1Electronic Communications SystemsChapter 1:Introduction to Electronic Communications

Chapter 1:Introduction to Electronic CommunicationsPart 1:Introduction

Electronic Communications Systems The transmission, reception, and processing

of information between two or more locations using electronic circuits.

Definition of terms Communication Refers to the sending and receiving of

information through a medium. Information Anything that conveys a thought or an idea, e.g.

speech, picture, video. Medium Any facility in which communication is made

possible, e.g. air, wire, optic fiber.

2A Brief History ofElectronic Communications1837. Samuel Morse invents the telegraph1847. Alexander Bain invents the facsimile1866. The first trans-Atlantic telegraph cable is

laid between the US and England1876. Alexander Graham Bell and Thomas

Watson invents the telephone1887. Heinrich Hertz discovers radio waves

A Brief History ofElectronic Communications1894. Guglielmo Marconi successfully transmits

the first wireless radio signals1901. First trans-Atlantic radio transmission1903. John Fleming invents the vacuum tube

diode (rectifier)1906. Reginald Fessenden invents AM, transmits

first AM broadcast

A Brief History ofElectronic Communications1908. Lee DeForest invents the vacuum tube

triode (amplifier)1920. First commercial AM broadcasts1923. Vladimir Zworykin invents television1933. Edwin Howard Armstrong invents the

superheterodyne receiver and FM1936. Commercial FM broadcasting commences

A Brief History ofElectronic Communications1939. First use of two-way radio systems (US)1940. Invention of RADAR. Perfected after

World War II1948. John von Neumann creates the first stored-

program electronic digital computer1949. Invention of the transistor (Bell

Laboratories Bardeen, Brattain, Shockley)

3A Brief History ofElectronic Communications1953. RCA/NBC transmits the first colored TV

broadcasts1959. Jack Kilby (Texas Instruments) invents the

integrated circuitFirst communications satellites tested (US)

1975. First personal computers (IBM)1977. First use of fiber optic cables

A Brief History ofElectronic Communications1978. First cellular telephone network (Motorola)1990s. Adoption and growth of computer

networking (LANs, Internet, and the WorldWide Web)

2000 to present. 3G cellular networks, wireless LANs, digital broadcasting, high-speed fiber-optic communications

Block Diagram of an Ideal Communications System

INFORMATION SOURCE

(MESSAGE)

TRANSMITTER

DESTINATION(RECEIVED

INFORMATION)

RECEIVER

MEDIUM

Message Physical manifestation of the information. Two distinct types of messages: Analog a physical quantity that varies

continuously with time. Digital an ordered sequence of symbols selected

from a finite set of discrete elements.

4Transmitter Processes the message (input signal) to

produce a signal suitable to the characteristics of the transmission medium.

In modern communications systems, the input signal undergoes modulation before transmission.

Block Diagram of a Transmitter

AUDIO

AMPLIFIERMODULATOR

RF

AMPLIFIER

RF

OSCILLATOR

source

Modulation The process of superimposing a low-

frequency information wave on a high-frequency carrier signal.

Varying the properties of a carrier wave with respect to the properties of the information wave.

Examples of Analog Modulation Techniques

5Why Modulate? For efficient transmission Transmitting low-frequency

electromagnetic energy is extremely difficult.

For frequency assignment Several information sources can be tuned

to different frequencies to avoid mix up. This is also known as multiplexing.

Receiver Operates on the received signal to extract

information from the carrier wave. This process is known as demodulation.

Receiver

RF

AMPLIFIERDEMODULATOR

AUDIO

AMPLIFIER

RF

OSCILLATOR

destination

Modes of Transmission Simplex (SX) Unidirectional (one-way) transmission. Examples: broadcasting, cable TV, paging

services, telemetry. Half Duplex (HDX) Bidirectional (two-way) transmission, but not at

the same time. Examples :amateur radio, citizens band radio.

6Modes of Transmission Full Duplex (FDX) Simultaneous bidirectional transmission. Example: telephone system

Full/Full Duplex (F/FDX) One station may transmit to a second station and

receive from a third station at the same time. Example: data communication circuits

Medium The physical facility wherein the transmission

of information takes place. Could be wired, wireless (over the air), or

optic fiber. Composed of channels.

The Electromagnetic Spectrum The Electromagnetic Spectrum

7MEDIUM

Channel Range of frequencies allocated for a particular

service or transmissionCHANNEL 1

CHANNEL 2

CHANNEL n

.

.

.

Bandwidth The difference between the highest and lowest

frequencies contained in the information signal. The difference between the highest and lowest

frequencies that a communications channel will allow to pass.

The bandwidth of the channel must be large enough to pass all significant information frequencies.

Bandwidth Example 1: The human ear can hear

frequencies from 20 Hz up to 20 kHz. What is the bandwidth of this (audio) information?Answer: 19.98 kHz (~20 kHz)

Example 2: Telephone circuits bandpass voice information between 300 Hz and 3.4 kHz. What is the bandwidth of the circuit?Answer: 3.1 kHz

Information Capacity The theoretical study of the efficient use of

bandwidth to propagate information is called information theory.

In a data communications system, information capacity is the measure of the amount of information that can be propagated, and is a function of bandwidth and the transmission time.

8Information Capacity Information capacity represents the number of

independent symbols that can be carried through a system in a given amount of time.

The most basic unit of digital information is called a binary digit or bit.

Therefore the amount of digital information carried per unit time is measured as bits per second or bps, and is referred to as the bitrate.

Information Capacity According to R. Hartley (Hartleys Law, 1928)

tBI where:

I information capacity, bpsB bandwidth, HzT transmission time, s

Information Capacity Claude Shannon related the information

capacity to the bandwidth and signal-to-noise ratio (Shannon limit, 1948)

NSBI 1log2where:

I information capacity, bpsB bandwidth, HzS/N signal-to-noise ratio

NSBI 1log32.3 10or

Information Capacity Example: For a standard telephone circuit

with a signal-to-noise power ratio of 1000 and a bandwidth of 2.7 kHz, determine the Shannon limit for information capacity.Answer:

10001log)2700(32.3 10 IFrom the previous equation:

I = 26.9 kbps

9Chapter 1:Introduction to Electronic CommunicationsPart 2:Power Measurements and Noise Analysis

The Decibel A logarithmic unit used to measure ratios

(voltage, power, sound pressure, etc.). Originally used to measure differences in

power between source and receiver sides in a telephone circuit.

Named after Alexander Graham Bell, the inventor of the telephone.

The Decibel

ref

measP

P

PA 10)dB( log10

where:AP(dB) power gain, dBPmeas quantity to be measured, wattsPref reference power, watts

Decibels are used to denote relative magnitudesbetween two quantities (Pmeas vs. Pref)

refmeas PP PA 10)dB(10The Decibel Example 1: Solve for the following relationships

1. 10 W versus 1 W2. 1 W versus 10 W3. 100 mW versus 1 W4. 100 mW versus 1 mW5. 2 W versus 1 W6. 0.5 W versus 1W7. 4 W versus 1 W8. 250 mW versus 1 W

10 dB-10 dB-10 dB20 dBm3 dB

-3 dB6 dB

-6 dB

10

The Decibel Example 2: Given the cascaded amplifier below,

determine the following: Amplifier (system) voltage gain Output voltage in dBV Amplifier (system) power gain Power across Ro in dBm

Assume Ri = Ro = 1 k for all amplifiers

Av1 = 20 Av2 = 15 Av3 = 0.4vi = 20 mV vo

1207.604 dBV144007.604 dBm

Noise Specifically, electrical noise, is any

undesirable electrical energy that falls within the passband of the signal.

Noise causes distortions in the signal which may affect reception and/or intelligibility of the demodulated signal.

Noise Noise can fall into two general categories: Uncorrelated noise is present regardless

whether there is a signal or not. It can either be external (generated from outside of the system) or internal (generated by components from within the system, e.g. resistors, transistors).

Correlated Noise produced by the system (internal) due to the presence of a signal. It is caused by nonlinearities in a components behavior.

External Noise Atmospheric Noise Also termed as static Caused by lightning and thunderstorms Consists of impulses Spread across a wide range of frequencies Less severe at frequencies above 30 MHz

11

External Noise Extraterrestrial Noise Also called space noise. Two types of space noise

are worth discussing: Solar Noise

The sun is a large body at a very high temperature emitting constant noise radiation called thermal or black-body radiation.

Aside from this quiet condition, the sun also has peaks in its activity in the form of solar flares and sunspots.

This solar cycle repeats approximately every 11 years, with a super-cycle every 100 years.

External Noise Extraterrestrial Noise

Cosmic Noise Distant stars are themselves suns, which emit the same

kind of radiation as our Sun Though not as powerful, they make up for it in numbers There are also quasars and pulsars Also called galactic noise

Industrial Noise Sources include engine ignition, electric motors and

switching equipment, high-voltage lines, arc lamps Comes in the form of impulse noise Occurs within the range of 1 to 600 MHz

Internal Noise Transit-time noise If the time taken by an electron to travel from the

emitter to the collector of a transistor becomes significant to the period of the signal being amplified, transit-time effect takes place, and the noise input admittance of the transistor increases.

Internal Noise Shot Noise Caused by the shot effect. First observed by

W. Schottky in 1918 in a vacuum tube triode. Present in virtually all active devices Caused by the random variations in the arrival

of carriers at the output electrode of an amplifying device.

Appears a randomly varying noise current superimposed on the output

12

Internal Noise

Biqi pn 2

where:

in rms shot noise current

q charge of an electron, 1.6x10-19 C

ip direct diode current

B bandwidth of interest

Shot Noise

Internal Noise Thermal Agitation Noise Also referred to as thermal noise, agitation noise,

white noise, or Johnson noise. Due to the rapid and random motion of molecules

(atoms and electrons) inside the component itself.

KTBTBPn where:

K Boltzmanns constant, 1.38x10-23 J/K

T absolute temperature, K

B bandwidth of interest

Pn maximum noise power output of a resistor

Internal Noise

R

Vn

RLV Ln R

VP2

R

VPn2

RVn

2)2/(R

Vn4

2

nn RPV 42 RKTB4

RKTBVn 4

Thermal Agitation Noise

Figure 1.1. A resistance noise generator

For maximum power transfer, RL = R. Therefore

Internal Noise Thermal Agitation Noise Example: An amplifier operating over the

frequency range from 18 to 20 MHz as a 10-kinput resistor. What is the rms noise voltage at the input to this amplifier if the ambient temperature is 27C?Answer:

]10)1820][()27327)[(1038.1)(1010(4 6233 HzKv KJn From the previous equation:

vn = 18.1989 V

13

Internal Noise Harmonic Distortion One type of correlated noise. It is the unwanted harmonics of a signal that are

created when amplified in a nonlinear device (e.g. a transistor amplifier).

These harmonics add to the original signal, causing amplitude distortion.

Harmonic distortion is calculated as the ratio of the rms amplitude of the nth harmonic frequency to the rms amplitude of the fundamental frequency.

Internal Noise Harmonic Distortion The Total Harmonic Distrortion (THD) is the

ratio of the quadratic sum of the rms amplitudes of the higher harmonics to the rms amplitude of the fundamental frequency.

100%1

223

22

V

VVVTHD n

where:

%THD percent total harmonic distortion

V2, V3, Vn rms amplitudes of higher harmonics(2nd up to nth harmonic)

V1 rms amplitude of fundamental frequency

Internal Noise Harmonic Distortion Example: Determine the percent 2nd-order, percent

3rd-order, and total harmonic distortion of the signals shown below:

200 400 800 1600100

1

2

3

V

r

m

s

Frequency (Hz)

Internal Noise Harmonic Distortion

Answer:100

3

2100order- 2%1

2 V

Vnd

1003

1100order-3%1

3 V

Vrd

%667.66

%333.33

1003

12100%22

1

23

22

V

VVTHD %536.74

14

Internal Noise Intermodulation Noise Also called intermodulation distortion. It is caused by unwanted cross product (sums and

difference) frequencies created when two or more signals mix or are amplified in a nonlinear device, such as a large scale amplifier.

It is impossible to measure all the intermodulationcomponents produced when two or more frequencies mix in a nonlinear device.

Internal Noise Intermodulation Noise For comparison purposes, a common method

used to measure intermodulation distortion is percent second-order intermodulation distortion, which is the ratio of the total rms amplitude of the second order cross products to the combined rmsamplitude of the original input frequencies.

Internal Noise Intermodulation Noise To measure second-order intermodulation

distortion, four test frequencies are used: Two are designated as the A-band (fa1 and fa2), and Two are B-band frequencies (fb1 and fb2).

The second-order cross products (2A B) are: 2fa1 fb1 2fa1 fb2

2fa2 fb1 2fa2 fb2

(fa1 + fa2) fb1 (fa1 + fa2) fb2

Internal Noise Intermodulation Noise The second-order intermodulation distortion

(%2nd-order IMD) is given as:

100IMDorder 2%2

2

m

f

nn

mV

Vnd

where:

Vn rms amplitudes of intermodulation components

Vfm rms amplitude of input frequencies

15

Internal Noise Intermodulation Noise Example: Determine the intermodulation

components of the A-band and B-band frequencies, and their percent 2nd-order intermodulation distortion.

2

4

6

V

r

m

s

Frequency (MHz)

0.8 0.9 1.0 1.6 2.01.2

B-band

0.8560.863

1.374

1.385

A-band

Intermodulationcomponents

Internal Noise Intermodulation Noise

Answer: 1.885 MHz, 1.892 MHz, 1.896 MHz,1.903 MHz, 1.907 MHz, 1.914 MHz

Frequency (MHz)

1

2

V

r

m

s

1.850 1.9501.860 1.870 1.880 1.890 1. 900 1. 910 1. 920 1. 930 1. 940

1.9141.892

1.896

1.903

1.907

1.885

1006456

212122IMDorder 2%2222

222222

nd %911.39

Interference A form of external noise and as the name

implies, means to disturb or detract from. This happens when information signals from

one source produce frequencies that fall outside their allocated bandwidth and interfere with information signals from another source.

Most interference occurs when harmonics or cross products from one source fall into the passband of a neighboring channel.

Chapter 1:Introduction to Electronic CommunicationsPart 3:Noise Calculations

16

Signal-to-Noise Power Ratio The ratio of the signal power to the noise

power at the same point in the circuit.

n

s

P

P

N

S where:Ps signal powerPn noise power

In decibels:

n

s

P

PdBN

S log10)(

Example: For an amplifier with a input signal voltage of 4V and a noise voltage of 5mV at the input, determine the signal to noise ratio in decibels across the input when Rin = 100k.

n

s

RV

RV

V

VdBN

S

inn

ins

log20log10)(2

2

V

V

005.0

4log20 dB062.58

Noise Factor and Noise Figure Ratio of the S/N supplied to the input of an

amplifier to the S/N of the output resistor. A measure of the amount of noise a certain

device introduces into a system. Should ideally be 1. Noise figure (NF) is the noise factor (F)

expressed in decibels.

NS

NS

Foutput

input N

SN

SdBNF

output

input log10)(

Noise Factor and Noise Figure

Ri VoVi

Amplifier with gain A

Ro

AVi

Ideal amplifier (noiseless)

VoVi

Amplifier with gain A

AVi

RiVni

RoVno

Practical amplifier

oo

ii

NS

NS

F

RiVni

ii

ii

NASA

NS

1

oo

ii

NS

NS

F )( Ai

i

ii

NNASA

NS

ANNS

NS

Ai

i

ii

i

Ai

NA

NNF

Noise Factor and Noise Figure Example: Given an non-ideal amplifier with the

following parametersInput signal power = 2 x 1010 WInput noise power = 2 x 1018 WPower gain = 1,000,000Amplifier noise = 6 x 1012 W

Determine the following:a) Input S/N in dBb) Output S/N in dBc) Noise factor and noise figure

100,000,000 (80 dB)25,000,000 (74 dB)4 (6 dB)

17

Noise Factor and Noise Figure For amplifiers in cascade, the noise factor is

computed using Friisss formula

12121

3

1

21

111

n

nT

AAA

F

AA

F

A

FFF

AP1F1

AP3F3

APnFn

input outputAP2F2

Noise Factor and Noise Figure Example: Calculate the total noise figure for three

similar cascaded amplifiers having individual noise figures and power gains of 3 dB and 10 dB.Answer:Convert first the noise figure and power gain into a ratio

210103 F 10101010 pA

)10)(10(

12

10

122log10log10 TT FNF

dB243.3

Equivalent Noise Temperature Since the noise power is proportional to the

temperature, an amplifier tends to get noisieras the temperature increases.

As a result, the noise factor of the amplifier also increases. Conversely, amplifiers with large noise factors are said to be hotter.

The temperature at which this amplifier is operating at is said to be its equivalent noise temperature.

Equivalent Noise Temperature

Example: Determine the followinga) Noise figure at Teq = 70Kb) Teq for a noise figure of 6 dB

KTBPn FromThe noise factor can be expressed as

0

1T

TF eqwhere:

Teq equivalent noise temperature, K

T0 reference temperature, 290K (17 C)

0.9391 dB (1 dB)870 K