COMM 704: Communication Systems

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1 COMM 704: Communication Lecture 1: Introduction Dr. Mohamed Abd El Ghany,

2 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

3 Text and Reference Books Wayne Tomasi, Electronic Communication, Prentice Hall, ISBN: Frank R. Dungan, Electronic communications systems, PWS Publishers, ISBN x William schweber, Electronic Communications systems: Acomplete Course, Prentice Hall, ISBN Behzad Razavi, RF Microelectronics, Prentice Hall PTR, ISBN

4 Prerequisites Communication Microelectronics (ELCT 508) Signal and (COMM 401) Modulation I (COMM 601) Modulation II (702) 4

5 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% 5

6 Course Outline Introduction Multipliers Filters Oscillators Power amplifiers AM/FM modulation Transceiver architectures Television Satellite communications systems 6

7 Course Outline Introduction Multipliers Filters Oscillators Power amplifiers AM/FM modulation Transceiver architectures Television Satellite communications systems 7

8 Radio Communication Services Radio broadcasting TV broadcasting Satellite communication Mobile telephony Internet And more. 8

9 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

10 Types of communication A A A B B 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 10

11 Antenna Design Antenna dimension ~ λ/2 For voice signal (f~ 3KHz) λ = c/f = (3x10 8 )/(3x10 3 )= 100 km D = λ /2 = 50 km! Impossible to realize 11

signal(carrier wave), which is more suitable to transmit.

12 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 12

13 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. 13

14 Modulation Types Modulation Analog Digital Amplitude Modulation (AM) Amplitude Shift Key (ASK) Frequency Modulation (FM) Frequency Shift Key (FSK) Phase Modulation (PM) Phase Shift Key (PSK) 14

AM/FM Modulation In the AM process, the alternating current from the microphone modulates the carrier wave by causing carrier wave s amplitude or

15 AM/FM Modulation 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. In the FM process, the alternating current from the microphone modulates the carrier wave by changing carrier wave s frequency. 15

16 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

17 Carrier Frequency Bands Name Frequency Range Wave Length ELF 300Hz to 3KHz 100Km to 1000Km VLF 3KHz to 30KHz 10Km to 100Km LF 30KHz to 300KHz 1Km to 10Km Application Navigation, long distance communication with ships Navigation, long distance communication 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) 17

18 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 18

19 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 19

20 FM Transmitter AF Amplifier Voltage Controlled Oscillator Power Amplifier 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. 20

AM Superheterodyne Receiver RF Amplifier Mixer

using mixer and local oscillator components.

21 AM Superheterodyne Receiver RF Amplifier Mixer IF Amplifier Detector AF Amplifier Local Oscillator 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. 21

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

22 FM Superheterodyne Receiver RF Amplifier Mixer IF Amplifier Limiter Discriminator AF Amplifier Local Oscillator 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. 22

23 Case Studies The main components of the Motorola s FM receiver are: -Antenna -LC matching network -Mixer -Bandpass Filter -Voltage Controlled Oscillator -Crystal Oscillator -Limiter Simplified architecture of Motorola s FM receiver 23

24 Case Studies Philips DECT Transceiver 24

25 Characteristics of Tuned LC Circuits For any series or parallel LC circuit, the inductive reactance X L and Capacitance X c will be equal at some frequency. The frequency at which X L =X c 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. 25

26 Series Resonant LC Circuit When the generator frequency is above or below the resonant frequency, the net reactance X is no longer zero and Z t increases. Above the resonant frequency, X L >X c and the net reactance X is inductive. Below the resonant frequency, X c >X L and the net reactance X is capacitive. 26

27 Series Resonant LC Circuit 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 X L at f o For example: if L is doubled and C is halved, then f o does not change, but X L and X c each double in value. Assuming the series resistance remains the same, Q doubles. 27

28 Series Resonant LC Circuit 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. 28

29 Series Resonant LC Circuit 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. 29

30 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 X L and X c being equal. Since the inductive current I L and the capacitance current I c are 180 out of phase, the net line current equals zero at resonant frequency With a total line current I T of zero, the tank impedance Z tank approaches infinity at the resonant frequency. 30

31 Parallel Resonant LC Circuit Practical LC Tank Circuit In a practical LC tank circuit, a coil always contains amount of internal resistance. I L is always slightly less than I C at f o. The net current I T is never exactly zero, and as a result the tank impedance is never actually infinity. 31

32 Parallel Resonant LC Circuit Tank Impedance at the resonant frequency Since Q = X L /r s, the equation can be reduced to : Since X C = X L at f o, Z tank is usually stated as If Q 10, which is usually the case, then the following approximations can be made 32

33 Parallel Resonant LC Circuit Q of a parallel resonant circuit Or Bandwidth of a parallel resonant circuit The bandwidth includes the frequencies extending from f 1 to f 2 (the edge frequencies). f1 and f2 are defined as those frequencies at which Z tank is reduced to 70.7% of its maximum value at f o. 33

34 Parallel Resonant LC Circuit Adding external load Decrease Q Increase BW R L R L reduces the sharpness of the resonant effect. 34

35 Problem For the tank circuit shown in Fig.1, calculate the following values at the resonant frequency f o : a) X L, X C, I L, I C, Q, Z tank and I T b) The bandwidth and edge frequencies V A = 300 mv L= 200 µh r s = 25 Ω C= 75 pf c) Assume that a 100KΩ load R L is placed in parallel with the tank, calculate Q overall and BW Fig.1 35

36 Solution a) 36

37 Solution b) 37

38 Solution c) Since Z tank < 10 R L 38

PRINCIPLES OF COMMUNICATION SYSTEMS. Lecture 1- Introduction Elements, Modulation, Demodulation, Frequency Spectrum

PRINCIPLES OF COMMUNICATION SYSTEMS Lecture 1- Introduction Elements, Modulation, Demodulation, Frequency Spectrum Topic covered Introduction to subject Elements of Communication system Modulation General