1 INTRODUCTION

1.1 COMMUNICATION SYSTEMS

COMMUNICATION a process by which information is exchanged between individuals through a common system of symbols a technique for expressing ideas effectively a system of routes for moving troops, supplies, and vehicles

Communication is the transfer of information from one point in space and time to another point. The block diagram of a communication system is shown in Figure 1.1.

Figure 1.1 A block diagram of a communication systems

Transmitter - couples the message onto the channel using high frequency signalsReceiver - restores the signal to its original formChannel - the medium used for transmission of signalsModulation - the process of shifting the frequency spectrum of a message signal to a frequency range in which more efficient transmission can be achievedDemodulation - the process of shifting the frequency spectrum back to the original baseband frequency range and reconstructing the original form, if necessaryBaseband - refers to the lower portion of the over-all electromagnetic spectrum (Figure 1.2)

Baseband versus Passband (Lui, Principles and Applications of Optical Communications. Irwin, Times Mirror Higher Education Group, Inc, Chicago, 1996)

A signal can be transmitted in different frequency bands. If the signal is transmitted over its original frequency band, the transmission is called baseband transmission. On the other hand, if the signal is shifted to a frequency band higher than its original baseband, it is called passband transmission.

There are several reasons to shift a baseband signal to passband. First, some transmission media have either a large loss or high noise at low frequencies. For example, optical fibers have a cut-off frequency below which electromagnetic waves have a high loss. Therefore, we need to convert a baseband signal to lightwave for transmission over optical fibers. Similarly, in seawater communications, either extremely low frequency (ELF) of a few hundred hertz or blue light in the visible frequency range is chosen because of the low attenuation (this is why seawater is blue).

Another reason for passband transmission is to multiplex multiple signals in the same transmission medium. For example, AM/FM radio and TV channels are multiplexed in the frequency domain by a process called frequency division multiplexing (FDM), where each channel is centered around a pre-assigned carrier

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SOURCE

CHANNEL NOISE

DESTINATION

TRANSMITTER(modulator)

RECEIVER(demodulator)

frequency. AM, FM, and TV are in the frequency ranges of 530-1700 kHz, 88-108 MHz, and 54-88 MHz plus 120-600 MHz, respectively. Therefore, the higher carrier frequency, the more information it can carry.

Electromagnetic spectrum

←longer wavelengths         higher frequencies →Audio · Radio · Microwave · Terahertz radiation · Infrared · Visible · Ultraviolet · X-rays · Gamma rays 

Figure 1.2 Electromagnetic spectrum

Table 1.1 Spectrum of Communication Systems

Frequency Designation Transmission Media

PropagationModes

Applications

Infrared1 THz – 430 THz

Optical fibers Laser beam Wideband DataMultimediaATM

Super High Frequency (SHF)3 GHz – 30 GHz

Waveguides Line-of-Sight (LOS) Radio

SatelliteMicrowave Radar Navigational

Ultra High F’cy (UHF)300 MHz – 3000 MHz

Waveguides/ Co-axial cable LOS Radio UHF TVMobile

Very High F’cy (VHF)30 MHz – 300 MHz

Co-axial cable LOS Radio MobileVHF TV, FM

High F’cy (HF)3 MHz – 30 MHz

Co-axial cable Skywave Radio CB Amateur RadioCivil Defense

Medium F’cy (MF)300 kHz – 3000 kHz

Co-axial cable Groundwave Radio AM

Low F’cy (LF) Wire pairs Groundwave Radio Aeronautical

2

Microwave

3 GHz 30 GHz 300 GHz 3 THz 300 THz30 THz 3000 THz

InfraredVisible Ultra-

violet

f

Satellitecommunications

Opticalfrequencypassband

Millimeterwave

Audiofrequency

(AF)

30 Hz 300 Hz 3 kHz 30 kHz 3 MHz300 kHz 30 MHz 300 MHz 3 GHz

Very LowF’cy (VLF)

LowF’cy (LF)

MediumF’cy (MF)

HighF’cy (HF)

Very HighF’cy

(VHF)

Ultra HighF’cy

(UHF)

f

Audiobaseband

Video baseband

AMpassband

FMpassband

TVpassbandHigh-fidelity

audio baseband

30 kHz – 300 kHz Transoceanic RadioVery Low F’cy (VLF)3 kHz – 30 kHz

Wire pairs Groundwave Radio Telephone Telegraph

Audio F’cy (AF)20 Hz – 20 kHz

Wire pairs

Table 1.2 A CHRONOLOGY OF ELECTRICAL COMMUNICATION

1800-1837 Preliminary DevelopmentsVolta discovers the battery; the mathematical treatises by Fourier, Cauchy, and Laplace; experiments on electrical and magnetism by Oersted, Ampere, Faraday, and Henry; Ohm’s Law (1826); early telegraph systems by Gauss, Weber, and Wheatstone

1838-1866 TelegraphyMorse perfects his system; Steinhill finds that the earth can be used for a current path; commercial service initiated (1844); multiplexing techniques devised; William Thomson (Lord Kelvin) calculates the pulse response of a telegraph line (1855); transatlantic cables installed by Cyrus Field and associates

1845 Kirchhoff’s circuit laws enunciated1864 Maxwell’s equation predicts electromagnetic radiation1876-1899 Telephony

Acoustic transducer perfected by Alexander Graham Bell, after earlier attempts by Reis; first telephone exchange, in New Haven, with eight lines (1878); Edison’s carbon-button transducer; cable circuits introduced; Strowger devises automatic step-by-step switching (1887); the theory of cable loading by Heaviside, Pupin, and Campbell

1887-1907 Wireless telegraphyHeinrich Hertz verifies Maxwell’s theory; demonstrations by Marconi and Popov; Marconi patents a complete wireless telegraph system (1897); the theory of tuning circuits developed by Sir Oliver Lodge; commercial service begins, including ship-to-shore and transatlantic systems

1892-1899 Oliver Heaviside’s publication on operational calculus, circuits, and electromagnetics1904-1920 Communication electronics

Lee De Forrest invents the Audion (triode) based on Fleming’s diode; basic filter types devised by G. A. Campbell and others; experiments with AM radio broadcasting; transcontinental telephone line with electronic repeaters completed by the Bell System (1915); multiplexed carrier telephony introduced; E. H. Armstrong perfects the superheterodyne radio receiver (1918); first commercial broadcasting station, KDKA, Pittsburgh

1920-1928 Transmission theoryLandmark papers on the theory of signal transmission and noise by J. R. Carson, H. Nyquist, J. B. Johnson, and R. V. L. Hartley

1923-1938 TelevisionMechanical image-formation system demonstrated by Baird and Jenkins; theoretical analysis of bandwidth requirements; Farnsworth and Zworykin propose electronic systems; vacuum cathode-ray tubes perfected by DuMont and others; field tests and experimental broadcast begin

1931 Teletypewriter service initiated1934 H. S. Black develops the negative-feedback amplifier1936 Armstrong’s paper states the case for FM radio1937 Alec Reeves conceives pulse code modulation1938-1945 World War II

Radar and microwave systems developed; FM used extensively for military communications; improved electronics, hardware, and theory in all areas

1944-1947 Statistical communication theoryRice develops a mathematical representation of noise; Weiner, Kolmogoroff, and Kotel’nikov apply statistical methods to signal detection

1948-1951 Information theory and codingC. E. Shannon publishes the founding papers of information theory; Hamming and Golay devise error-correcting codes

1948-1951 Transistor devices invented by Bardeen, Brattain, and Shockley1950 Time-division multiplexing applied to telephony1953 Color TV standards established in the United States

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1955 J. R. Pierce proposes satellite communication systems1956 First transoceanic telephone cable (36 voice channels)1958 Long-distance data transmission system developed for military purposes1960 Maiman demonstrates the first laser1961 Integrated circuits go into commercial production1962 Satellite communication begins with Telstar I1962-1966 High-speed digital communication

Data transmission service offered commercially; wideband channels designed for digital signalling; pulse code modulation proves feasible for voice and TV transmission; major breakthroughs in the theory and implementation of digital transmission, including error-control coding methods by Viterbi and others, and the development of adaptive equalization by Lucky and co-workers

1963 Solid-state microwave oscillators perfected by Gunn1964 Fully electronic telephone switching system (No. 1 ESS) goes into service1965 Mariner IV transmits pictures from Mars to earth1966-1975 Wideband communication systems

Cable TV systems; commercial satellite relay service becomes available; optical links using lasers and fiber optics; the forerunner of the Internet, ARPANET was created in 1969;

1975-1985 Integrated-circuit communication modules; high-frequency power MOS devices; digital signal processing with microprocessors; filter circuits using switched capacitors and surface acoustic waves; rate distortion theory and predictive coding applied to data compression

1983 TCP/IP became the official protocol of ARPANET/Internet1985 to present Gigabit Networks, B-ISDN or ATM Networks, Digital TVbeyond 2000 Third- and fourth-generation wireless systems (Advanced mobile communications)

International Mobile Telecommunications (IMT)-2000; Wireless ATM (WATM)

Wireless communications networks today are based largely on first-generation analog and second-generation digital technologies. The specifications and standards for the new generation of wireless technologies (known as IMT-2000) are being developed by the ITU. The third-generation wireless systems will work in a range of service environments, from in-building to global. These systems will be offered in the 2-GHz frequency bands. (Shafi, et al., “Wireless Communications in the Twenty-First Century: A Perspective”, Proceedings of the IEEE, vol. 85, no. 10, October 1997, 1622-1638)

1.2 RADIO FREQUENCY AMPLIFIERS AND OSCILLATORS

1.2.1 TUNED AMPLIFIERS

Figure 1.3 A simple block diagram of a tuned amplifier

1.2.1.1 Series Resonance

L

R

CZ

4

ResonanceCircuit

AmplifierCircuit

ResonanceCircuit

Input Output

Z R j Lj C

R j LC

1 1

At resonance 0, Z = R, which is its minimum value. Hence,

0

0

10L

C

fLC

0

1

2

1.2.1.2 Parallel Resonance

Parallel resonance circuits are used when a high-impedance tuned circuits are required.

R L C

Z

fLC

0

1

2

5

V1

V2

V3

f3 fo f1 f2

f

V V

fo fL fH

f

Z

1.2.1.3 Quality Factor and Bandwidth

Inductors store energy in the magnetic field surrounding the device. Capacitors store energy in the space between electrodes. The energy is stored during one-half of the ac cycle and returned during the other half. Any energy lost during the cycle is associated with a dissipative resistance.

The sharpness of the response curve of any resonant circuit is determined by the maximum amount of energy that can be stored in the circuit, compared with the energy that is lost during one complete period of the response. This sharpness of the response curve is related the parameter called the quality factor, Q.

Since energy-storage devices must lose the least amount of energy, the higher the Q of the resonant circuit means that the capacitors and inductors used have good “quality”.

The width of the resonant response curve that can provide the required output is between the half-power frequencies, fH and fL. This width is usually called the bandwidth, B.

B f fH L

The bandwidth is related to the quality factor Q by the equation

Bf

Q 0

Q RCR

L0 00

BRC

1

2

R L CRB

v i

v

v

hfeZ

hiei

0

Designing the circuit to have C >> Cbe + Cbc and R <<1

hoe,

6

maximum energy stored . amount of enegy lost per cycle

Q =

YZ R j L

j CR

j RCR

j L

1 1 1 11

YR

jQ

11 0

0

0

v

v

hfe

hieR

jQi

0

00

0

1

1

vv

vv

Qi

i

si

si

si

0

002

0

0

2

0 1

1 0

2

0

0

2

Qf

f

f

fsi

si

1.2.2 OSCILLATORS

Basic Configuration of a Resonant Circuit Oscillator

Oscillator Circuits

7

Gain at = o

Gain at = si

A

X1 X2

X3

RFC

C 1

C 2

V oL

R E C E

R 2

R 1

C C

V CC

RFC

C V oL

R E C E

R 2

R 1

C C

V CC

Colpitts Oscillator Hartley Oscillator

1.3 FREQUENCY TRANSLATION AND FILTER RESPONSES

1.3.1 Frequency Translation

v t V f( ) ( )

v t e V f fj tc

c( ) ( )

v t t V f f V f fc c c( )cos 1

2

v t tj

V f f V f fc c c( ) sin 1

2

1 ( )f

cos c c ct f f f f 1

2

sin c c ctj

f f f f 1

2

8

1

1/2

fc

j/2

- fc

Tank Circuit

1

W -W

1

fc

j/2

- fc

1/2

fc

1.3.2 Filter Response

1.3.2.1 Low Pass Filter

1.3.2.2 High Pass Filter

1.3.2.3 Bandpass Filter

1.3.2.4 Block Diagram Examples

1) The system below is a simplified speech scrambler used to ensure communication privacy and foil wiretapping. Analyze its operation by sketching the spectrum at each stage, taking X(f) as shown. Draw also a descrambler for the system.

2) Consider the system skeched in the figure below, F1 (f) and F2 (f) as shown. a) Sketch the transform of f3 (t).b) Sketch the transform of g (t).c) Draw one possible realization of a complete receiver.

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HPFfC0 = 20 kHz

LPFB = 20 kHzx(t)

25 kHz20 kHz

X(f)

f (kHz)5-5

BPFfo = fC

B = W

fc fL fH- fH - fL

W

A

- fc

f

LPFfc o , B = fL

HPFfc o = fL

fL

fH- fH

- fL

A

A

f

f

3) Draw the spectrum of the different time-domain values.

1.3.3 Bandpass Systems

The simplest bandpass system is the parallel resonant or tuned circuit. Since practical tuned circuits usually have 10 < Q < 100, the 3-dB (half-power) bandwidth falls between 1% and 10% of the center frequency value.

0.01 < B

fc

< 0.1

10

90PHASESHIFT

HPFfc o = fc

LPFfc o = fc

a(t)

b(t)

c(t) d(t) e(t)f(t)

cos c t

cos c t

f1 (t)

25 kHz

10 kHz

f (kHz)

F1 (f)

5-5

f2 (t)

1

5

F2 (f)

f (kHz)-5

1

1% < B

fc

< 10%

For instance, the antennas in a radio system produce considerable distortion unless the frequency range is small compared to fc. Moreover, designing a reasonably distortionless bandpass amplifier turns out to be quite difficult if B is either very large or very small compared to fc. As a rough rule of thumb, the fractional bandwidth B/fc must be kept within the range considered.

1.4 MODULATION

Modulation is the systematic alteration of a high-frequency carrier wave in accordance with the instantaneous value of the modulating signal. The modulating signal that varies the parameters of the carrier is usually the baseband signal. The carrier wave after modulation is also called the modulated signal or the transmitted signal.

A carrier wave may be represented by the equations

e(t) = Em cos (c t + )

c(t) = Ac cos (c t + )

where e(t) and c(t) is the instantaneous value of the carrierEm and Ac is the maximum amplitudec is the angular velocity of the carrier (2fc ) is its phase angle

1.4.1 Why Modulate?

Modulation is required to match the signal to the transmission medium. Some of the major reasons why modulation is required are:

Modulation for ease of radiation Modulation for frequency assignment and multiplexing Modulation to reduce noise and interference

1.4.1 General Types of Modulation

Continuous Wave (CW) Modulation - carrier is a sinusoidal waveform

Pulse Modulation- carrier is a periodic train of pulses

Digital Modulation- modulation of binary symbols

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