CT0004NI - L.02 - Analogue & Digital Signals - pp 1/36 Analogue & Digital Signals Saroj Regmi Lecture 02 CT0004NI Principles of Comms Systems By: Dr. N

CT0004NI - L.02 - Analogue & Digital Signals - pp 1/36

Analogue & Digital Signals

Saroj Regmi

Lecture 02

CT0004NICT0004NIPrinciples of Comms SystemsPrinciples of Comms Systems

By: Dr. N. Ioannides (Feb. 2010)


• Introduction to the Module: CT0004NI.

• Introduction to Telecommunications.

• Telecommunications have made great progress over the past 150 years.

In the information age, much of the communication we receive each day is mediated in some way by technology.

• There are a number of organizations responsible for standards.

Standards are important because they ensure compatibility, inter-operability of products and reliability.

• Discussed Social Implications of Telecommunications.

Last Lecture: 01Last Lecture: 01Introduction to TelecommunicationsIntroduction to Telecommunications



• Signals.

• Voltage, Resistance and Current.

• Open and Closed Circuits.

• Analogue vs Digital Signals.

• Analogue Signal Characteristics.

• Digital Signal Characteristics.

• Digital Binary Codes.

• Data Coding Noise Immunity.

• Morse and ASCII Codes.

Today’s Lecture: 02Today’s Lecture: 02Analogue & Digital SignalsAnalogue & Digital Signals



SignalsSignals

• Signal Definitions:

A signal is an electrical voltage or current which varies with time and is used to carry messages or information from one point to another.

A signal is an electric current or electromagnetic field used to convey data from one place to another.

• Forms of Signal:

Direct Current (DC): The simplest form of signal (switched on and off).

Alternating Current (AC): More complex signals (vary continuously).



• Voltage ( V ): an electrical force, or pressure, that occurs when electrons and protons are separated.

The unit of voltage measurement is the Volt (V).

• Resistance ( R ): the opposition to the movement of electrons through materials.

The unit of resistance measurement is the Ohm (Ω).

• Current ( I ): Caused by the flow of free electrons in a circuit.

The unit of current measurement is the Ampere (A).

The Ampere is defined as the number of charges per second that pass by a point along a path.

Voltage, Resistance & CurrentVoltage, Resistance & Current



Open vs Closed CircuitsOpen vs Closed Circuits

• Electrons ONLY flow in CLOSED circuits, or COMPLETE loops.

• Example: The Torchlight.

Open Switch / Open CircuitCurrent does NOT flow !

Closed Switch / Closed CircuitCurrent DOES flow !



Current FlowCurrent Flow

• Current can flow in one of two ways: Direct Current (DC):

DC always flows in the same direction, and DC voltages always have the same polarity.

Alternating Current (AC): AC varies over time by changing its polarity, or direction.



Current vs Electron FlowCurrent vs Electron Flow

• For AC and DC electrical systems:

Current Flow: always from a positive terminal to a negative terminal.

Electron Flow: always from a negatively charged source to a positively charged source.

Current flow

Electron flow



Ohm’s LawOhm’s Law

• Identifies the relationship among Voltage, Resistance and Current:

For DC Systems:

)(VRIV

)(AR

VI

)( I

VR

• Note:

Ohm’s Law considers current which flows from the positive towards the negative terminal.



Analogue SignalsAnalogue Signals

• Analogue signals vary continuously with time:

• Signal variations are very important and they all mean something, i.e. they represent information.



• Digital signals vary abruptly and change between distinct voltage or current levels.

Digital SignalsDigital Signals

• The most widely used form of a digital signal is binary (2 states).

Usually represented as 0 Volts (0V) and 5 Volts (+5V),

Also as: 0 & 1, Low & High, OFF & ON, False & True.

• Information is represented as a train of pulses arranged in specific combinations.



Analogue vs Digital SignalsAnalogue vs Digital Signals

• Analogue Signals:

Noise added to the analogue signal can greatly affect the accuracy of the information.

Limiting the level of noise is not an easy or inexpensive process.

• Digital Signals:

A digital waveform is much less sensitive to noise than an analogue signal.

Less cross-talk (co-channel interference).

Lower distortion levels.

Faded signals are more easily recreated.

Greater transmission efficiency.



• The fundamental building block of all communications systems is the sinusoidal waveform:

Analogue Signal CharacteristicsAnalogue Signal Characteristics



• Signal Strength:

Displacement (d): The signal strength at any specific point in time.

Amplitude Peak (Ap): The signal strength at the positive or negative maxima.

Amplitude Peak-to-Peak (Ap-p): The signal strength from the negative maximum to the positive maximum.

Analogue Signal Characteristics (…2)Analogue Signal Characteristics (…2)



D: distancet: time

)/( smt

D

)(1

HzT

f

)(1

sf

T Periodic Time (T): The time it takes to

completely generate a full cycle of a sine wave. Measured in seconds (s).

Frequency (f): The number of complete waveform cycles included over one (1) second. Measured in Hertz (Hz).

Velocity (v): How fast an electromagnetic wave (signal) is travelling. Measured in meters per second (m/s).


• Time specific:




)/( smT

)(m

f

)(Hzf

)/( smf

Wavelength ( ): The distance that a single and complete sine wave would have travelled away from the source generating it. Depends on the signal velocity. Measured in meters (m).

• Distance specific:



Signal VelocitySignal Velocity

)/( smc

r

coax

c = speed of light = 3 x 108 m/s

• The velocity of a signal depends on the communications medium:

• Examples:

Air: = 1 vair = 3 x 108 m/s

)/( smn

cfibre

r

r

r

For a coaxial cable, on the dielectric constant ( ) of the insulator:

For an optical fibre, on the refractive index (n) of the glass core:

Fibre: n = 1.46 (glass) vfibre = 2.055 x 108 m/s

Coaxial Cable: = 2.25 (polythene)vcoax = 2 x 108 m/sBy: Dr. N. Ioannides (Feb.

2010)


Binary Digital SignalsBinary Digital Signals

• Signals arising from computer type equipment designed to transmit information in coded form.

• Bits: The individual 1's and 0's.

• Bit Slot Duration, : The time required by each bit to be transmitted.

0 1 0 1 0 1 0 1



: the 1 bit slot duration,

T : the periodic time,

T - :the 0 bit slot duration.

Binary Signal RatiosBinary Signal Ratios

• Ratios in Binary Signals enable us to see the relationship between a bit set at 1 and another set at 0.

0 1 0 1

T



• The ratio between the 1's and the periodic time of one cycle.

Expressed as a percentage of period.

?:1: TRatioSpaceMark

%?: TCycleDuty

• The ratio between the 1's and the 0's.

used in Morse Code.

Mark Space RatioMark Space Ratio

Duty-Cycle RatioDuty-Cycle Ratio



What is the mark-space ratio and the duty-cycle of a binary transmission with periodic time T = 1 ms when = 200 µs ?

T = 1 ms

= 200 µs = 0.2 ms

4:1

4:1108:10200

)10200101(:10200

:

46

636

RatioSpaceMark

TRatioSpaceMark

%205:1

5:1101:10200

:36

CycleDuty

TCycleDuty

Binary Signal Ratios: ExampleBinary Signal Ratios: Example



Data CodesData Codes

• Codes have always been in widespread use even since mankind’s early history:

From the use of hand signals to mirror flashing signals across the land, to smoke signals of the American Indians, information has been coded and sent by a variety of means.

• Data codes are the way in which bits are grouped together to represent different symbols.

• The sender and receiver must use the same code in order to communicate properly.

• ASCII is the most widespread data code in use today:

7-bit code that can represent a total of 128 symbols (27 = 128).

Limited by the number of symbols it can represent.By: Dr. N. Ioannides (Feb. 2010)


Number of bits in a Data CodeNumber of bits in a Data Code

• The number of bits in a code will dictate the maximum number of symbols which can be represented.

• Alternatively, the maximum number of symbols required will dictate the number of bits that must be included in a code.

MnorMn2log2

n = number of bits in a code

M = number of symbols represented



Number of bits in a Data Code (…2)Number of bits in a Data Code (…2)

CodeCode bits in Codebits in Code Maximum Number of SymbolsMaximum Number of Symbols

BCD 4 24 = 16

BAUDOT 5 25 = 32

? 6 26 = 64

ASCII 7 27 = 128

EBCDIC 8 28 = 256

? 10 210 = 1024

UNICODE 16 216 = 65536



Data Coding Noise ImmunityData Coding Noise Immunity

• Binary coding is more immune to noise than any other form of coding.

The Binary Code: The Decimal Code:

0V

10V

Threshold = 0.5V

Threshold = 5V

• Consider the example below:



Morse CodeMorse Code

• One of the first character codes developed.

• A crude but effective way of transmitting characters over a telegraph circuit.

• Developed with a telegraph operator in mind who:

sent combinations of dots (short beep) and dashes (long beep),

paused for a short time between letters.

• Unsuitable for computer communications:

due to the extra time required between the transmission of each character,

limiting number of characters that could be represented.



Morse Code (…2)Morse Code (…2)



ASCIIASCII(American Standard Code for Information Interchange)(American Standard Code for Information Interchange)

• The ASCII code is the most popular code for serial data communications today.

• It is a 7-bit code allowing up to 128 combinations (27 = 128), and thus supports upper and lowercase characters, numeric digits, punctuation symbols, and special codes.

• ASCII is also used as the data code for keyboards in computers.

• Control codes are used and are represented as symbols.

Used in binary synchronous communication, and device control codes in communicating with devices such as printers or terminals.



The 7-bit ASCII CodeThe 7-bit ASCII Code© C

i s c

o S

y s

t e m

s , I n

c

.



SummarySummary

• Signals.

• Voltage, Resistance and Current.

• Open and Closed Circuits.

• Analogue vs Digital Signals.

• Analogue Signal Characteristics.

• Digital Signal Characteristics.

• Digital Binary Codes.

• Data Coding Noise Immunity.

• Morse and ASCII Codes.



Tutorial QuestionsTutorial Questions

1. What is the Voltage (V) in a closed circuit with Resistance R = 1.5 kΩ if the current (I) is:

a. 1A, b. 5A, c. 2.5A, d. 0.5A, e. 0.25A.

1. What is the Current (I) in a closed circuit with Resistance R = 5.75 kΩ if the Voltage (V) is:

a. 1.5V, b. 2.5V, c. 3.3V, d. 5V, e. 12V.

1. What is the Resistance (R) in a closed circuit with Voltage V = 5 V if the current (I) is:

a. 0.1A, b. 0.5A, c. 2.5A, d. 15mA, e. 0.25mA.

1. What is the Frequency (F) of a signal if its Period (T) is:

a. 1s, b. 15ms, c. 250ms, d. 75μs, e. 0.5μs.

1. What is the Period (T) of a signal if its Frequency (F) is:

a. 100Hz, b. 1.5kHz, c. 250kHz, d. 3MHz, e. 1GHz.



1. What are the Frequency (F) and Period (T) of a signal travelling in free space if the wavelength ( ) is:

a. 100m, b. 1.5km, c. 20mm, d. 325μm, e. 1.5nm.

1. What is the velocity (vcoax) of a signal travelling in a coaxial cable if the dielectric constant ( ) of the cable is:

a. 1.5, b. 1.75, c. 2.5, d. 3.1, e. 4.

1. What is the velocity (vf-o) of a signal travelling in a fibre optic cable if the refractive index (n) of the fibre is:

a. 1.2, b. 1.35, c. 2, d. 2.25, e. 3.

1. What is the mark-space ratio and the duty-cycle of a binary transmission with periodic time T = 1ms if:

a. = 50µs, b. = 100µs, c. = 250µs, d. = 500µs

Tutorial Questions (…2)Tutorial Questions (…2)

r



1. How many different character combinations would codes with the following number of bits be able to represent?

a. 5, b. 9, c. 11, d. 13, e. 20, f. 28.

2. How many bits are required per code if they must represent the following number of different character combinations?

a. 11, b. 56, c. 120, d. 260, e. 1025, f. 55555.

1. Code using Morse code the words:

hello and goodbye.

1. Code the following sentence using ASCII codes.

This module is: CT0004NI Principles of Communication Systems!

Tutorial Questions (…3)Tutorial Questions (…3)



Scientific NotationScientific Notation

• Commonly used in Physics, Chemistry and Engineering.

• A way of separating the range of expressible numbers from their precision:

Examples: 3.14 = 0.314 × 101 = 3.14 × 100= 3.14

0.000001 = 0.1 × 10-5 = 1.0 × 10-6 = 1 µ

1941 = 0.1941 × 104 = 1.941 × 103= 1.941 k

erfn f: fraction or mantissa,

r: radix or base number,

e: +ve or -ve integer called the exponent.

• The range is determined by the number of digits in the exponent.

• The precision is determined by the number of digits in the fraction.By: Dr. N. Ioannides (Feb.

2010)


Decimal Prefixes of Units in EngineeringDecimal Prefixes of Units in Engineering

Multiplication FactorMultiplication Factor PrefixPrefix SymbSymbolol

1 000 000 000 000 000 000 1018 exa E

1 000 000 000 000 000 1015 peta P

1 000 000 000 000 1012 tera T

1 000 000 000 109 giga G

1 000 000 106 mega M

1 000 103 kilo k

1 100 - -

0.001 10-3 milli m

0.000 001 10-6 micro μ

0.000 000 001 10-9 nano n

0.000 000 000 001 10-12 pico p

0.000 000 000 000 001 10-15 femto f

0.000 000 000 000 000 001 10-18 atto aBy: Dr. N. Ioannides (Feb. 2010)

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CT0004NI - L.02 - Analogue & Digital Signals - pp 1/36 Analogue & Digital Signals Saroj Regmi Lecture 02 CT0004NI Principles of Comms Systems By: Dr. N