Introduction to Communication Engineering

Introduction to Communication Engineering

Significance of Human Communication
Communication is the process of exchanging information. Main barriers are language and distance. Contemporary society’s emphasis is now the accumulation, packaging, and exchange of information.

Significance of Human Communication
Methods of communication: Face to face Signals Written word (letters) Electrical innovations: Telegraph Telephone Radio Television Internet (computer)

Communication Systems
Basic components: Data Source (where the data originates) Transmitter (device used to transmit data) Transmission Medium (cables or non cable) Receiver (device used to receive data) Destination (where the data will be placed) Noise degrades or interferes with transmitted information.

A general model of all communication systems.

Examples of Communication

Transmitter The transmitter is a collection of electronic components and circuits that converts the electrical signal into a signal suitable for transmission over a given medium. Transmitters are made up of oscillators, amplifiers, tuned circuits and filters, modulators, frequency mixers, frequency synthesizers, and other circuits.

Communication Channel The communication channel is the medium by which the electronic signal is sent from one place to another. Types of media include Electrical conductors Optical media Free space System-specific media (e.g., water is the medium for sonar).

Transmission Media Speed
Bandwidth: The amount of data which can be transmitted on a medium over a fixed amount of time (second). It is measured on Bits per Second or Baud Bits per Second (bps): A measure of transmission speed. The number of bits (0 0r 1) which can be transmitted in a second Baud Rate: Is a measure of how fast a change of state occurs (i.e. a change from 0 to 1)

Receivers A receiver is a collection of electronic components and circuits that accepts the transmitted message from the channel and converts it back into a form understandable by humans. Receivers contain amplifiers, oscillators, mixers, tuned circuits and filters, and a demodulator or detector that recovers the original intelligence signal from the modulated carrier.

Transceivers A transceiver is an electronic unit that incorporates circuits that both send and receive signals. Examples are: Telephones Fax machines Handheld CB radios Cell phones Computer modems

Attenuation Signal attenuation, or degradation, exists in all media of wireless transmission. It is proportional to the square of the distance between the transmitter and receiver.

Noise Noise is random, undesirable electronic energy that enters the communication system via the communicating medium and interferes with the transmitted message.

Types of Electronic Communication
Electronic communications are classified according to whether they are One-way (simplex) two-way (full duplex or half duplex) transmissions Analog or digital signals.

Simplex The simplest method of electronic communication is referred to as simplex. This type of communication is one-way. Examples are: Radio TV broadcasting Beeper (personal receiver)

Full Duplex Most electronic communication is two-way and is referred to as duplex. When people can talk and listen simultaneously, it is called full duplex. The telephone is an example of this type of communication.

Half Duplex The form of two-way communication in which only one party transmits at a time is known as half duplex. Examples are: Police, military, etc. radio transmissions Citizen band (CB) Family radio Amateur radio

Analog Signals An analog signal is a smoothly and continuously varying voltage or current. Examples are: Sine wave Voice Video (TV)

Analog signals (a) Sine wave “tone.” (b) Voice. (c) Video (TV) signal.

Digital Signals Digital signals change in steps or in discrete increments. Most digital signals use binary or two-state codes. Examples are: Telegraph (Morse code) Continuous wave (CW) code Serial binary code (used in computers)

Digital signals (a) Telegraph (Morse code). (b) Continuous-wave (CW) code. (c) Serial binary code.

Digital Signals Many transmissions are of signals that originate in digital form but must be converted to analog form to match the transmission medium. Digital data over the telephone network. Analog signals. They are first digitized with an analog-to-digital (A/D) converter. The data can then be transmitted and processed by computers and other digital circuits.

Modulation and Multiplexing
Modulation and multiplexing are electronic techniques for transmitting information efficiently from one place to another. Modulation makes the information signal more compatible with the medium. Multiplexing allows more than one signal to be transmitted concurrently over a single medium.

Modulation and Multiplexing
Multiplexing at the transmitter.

Signal A signal is a pattern of variation that carry information. Signals are represented mathematically as a function of one or more independent variable Signal can be:- Electrical signals like voltages, current and EM field in circuit Acoustic signals like audio or speech signals (analog or digital) Video signals like intensity variation in an image Biological signal like sequence of bases in gene Noise which will be treated as unwanted signal

Signal classification
Continuous-time and Discrete-time (Analog/Digital) Energy and Power Real and Complex Periodic and Non-periodic Analog and Digital Even and Odd Deterministic and Random

Analog and Digital Signals
Digital Electronics TM 1.2 Introduction to Analog Analog and Digital Signals Continuous Infinite range of values More exact values, but more difficult to work with Analog Signals Discrete Finite range of values (2) Not as exact as analog, but easier to work with Digital Signals Example: A digital thermostat in a room displays a temperature of 72. An analog thermometer measures the room temperature at . The analog value is continuous and more accurate, but the digital value is more than adequate for the application and significantly easier to process electronically. This slide defines analog and digital signals and gives several examples of each. Project Lead The Way, Inc. Copyright 2009

Example of Analog Signals
Analog and Digital Signals Digital Electronics TM 1.2 Introduction to Analog Example of Analog Signals An analog signal can be any time-varying signal. Minimum and maximum values can be either positive or negative. They can be periodic (repeating) or non-periodic. Sine waves and square waves are two common analog signals. Note that this square wave is not a digital signal because its minimum value is negative. 0 volts Sine Wave Square Wave (not digital) Random-Periodic Examples of common analog signals. Project Lead The Way, Inc. Copyright 2009

Parts of an Analog Signal
Analog and Digital Signals Digital Electronics TM 1.2 Introduction to Analog Parts of an Analog Signal Amplitude (peak-to-peak) (peak) Period (T) Frequency: Parts of an analog signal: amplitude, period, & frequency. Project Lead The Way, Inc. Copyright 2009

Analog and Digital Signals
Digital Electronics TM 1.2 Introduction to Analog Logic Levels Before examining digital signals, we must define logic levels. A logic level is a voltage level that represents a defined digital state. Logic HIGH: The higher of two voltages, typically 5 volts Logic LOW: The lower of two voltages, typically 0 volts 5.0 v Logic High This slide introduces the concept of logic levels, gives the range of acceptable voltages for a logic high & low, and lists other common terms used to describe logic levels. Logic Level Voltage True/False On/Off 0/1 HIGH 5 volts True On 1 LOW 0 volts False Off 2.0 v Invalid Logic Level 0.8 v 0.0 v Logic Low Project Lead The Way, Inc. Copyright 2009

Example of Digital Signals
Analog and Digital Signals Digital Electronics TM 1.2 Introduction to Analog Example of Digital Signals Digital signal are commonly referred to as square waves or clock signals. Their minimum value must be 0 volts, and their maximum value must be 5 volts. They can be periodic (repeating) or non-periodic. The time the signal is high (tH) can vary anywhere from 1% of the period to 99% of the period. 5 volts Examples of common digital signals. 0 volts Project Lead The Way, Inc. Copyright 2009

Parts of a Digital Signal
Analog and Digital Signals Digital Electronics TM 1.2 Introduction to Analog Parts of a Digital Signal Amplitude: For digital signals, this will ALWAYS be 5 volts. Period: The time it takes for a periodic signal to repeat. (seconds) Frequency: A measure of the number of occurrences of the signal per second. (Hertz, Hz) Time High (tH): The time the signal is at 5 v. Time Low (tL): The time the signal is at 0 v. Duty Cycle: The ratio of tH to the total period (T). Rising Edge: A 0-to-1 transition of the signal. Falling Edge: A 1-to-0 transition of the signal. Falling Edge Amplitude Time High (tH) Time Low (tL) Rising Edge Period (T) The parts of a digital signal: amplitude, period & frequency, time high, time low, duty cycle, rising & falling edge. Frequency: Project Lead The Way, Inc. Copyright 2009

Deterministic and Random signal
A signal is deterministic whose future values can be predicted accurately. (there is no uncertainty) Example: Behavior of these signals is predictable w.r.t time There is no uncertainty with respect to its value at any time. These signals can be expressed mathematically. For example x(t) = sin(3t) is deterministic signal.

A signal is random whose future values can NOT be predicted with complete accuracy Behavior of these signals is random i.e. not predictable w.r.t time. There is an uncertainty with respect to its value at any time. These signals can’t be expressed mathematically. For example (Thermal) Noise generated is non deterministic signal.

Random signals whose future values can be statistically determined based on the past values are correlated signals. Random signals whose future values can NOT be statistically determined from past values are uncorrelated signals and are more random than correlated signals.

Periodic and Non-periodic Signals
A signal is said to be periodic if it repeats after a fixed time period. Given x(t) is a continuous-time signal x (t) is periodic if x(t) = x(t+Tₒ) for any T and any integer n Example x(t) = A cos(wt) x(t+Tₒ) = A cos[w(t+Tₒ)] = A cos(wt+wTₒ) = A cos(wt+2p) = A cos(wt) Note: Tₒ =1/fₒ ; w=2pfₒ

Periodic and Non-periodic Signals Contd.
A signal is said to be non periodic if it does not repeat. For non-periodic signals x(t) ≠ x(t+Tₒ) A non-periodic signal is assumed to have a period T = ∞ Example of non periodic signal is an exponential signal

Periodic and Non-periodic Signals Contd.

Scaling of signal

The Electromagnetic Spectrum
EM wave is a signal made of oscillating electric and magnetic fields. The range of electromagnetic signals encompassing all frequencies is referred to as the electromagnetic spectrum.

Frequency and Wavelength: Frequency A signal is located on the frequency spectrum according to its frequency and wavelength. Frequency is the number of cycles of a repetitive wave that occur in a given period of time. A cycle consists of two voltage polarity reversals, current reversals, or electromagnetic field oscillations. Frequency is measured in cycles per second (cps). The unit of frequency is the hertz (Hz).

Frequency and Wavelength: Wavelength Wavelength is the distance occupied by one cycle of a wave and is usually expressed in meters. Wavelength is also the distance traveled by an electromagnetic wave during the time of one cycle. The wavelength of a signal is represented by the Greek letter lambda (λ).

Frequency and wavelength. (a) One cycle. (b) One wavelength.

Frequency and Wavelength: Wavelength Wavelength (λ) = speed of light ÷ frequency Speed of light = 3 × 108 meters/second Therefore: λ = 3 × 108 / f Example: What is the wavelength if the frequency is 4MHz? λ = 3 × 108 / 4 MHz = 75 meters (m)

Frequency Ranges from 30 Hz to 300 GHz The electromagnetic spectrum is divided into segments: Extremely Low Frequencies (ELF) 30–300 Hz. Voice Frequencies (VF) 300–3000 Hz. Very Low Frequencies (VLF) include the higher end of the human hearing range up to about 20 kHz. Low Frequencies (LF) 30–300 kHz. Medium Frequencies (MF) 300–3000 kHz AM radio 535–1650 kHz.

Frequency Ranges from 30 Hz to 300 GHz High Frequencies (HF) (short waves; VOA, BBC broadcasts; government and military two-way communication; amateur radio, CB. 3–30 MHz Very High Frequencies (VHF) FM radio broadcasting (88–108 MHz), television channels 2–13. 30–300 MHz Ultra High Frequencies (UHF) TV channels 14–67, cellular phones, military communication. 300–3000 MHz

Frequency Ranges from 30 Hz to 300 GHz Microwaves and Super High Frequencies (SHF) Satellite communication, radar, wireless LANs, microwave ovens 1–30 GHz Extremely High Frequencies (EHF) Satellite communication, computer data, radar 30–300 GHz

Optical Spectrum The optical spectrum exists directly above the millimeter wave region. Three types of light waves are: Infrared Visible spectrum Ultraviolet

Optical Spectrum: Infrared Infrared radiation is produced by any physical equipment that generates heat, including our bodies. Infrared is used: In astronomy, to detect stars and other physical bodies in the universe, For guidance in weapons systems, where the heat radiated from airplanes or missiles can be detected and used to guide missiles to targets. In most new TV remote-control units, where special coded signals are transmitted by an infrared LED to the TV receiver to change channels, set the volume, and perform other functions. In some of the newer wireless LANs and all fiber-optic communication.

Optical Spectrum: The Visible Spectrum Just above the infrared region is the visible spectrum we refer to as light. Red is low-frequency or long-wavelength light Violet is high-frequency or short-wavelength light. Light waves’ very high frequency enables them to handle a tremendous amount of information (the bandwidth of the baseband signals can be very wide).

Optical Spectrum: Ultraviolet Ultraviolet is not used for communication Its primary use is medical.

Gain, Attenuation, and Decibels
Most circuits in electronic communication are used to manipulate signals to produce a desired result. All signal processing circuits involve: Gain Attenuation

Gain means amplification. It is the ratio of a circuit’s output to its input. AV = = output input Vout Vin An amplifier has gain.

Most amplifiers are also power amplifiers, so the same procedure can be used to calculate power gain AP where Pin is the power input and Pout is the power output. Power gain (Ap) = Pout / Pin Example: The power output of an amplifier is 6 watts (W). The power gain is 80. What is the input power? Ap = Pout / Pin therefore Pin = Pout / Ap Pin = 6 / 80 = W = 75 mW

An amplifier is cascaded when two or more stages are connected together. The overall gain is the product of the individual circuit gains. Example: Three cascaded amplifiers have power gains of 5, 2, and 17. The input power is 40 mW. What is the output power? Ap = A1 × A2 × A3 = 5 × 2 × 17 = 170 Ap = Pout / Pin therefore Pout = ApPin Pout = 170 (40 × 10-3) = 6.8W

Attenuation refers to a loss introduced by a circuit or component. If the output signal is lower in amplitude than the input, the circuit has loss or attenuation. The letter A is used to represent attenuation Attenuation A = output/input = Vout/Vin Circuits that introduce attenuation have a gain that is less than 1. With cascaded circuits, the total attenuation is the product of the individual attenuations.

Total attenuation is the product of individual attenuations of each cascaded circuit.

The decibel (dB) is a unit of measure used to express the gain or loss of a circuit. The decibel was originally created to express hearing response. A decibel is one-tenth of a bel. When gain and attenuation are both converted into decibels, the overall gain or attenuation of a circuit can be computed by adding individual gains or attenuations, expressed in decibels.

Decibels: Decibel Calculations Voltage Gain or Attenuation dB = 20 log Vout/ Vin Current Gain or Attenuation dB = 20 log Iout/ Iin Power Gain or Attenuation dB = 10 log Pout/ Pin

Decibels: Decibel Calculations Example: An amplifier has an input of 3 mV and an output of 5 V. What is the gain in decibels? dB = 20 log 5/0.003 = 20 log = 20 (3.22) = 64.4

Decibels: Decibel Calculations Example: A filter has a power input of 50 mW and an output of 2 mW. What is the gain or attenuation? dB = 10 log (2/50) = 10 log (0.04) = 10 (−1.398) = −13.98 If the decibel figure is positive, that denotes a gain.

Bandwidth Bandwidth (BW) is that portion of the electromagnetic spectrum occupied by a signal. It is the difference between the upper and lower frequency limits of the signal. Channel bandwidth refers to the range of frequencies required to transmit the desired information.

Bandwidth requirements for various signals
Telegraph Signals:- Telegraph speed is often expressed in terms of the reciprocal of the duration in the seconds, of the shorted signaling element termed as band. The shortest time element is of 20 milliseconds. Therefore the Bandwidth required is, BW = 1/20X10^-3 = 50Hz

Voice/Speech signal:-
Voice signal ranges from 300Hz to 3000Hz so the Bandwidth required is 2700 Hz. Music Signal:- Musical Instruments produce harmonics whose orders and amplitude decides the quality of musical output. For this purpose a frequency range of 30Hz to 15 KHz is needed for high quality music transmission. Therefore for music transmission bandwidth required is, KHz ( )

Television Signals:- For television signals bandwidth required is 6 MHz For digital data transmission:- Many digital data transmission utilize telephone channels, the bandwidth of the telephone is an appropriate consideration. The faster the rate of data transmission, the greater the bandwidth required. The telephone channel occupies the frequency range of 300 Hz to 3400 Hz.

When data is sent over telephone channel, the speed must be limited to ensure that the bandwidth required by the data transmission will not exceed the telephone channel bandwidth. The data rates of common systems are limited to a maximum rate of about 10,800 bits per second (bps) for a telephone channel.

Noise What is Noise? With reference to an electrical system, noise may be defined as any unwanted form of energy which tends to interfere with proper reception and reproduction of wanted signal. OR Noise is random, undesirable electrical energy that enters the communications system via the communicating medium and interferes with the transmitted message. However, some noise is also produced in the receiver.

Classification of Noise
Noise may be put into following two categories. External noises, i.e. noise whose sources are external. External noise may be classified into the following three types: Atmospheric noises Extraterrestrial noises Man-made noises or industrial noises.

Thermal noise or white noise or Johnson noise Shot noise.
Internal noise in communication:- Noises which get, generated within the receiver or communication system. Internal noise may be put into the following four categories. Thermal noise or white noise or Johnson noise Shot noise. Transit time noise Miscellaneous internal noise.

External noise cannot be reduced except by changing the location of the receiver or the entire system. Internal noise on the other hand can be easily evaluated Mathematically and can be reduced to a great extent by proper design. As already said, because of the fact that internal noise can be reduced to a great extent, study of noise characteristics is a very important part of the communication engineering.

Atmospheric Noise Atmospheric noise or static is caused by lighting discharges in thunderstorms and other natural electrical disturbances occurring in the atmosphere. These electrical impulses are random in nature. Hence the energy is spread over the complete frequency spectrum used for radio communication. Atmospheric noise accordingly consists of spurious radio signals with components spread over a wide frequency range.

These spurious radio waves constituting the noise get propagated over the earth in the same fashion as the desired radio waves of the same frequency. Accordingly at a given receiving point, the receiving antenna picks up not only the signal but also the static from all the thunderstorms, local or remote. The field strength of atmospheric noise varies approximately inversely with the frequency

Thus large atmospheric noise is generated in low and medium frequency (broadcast) bands while very little noise is generated in the VHF and UHF bands. Further VHF and UHF components of noise are limited to the line-of-sight (less than about 80 Km) propagation. For these two-reasons, the atmospheric noise becomes less severe at Frequencies exceeding about 30 MHz

Extraterrestrial Noise
There are numerous types of extraterrestrial noise or space noises depending on their sources. However, these may be put into following two subgroups. Solar noise Cosmic noise

Solar Noise This is the electrical noise emanating from the sun.
Under quite conditions, there is a steady radiation of noise from the sun. This results because sun is a large body at a very high temperature and radiates electrical energy in the form of noise over a very (exceeding 6000°C on the surface), wide frequency spectrum including the spectrum used for radio communication.

The intensity produced by the sun varies with time
The intensity produced by the sun varies with time. In fact, the sun has a repeating 11-Year noise cycle. During the peak of the cycle, the sun produces some amount of noise that causes tremendous radio signal interference, making many frequencies unusable for communications. During other years the noise is at a minimum level.

Cosmic noise Distant stars are also suns and have high temperatures. These stars, therefore, radiate noise in the same way as our sun. The noise received from these distant stars is thermal noise (or black body noise) and is distributing almost uniformly over the entire sky. We also receive noise from the center of our own galaxy (The Milky Way) from other distant galaxies and from other virtual point sources such as quasars and pulsars.

Man-Made Noise (Industrial Noise)
It is the electrical noise produced by such sources as automobiles and aircraft ignition, electrical motors and switch gears, leakage from high voltage lines, fluorescent lights, and numerous other heavy electrical machines. Such noises are produced by the arc discharge taking place during operation of these machines. Such man-made noise is most intensive in industrial and densely populated areas. Man-made noise in such areas far exceeds all other sources of noise in the frequency range extending from about 1 MHz to 600 MHz

Internal Noise Thermal Noise
Conductors contain a large number of 'free" electrons and "ions" strongly bound by molecular forces. The ions vibrate randomly about their normal (average) positions, however, this vibration being a function of the temperature. Continuous collisions between the electrons and the vibrating ions take place. Thus there is a continuous transfer of energy between the ions and electrons.

The movement of free electrons constitutes a current which is purely random in nature and over a long time averages zero. There is a random motion of the electrons which give rise to noise voltage called thermal or thermal agitation or Johnson noise. “Thus noise generated in any resistance due to random motion of electrons is called thermal noise or white or Johnson noise.” The analysis of thermal noise is based on the Kinetic theory.

At -273°C (or zero degree Kelvin) the kinetic energy of the particles of a body becomes zero .Thus we can relate the noise power generated by a resistor to be proportional to its absolute temperature and the bandwidth over which it is measured. From the above discussion we can write down. Pn ∝ TB Pn = KTB (1) Where Pn = Maximum noise power output of a resistor. K = Boltzmann’s constant = 1.38 x10-23 joules I Kelvin. T = Absolute temperature. B = Bandwidth over which noise is measured.

From equation (1), an equivalent circuit can be drawn as shown in below figure

From equation (2), we see that the square of the rms noise voltage is proportional to the absolute temperature, the value of the resistor, and the bandwidth over which it is measured. En is quite independent of the Frequency. Example R.F. amplifier is saving an input resistor of 8Kohm and works in the frequency range of 12 to 15.5 MHz Calculate the rms noise voltage at the input to this amplifier at an ambient temperature of 17oC? Solution:

Shot Noise It produces by the random arrival of electrons or holes at the output element, at the plate in a tube, or at the collector or drain in a transistor. Shot noise is also produced by the random movement of electrons or holes across a PN junction.

Even through current flow is established by external bias voltages, there will still be some random movement of electrons or holes due to discontinuities in the device. An example of such a discontinuity is the contact between the copper lead and the semiconductor materials. The interface between the two creates a discontinuity that causes random movement of the current carriers.

Transit Time Noise Another kind of noise that occurs in transistors is called transit time noise. Transit time is the duration of time that it takes for a current carrier such as a hole or current to move from the input to the output. The devices themselves are very tiny, so the distances involved are minimal. Yet the time it takes for the current carriers to move even a short distance is finite.

At some frequencies this time is negligible
At some frequencies this time is negligible. But when the frequency of operation is high and the signal being processed is the magnitude as the transit time, then problem can occur. The transit time shows up as a kind of random noise within the device, and this is directly proportional to the frequency of operation. It is also called as high frequency noise.

MISCELLANEOUS INTERNAL NOISES Flicker Noise (Low frequency Noise)
Flicker noise or modulation noise is the one appearing in transistors operating at low audio frequencies(below few KHz). Flicker noise is proportional to the emitter current and junction temperature. However, this noise is inversely proportional to the frequency hence it is sometime referred as 1/f noise. Hence it may be neglected at frequencies above about 500 Hz and it, Therefore, possess no serious problem.

Transistor Thermal Noise Partition Noise
Within the transistor, thermal noise is caused by the emitter, base and collector internal resistances. Out of these three regions, the base region contributes maximum thermal noise. Partition Noise Partition noise occurs whenever current has to divide between two or more paths, and results from the random fluctuations in the division. It would be expected, therefore, that a diode would be less noisy than a transistor (all other factors being equal) If the third electrode draws current (i.e.., the base current). It is for this reason that the inputs of microwave receivers are often taken directly to diode mixers.

Correlated Noise Inter-modulation distortion
When two or more signals are amplified in non-linear device, such as large signal amplifiers some unwanted sum and difference frequencies are generated The distortion due to unwanted frequencies are called inter-modulation distortion The sum and difference frequencies are called cross products. Cross product = mf1 ± mf2 where f1,f2= fundamental frequencies m and n = positive integer between one and infinity

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