winter 2012

COMM 704: Communication Systems - GUC · Eng. Sara abd El Azim Grading: Assignments: 15% (3 x 5%)...

Documents

Transcript of COMM 704: Communication Systems - GUC · Eng. Sara abd El Azim Grading: Assignments: 15% (3 x 5%)...