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Архитектура Аудит Военная наука Иностранные языки Медицина Металлургия Метрология Образование Политология Производство Психология Стандартизация Технологии |
Unit 1 Radio engineering systemsСтр 1 из 6Следующая ⇒
Unit 1 Radio engineering systems 1. Memorize the words: To travel – распространяться transmitting range – дальность передачи a receiver – приемник a transmitter – передатчик a high-frequency oscillator – высокочастотный генератор колебаний an oscillatory circuit – колебательный контур a capacitor – конденсатор an amplifier – усилитель a detector – детектор, следящий механизм a rectifier – выпрямитель, детонатор the audio frequency – звуковая частота to couple together – соединять, спаривать by means of a switch – с помощью переключателя (коммутатора) means of communication – средства связи telegraph sending key – телеграфный ключ dots and dashes – точки и тире the mirror galvanometer – зеркальный гальванометр powdered carbon – порошковый углерод a far sensitive receiver – гораздо более чувствительный приемник wireless communication – беспроводная связь a transmitting / receiving coil – передающая / приемная катушка
2. Read the text and explain the operation principle of radio communication: Radio communication Radio communication is the transmission of high frequency energy from the transmitter to the receiver without wires. Radio is a device that transmits and receives signals and programs by electromagnetic waves. Since the process of radio communication includes transmission and reception of signals, the two necessary components of radio are a transmitter and a receiver. The first component of radio, the transmitter, is a device producing radio-frequency energy. The transmitter consists of a high-frequency oscillator including an oscillatory circuit (a coil and a capacitor) and one or more amplifiers. Electric oscillations are produced in the antenna of the transmitter. They travel in all directions. Electron lamps are used to amplify currents and give greater transmitting range and better reception. Radio waves are electric waves of very high frequency; they travel through space at a speed of light, and differ from other wave forms only n frequency (number of vibrations per second). The second important component of radio communication is the receiver, a device that receives waves sent out by a transmitter. Radio receiver demodulates these waves, and they are heard as speech, music or signals. To understand this process let us consider the principle of operation of these devices. A microphone is connected to the circuit of the transmitting antenna. When we speak into the microphone its resistance varies with the audio frequency. An alternating current is established in the microphone and antenna circuits, and its frequency is the same as the audio frequency. Oscillations of the same frequency are induced in the antenna and the oscillatory circuit of a receiver. These oscillations are in fact a high-frequency current. In order to reproduce the transmitted sound, this current modulated by audio frequency should be sent through the telephone, and a detector or rectifier should be connected to the telephone circuit. The audio frequency rectified current passes through the telephone and produces oscillations. These oscillations will reproduce the sounds produced at the transmitting station. The operation of a radio set will be the better, the more energy is received by its oscillatory circuit. The oscillatory circuit is also provided with a ground. It is important for god operation of the receiver. The antenna should be grounded by means of a switch. Internet radio, also known as web radio or net radio is an audio broadcasting service transmitted via the Internet. Internet radio services are usually accessible from anywhere in the world. This makes it popular among listeners with interests that are not adequately served by local radio stations. Internet radio services offer news, sorts, talk and various genres of music – everything that is available on traditional radio stations.
3. Look through the text and answer the questions: 1. What is radio communication? 2. What are the main components of radio? 3. What is a transmitter? 4. What does it consist of? 5. What is used for amplifying currents? 6. What happens in the microphone when we speak into it? 7. What kind of current is established in the microphone and antennas circuit? 8. What device should be connected to the circuit in order to reproduce the transmitted sounds? 9. By what means is the antenna grounded? 10. How are transmitted sounds reproduced in the receiver?
4. Find out synonyms between: Nouns: specialist; improvement; traffic; dot; instrument; speed; transport; operation; expert; wire; point; conversation; invention; wireless; communication; tool; state; link; power; connection; perfection; possibility; prize; oscillations; bonus; capacitors; vibrations; energy; radio; feasibility; country; rate; work; conductor; discovery; speech; condenser; device. Verbs: to provide; to produce; to establish; to demonstrate; to induce; to amplify; to involve; to build; to improve; to receive; to lay; to link; to continue; to invent; to use; to go on; to apply; to supply; to install; to intensify; to construct; to perfect; to connect; to put; to get; to advance; to include; to move forward; to excite; to show; to generate. Adjectives: distant; fast; important; modern; several; wonderful; various; intelligible; simple; different; quick; some; far; primitive; understandable; remarkable; present-day; significant.
5. Read the text and tell about developed communication systems:
Radio waves Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequenciesfrom 300 GHz to as low as 3 kHz, and corresponding wavelengths ranging from 1 millimeter (0.039 in) to 100 kilometers (62 mi). Like all other electromagnetic waves, they travel at the speed of light. Naturally, occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, communication satellites, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight. To prevent interference between different users, the artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunications Union (ITU). The radio spectrum is divided into a number of radio bands based on frequency, allocated to different uses. Radio waves were first predicted by mathematical work done in 1867 by Scottish mathematical physicist James Clerk Maxwell. Maxwell noticed wavelike properties of light and similarities in electrical and magnetic observations. He then proposed equations that described light waves and radio waves as waves of electromagnetism that travel in space, radiated by a charged particle as it undergoes acceleration. In 1887, Heinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory. Many inventions followed, making the use of radio waves to transfer information through space. The study of electromagnetic phenomena such as reflection, refraction, polarization, diffraction, and absorption is of critical importance in the study of how radio waves move in free space and over the surface of the Earth. Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others. Radio waves travel at the speed of light in a vacuum. When passing through an object, they are slowed according to that object's permeability and permittivity. The wavelength is the distance from one peak of the wave's electric field to the next, and is inversely proportional to the frequency of the wave. The distance a radio wave travels in one second, in a vacuum, is 299, 792, 458 meters (983, 571, 056 ft) which is the wavelength of a 1-hertz radio signal. A 1-megahertz radio signal has a wavelength of 299.8 meters (984 ft). In order to receive radio signals, for instance from AM/FM radio stations, a radio antenna must be used. However, since the antenna will pick up thousands of radio signals at a time, a radio tuner is necessary to tune in a particular signal. This is typically done via a resonator (in its simplest form, a circuit with a capacitor and an inductor). The resonator is configured to resonate at a particular frequency, allowing the tuner to amplify sine waves at that radio frequency and ignore other sine waves. Usually, either the inductor or the capacitor of the resonator is adjustable, allowing the user to change the frequency at which it resonates. The etymology of " radio" or " radiotelegraphy" reveals that it was called " wireless telegraphy", which was shortened to " wireless" in Britain. The prefix radio- in the sense of wireless transmission was first recorded in the word radio conductor, a description provided by the French physicist Edouard Branly in 1897. It is based on the verb to radiate (in Latin " radius" means " spoke of a wheel, beam of light, ray" ). The word " radio" also appears in a 1907 article by Lee De Forest. It was adopted by the United States Navy in 1912, to distinguish radio from several other wireless communication technologies, such as the photo phone. The term became common by the time of the first commercial broadcasts in the United States in the 1920s. (The noun " broadcasting" itself came from an agricultural term, meaning " scattering seeds widely." ) The term was adopted by other languages in Europe and Asia. British Commonwealth countries continued to use commonly the term " wireless" until the mid-20th century, though the magazine of the BBC in the UK has been called Radio Times ever since it was first published in the early 1920s. In recent years, the more general term " wireless" has gained renewed popularity through the rapid growth of short-range computer networking, e.g., Wireless Local Area Network (WLAN), Wi-Fi, and Bluetooth, as well as mobile telephony, e.g., GSM and UMTS. Today, the term " radio" specifies the actual type of transceiver device or chip, whereas " wireless" refers to the lack of physical connections; one talks about radio transceivers, but other talks about wireless devices and wireless sensor networks.
13. Translate the verbs and their derivatives: To communicate – communication; communicative; uncommunicative; communicator. To transmit – transmitter; transmission; transmitted; transmissible; transmitting (coil). To receive – receiver; reception; receptive; receptivity; receiving (coil). To follow – follower; following. To contribute – contribution; contributor; contributory. To invent – inventor; invention; invented. To implement – implementation; implemented. To retrieve – retrieval; retrievable; irretrievable. To improve – improvement; improver; improved; unimproved; improvable; unimprovable. To appear – to disappear; appearance; disappearance. To establish – to disestablish; established; establishment. To predict – predicted; prediction; predictor. To address – addressability; addressable; addressee; addressing; addressless; addressness. Sequence – sequent; sequential; sequencer; consequently. Function – functional; functionality; functionally/
14. Read the texts, study basic types of modulation and speak of them:
Basic types of modulation Today vast amounts of information are communicated using radio communications systems. Both analogue radio communications systems and digital or data radio communications links are used. However, one of the fundamental aspects of any radio communications transmission system is modulation, or the way in which the information is superimposed on the radio carrier. In order that a steady radio signal or " radio carrier" can carry information, it must be changed or modulated in one way so that the information can be conveyed from one place to another. There are very many ways in which a radio carrier can be modulated to carry a signal, each having its own advantages and disadvantages. The choice of modulation have a great impact on the radio communications system. Some forms are better suited to one kind of traffic whereas other forms of modulation will be more applicable in other instances. Choosing the correct form of modulation is a key decision in any radio communications system design. There are three main ways in which a radio communications or RF signal can be modulated: - Amplitude modulation, AM: as the name implies, this form of modulation involves modulating the amplitude or intensity of the signal.
Amplitude modulation was the first form of modulation to be used to broadcast sound, and although other forms of modulation are being increasingly used, amplitude modulation is still in widespread use. - Frequency modulation, FM: this form of modulation varies the frequency in line with the modulating signal.
Frequency modulation has the advantage that, as amplitude variations do not carry any information on the signal, it can be limited within the receiver to remove signal strength variations and noise. As a result, this form of modulation has been used for many applications including high quality analogue sound broadcasting. - Phase modulation, PM: as the name indicates, phase modulation varies the phase of the carrier in line with the modulating signal.
Phase modulation and frequency modulation have many similarities and are linked - one is the differential of the other. However, phase modulation lends itself to data transmissions, and as a result, its use has grown rapidly over recent years. Each type of modulation has its own advantages and disadvantages, and accordingly they are all used in different radio communications applications. In addition to the three main basic forms of modulation or modulation techniques, there are many variants of each type. Again, these modulation techniques are used in a variety of applications, some for analogue applications, and others for digital applications. Angle Modulation Angle modulation is a name given to forms of modulation that are based on altering the angle or phase of a sinusoidal carrier. Using angle modulation there is no change in the amplitude of the carrier. The two forms of modulation that fall into the angle modulation category are frequency modulation and phase modulation. Both types of angle modulation, namely, frequency modulation and phase modulation are linked because frequency is the derivative of phase, i.e. frequency is the rate of change of phase. Another way of looking at the link between the two types of modulation is that a frequency modulated signal can be generated by first integrating the modulating waveform and then using the result as the input to a phase modulator. Conversely, a phase modulated signal can be generated by first differentiating the modulating signal and then using the result as the input to a frequency modulator.
Signal bandwidth One key element of any signal is the bandwidth it occupies. This is important because it defines the channel bandwidth required, and hence the number of channels that can be accommodated within a given segment of radio spectrum. With pressure on the radio spectrum increasing, the radio signal bandwidth is an important feature of any type of radio emission or transmission. The bandwidth is governed by two major features: - The type of modulation: Some forms of modulation use their bandwidth more effectively than others. Accordingly, where spectrum usage is of importance, this alone may dictate the choice of modulation. - The bandwidth of the modulating signal: A law called Shannon's law determines the minimum bandwidth through which a signal can be transmitted. In general, the wider the bandwidth of the modulating signal, the wider the bandwidth required. Modulating signal type Apart from the form of modulation itself, the type of signal being used to modulate the carrier also has a bearing on the signal. Analogue and data are two very different forms of modulating signal and need to be treated differently. While different formats of actual modulation may be used, the type of signal being applied via the modulator also have a bearing on the signal. Signals for high quality stereo broadcasting will be treated differently to signals that provide digital telemetry for example. As a result, it is often important to know the signal type that needs to be carried by the RF carrier. By Ian Poole
Radio transmitters A radio transmitter consists of several elements that work together to generate radio waves that contain useful information such as audio, video, or digital data. - Power supply: provides the necessary electrical power to operate the transmitter. - Oscillator: creates alternating current at the frequency on which the transmitter will transmit. The oscillator usually generates a sine wave, which is referred to as a carrier wave. - Modulator: adds useful information to the carrier wave. There are two main ways to add this information. The first, called amplitude modulation or AM, makes slight increases or decreases to the intensity of the carrier wave. The second, called frequency modulation or FM, makes slight increases or decreases to the frequency of the carrier wave. - Amplifier: amplifies the modulated carrier wave to increase its power. The more powerful the amplifier, the more powerful the broadcast. - Antenna: converts the amplified signal to radio waves.
3. Read the text and define the role of antennas in radio communication: Radio antennas An antenna (or aerial) is an electrical device that converts electric power into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an electric current oscillating at radio frequency (i.e. a high frequency alternating current (AC)) to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, which is applied to a receiver to be amplified. Antennas are essential components of all equipment that uses radio. They are used in systems such as radio broadcasting, broadcast television, two-way radio, communications receivers, radar, cell phones, and satellite communications, as well as other devices such as garage door openers, wireless microphones, Bluetooth-enabled devices, wireless computer networks, baby monitors, and RFID tags on merchandise. Typically an antenna consists of an arrangement of metallic conductors (elements), electrically connected (often through a transmission line) to the receiver or transmitter. An oscillating current of electrons forced through the antenna by a transmitter will create an oscillating magnetic field around the antenna elements, while the charge of the electrons also creates an oscillating electric field along the elements. These time-varying fields radiate away from the antenna into space as a moving transverse electromagnetic field wave. Conversely, during reception, the oscillating electric and magnetic fields of an incoming radio wave exert force on the electrons in the antenna elements, causing them to move back and forth, creating oscillating currents in the antenna. Antennas can be designed to transmit and receive radio waves in all horizontal directions equally (omnidirectional antennas), or preferentially in a particular direction (directional or high gain antennas). In the latter case, an antenna may also include additional elements or surfaces with no electrical connection to the transmitter or receiver, such as parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation pattern. The first antennas were built in 1888 by German physicist Heinrich Hertz in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving.
4. Read the text and tell how radio receivers work: Radio receivers A radio receiver is the opposite of a radio transmitter. It uses an antenna to capture radio waves, processes those waves to extract only those waves that are vibrating at the desired frequency, extracts the audio signals that were added to those waves, amplifies the audio signals, and finally plays them on a speaker. - Antenna: captures the radio waves. Typically, the antenna is simply a length of wire. When this wire is exposed to radio waves, the waves induce a very small alternating current in the antenna. - RF amplifier: A sensitive amplifier that amplifies the very weak radio frequency (RF) signal from the antenna so that the signal can be processed by the tuner. - Tuner: A circuit that can extract signals of a particular frequency from a mix of signals of different frequencies. On its own, the antenna captures radio waves of all frequencies and sends them to the RF amplifier, which dutifully amplifies them all. Unless you want to listen to every radio channel at the same time, you need a circuit that can pick out just the signals for the channel you want to hear. That’s the role of the tuner. The tuner usually employs the combination of an inductor (for example, a coil) and a capacitor to form a circuit that resonates at a particular frequency. This frequency, called the resonant frequency, is determined by the values chosen for the coil and the capacitor. This type of circuit tends to block any AC signals at a frequency above or below the resonant frequency. You can adjust the resonant frequency by varying the amount of inductance in the coil or the capacitance of the capacitor. In simple radio receiver circuits, the tuning is adjusted by varying the number of turns of wire in the coil. More sophisticated tuners use a variable capacitor (also called a tuning capacitor) to vary the frequency. - Detector: Responsible for separating the audio information from the carrier wave. For AM signals, this can be done with a diode that just rectifies the alternating current signal. What is left after the diode has its way with the alternating current signal is a direct current signal that can be fed to an audio amplifier circuit. For FM signals, the detector circuit is a little more complicated. - Audio amplifier: This component's job is to amplify the weak signal that comes from the detector so that it can be heard. This can be done using a simple transistor amplifier circuit.
Of course, there are many variations on this basic radio receiver design. Many receivers include additional filtering and tuning circuits to better lock on to the intended frequency — or to produce better-quality audio output — and exclude other signals. Still, these basic elements are found in most receiver circuits. 5. Read the text and tell about the design and principle of operation of a super heterodyne receiver. Use the bloc diagram: Super heterodyne receiver In electronics, a super heterodyne receiver (often shortened to superhet) uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original radio carrier frequency. It was invented by US engineer Edwin Armstrong in 1918 during World War I. Virtually all modern radio receivers use the super heterodyne principle. At the cost of an extra frequency converter stage, the super heterodyne receiver provides superior selectivity and sensitivity compared with simpler designs.
Text A As it is known the history of radio transmitters dates back to 1895 when a great Russian scientist A. Popov transmitted the first radiogram. Since that time many Russian and foreign scientists contributed much to the theory of radio transmitting devices. The function of the radio transmitter is to convert the electrical power received from a primary source into radio-frequency energy modulated with a signal for transmission by means of electromagnetic waves through space. The radio transmitter consists of two principal components: the radio-frequency section and the audio-frequency one. The radio-frequency section produces radio-frequency power of continuous waves, the audio-frequency section being concerned with modulation of radio signals. The parameters of the radio transmitter are output power, frequency stability, efficiency and modulation. Radio transmitters are classified into many different types. When classifying them according to the service for which they are used, radio transmitters may be of communication, broadcast, radar and other types. Taking into consideration the type of transmitting signals, specialists subdivide radio transmitters into telegraph, telephone and pulse transmitters. According to the power consumed transmitters are of low power, medium power and other types. At last, they may be of fixed and mobile types. To meet the requirements of high transmission quality of radio transmitters much is being done for improving radio transmitters’ performance by developing new design of these devices.
Text B As it is known, that Russian scientist V. Siforov worked out the theory of radio receiving devices. However, A. Popov invented and demonstrated the first radio receiving set. Since that time, radio devices have been improving and perfecting. The receiver performs the function of converting the current in the receiving antenna into the intelligence contained in the transmission the man parameters of radio receivers are sensitivity, selectivity and fidelity. Sensitivity is a measure of the receiver’s ability to receive weak signals, as it is known that the farther an electromagnetic wave travels, the weaker is its energy. Selectivity is the ability of the receiver to reject undesirable signals. Fidelity is a measure of the receiver’s ability to reproduce clearly audio-frequency currents, which are in accordance with the modulation envelope of the received signals. However simple the radio receiver may be, it includes an antenna, an input tuning circuit, a detector and a pair of earphones. The principle of operation of the radio receiver is not very difficult to understand. The electromotive force is impressed upon the receiving antenna and produces a current; this current is a reproduction of the current of the transmitting antenna. There are various types of receivers, communication and broadcast receivers being the principal types of them. Communication receivers are used in radiotelephone and telegraph service, broadcast receivers finding application for the reception of sound and visual programs. Wherever radio receivers were applied, they must meet an important requirement as reliability in operation.
7. Answer the following questions: What is radio communication? What are the main components of radio communication? What does a transmitter consist of? What is used for amplifying currents? What happens in the microphone when we speak into it? What are the advantages of Internet radio? What new sciences could develop due to radio communication? What are distinguishing features of mathematical theory dealing with communication system? What does the modern communication theory stem from? What was Wienner’s contribution into the development of communication theory? A television picture Human perception of motion. A television system involves equipment located at the source of production, equipment located in the home of the viewer, and equipment used to convey the television signal from the producer to the viewer. The purpose of all of this equipment is to extend the human senses of vision and hearing beyond their natural limits of physical distance. A television system must be designed, therefore, to embrace the essential capabilities of these senses, particularly the sense of vision. The aspects of vision that must be considered include the ability of the human eye to distinguish the brightness, colors, details, sizes, shapes, and positions of objects in a scene before it. Aspects of hearing include the ability of the ear to distinguish the pitch, loudness, and distribution of sounds. In working to satisfy these capabilities, television systems must strike appropriate compromises between the quality of the desired image and the costs of reproducing it. They must also be designed to override, within reasonable limits, the effects of interference and to minimize visual and audial distortions in the transmission and... (200 of 21, 814 words).
Radar The word “radar” means Radio Determination and Ranging. Radar equipment is capable of determining by radio echoes the presence of objects, their direction, range and recognizing their character. Radar detects objects at a distance by reflecting radio waves off them. The delay caused by the echo measures he distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface. There are several types of radar sets, all of them consisting of six essential components, namely: a transmitter, a receiver, an antenna system, an indicator, a timer and, of course, a power supply. A radar set detects by sending out short powerful pulses of ultra-high frequency radio wave energy from a high power transmitter. The directional antenna takes this energy from the transmitter and radiates it in a beam (similar to that of a searchlight). As the transmitted energy strikes an object, a portion of it is reflected back. The receiver picks up the returning echo through its antenna and translates it into visual readable signals on a fluorescent screen. The appearance of these signals show the presence of an object within the field of view of radar. Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on ships and long-distance aircraft. General-purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so he receiver can determine the type of surface of the reflector. Search radars scan a wide area with pulses of short radio waves and sometimes use the Doppler effect to separate moving vehicles from clutter. Weather radars can even measure wind speed. 4. Coordinate the words given in the left column with their interpretation in the right:
Unit 4. Supplementary texts Communication systems Communication is the basic process of exchanging information. The basic components of an electronic communication system are: 1) Transmitter. 2) Communication channel. 3) Receiver. A Transmitter is a collection of electronic circuits designed to convert the information into a signal suitable for transmission over a given communication medium. A radio communication system sends signals by radio. Types of radio communication systems deployed depend on technology, standards, regulations, radio spectrum allocation, user requirements, service positioning, etc. The radio equipment involved in communication systems includes a transmitter and a receiver, each having an antenna and appropriate terminal equipment such as a microphone at the transmitter and a loudspeaker at the receiver in the case of a voice-communication system. The power consumed in a transmitting station varies depending on the distance of communication and the transmission conditions. The power received at the receiving station is usually only a tiny fraction of the transmitter's output, since communication depends on receiving the information, not the energy that was transmitted. Classical radio communications systems use frequency-division multiplexing (FDM) as a strategy to split up and share the available radio-frequency bandwidth for use by different parties communications concurrently. Modern radio communication systems include those that divide a radio-frequency band by time-division multiplexing (TDM) and code-division multiplexing (CDM) as alternatives to the classical FDM strategy. These systems offer different tradeoffs in supporting multiple users, beyond the FDM strategy that was ideal for broadcast radio but less so for applications such as mobile telephony. A radio communication system may send information only one way. For example, in broadcasting a single transmitter sends signals to many receivers. Two stations may take turns sending and receiving, using a single radio frequency; this is called " simplex." By using two radio frequencies, two stations may continuously and concurrently send and receive signals - this is called " duplex" operation. Do the following tasks: 1) Translate into Russian. 2) Write a summary.
What is Modulation? Modulation is the process of superimposing the information contents of a modulating signal on a carrier signal (which is of high frequency) by varying the characteristics of carrier signal according to the modulating signal.
Modulation is a process in which the base band signal modifies another high-frequency signal called the carrier. Types of Modulation We can modulate the information-bearing signal into two types namely.These are called Modulation Techniques. 1. Analog Modulation 2. Digital Modulation Analog modulation is the process of converting an analog input signal into a signal that is suitable for RF transmission. Digital modulation is the process of converting a digital bistream into an analog signal suitable for RF transmission. Modulation Index Modulation Index indicates the depth of modulation. As the amplitude of the modulating signal increases, modulation index increases. For amplitude modulation, the modulation index is given as
m = EmEc m = Amplitude of modulating signal Amplitude of the carrier.
m = δ fm
Analog modulation The Analog carrier signal is modulated by analog information signal so that information bearing analog signal can travel larger distance without the fear of loss due to absorption. The Analog modulation is of two types: 1) Amplitude Modulation 2) Angle Modulation The Angle modulation is further classified as Frequency modulation and Phase Modulation. Amplitude Modulation: In this type of modulation, the strength of the carrier signal is varied with the modulating signal. Frequency Modulation: In this type of modulation, the frequency of the carrier signal is varied with the modulating signal. Phase Modulation: In this type of modulation, the phase of the carrier signal is varied with the modulating signal. It is the variant of the frequency modulation. The analog carrier signal is modulated by digital information signal. It is also considered as digital to analog conversion. Do the following tasks: 1)Translate the texts. 2)Speak about types of modulation.
Channels of communications Old telephone wires are a challenging communications channel for modern digital communications. In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire or to a logical connection over a multiplexed medium such as a radio channel. A channel is used to convey an information signal, for example, a digital bit stream, from one or several senders (or transmitters) to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second. Communicating data from one location to another requires some form of pathway or medium. These pathways, called communication channels, use two types of media: cable (twisted-pair wire, cable, and fiber-optic cable) and broadcast (microwave, satellite, radio, and infrared). Cable or wire line media use physical wires of cables to transmit data and information. Twisted-pair wire and coaxial cables are made of copper, and fiber-optic cable is made of glass. In information theory, a channel refers to a theoretical channel model with certain error characteristics. In this more general view, a storage device is also a kind of channel, which can be sent to (written) and received from (read). A channel can take many forms. Examples of communications channels include: 1) A connection between initiating and terminating nodes of a circuit. 2) A single path provided by a transmission medium via either physical separation, such as by multipair cable or electrical separation, such as by frequency-division or time-division multiplexing. 3) A path for conveying electrical or electromagnetic signals, are usually distinguished from other parallel paths. It includes: -A storage that can communicate a message over time as well as space. -The portion of a storage medium, such as a track or band, that is accessible to a given reading or writing station or head. -A buffer from which messages can be 'put' and 'got'. In a communication system, the physical or logical link connects a data source to a data sink. 4) A specific radio frequency, pair or band of frequencies, are usually named with a letter, number, or code word, and often allocated by international agreement. A channel can be modelled physically by trying to calculate the physical processes, which modify the transmitted signal. For example in wireless communications the channel can be modelled by calculating the reflection off every object in the environment. A sequence of random numbers might also be added in to simulate external interference and/or electronic noise in the receiver. Statistically a communication channel is usually modelled as a triple consisting of an input alphabet, an output alphabet, and for each pair (i, o) of input and output elements a transition probability p (i, o). Semantically, the transition probability is the probability that the symbol o is received given that i was transmitted over the channel. Statistical and physical modelling can be combined. For example in wireless communications the channel is often modelled by a random attenuation (known as fading) of the transmitted signal, followed by additive noise. The attenuation term is a simplification of the underlying physical processes and captures the change in signal power over the course of the transmission. The noise in the model captures external interference and/or electronic noise in the receiver. If the attenuation term is complex it also describes the relative time a signal takes to get through the channel. The statistics of the random attenuation are decided by previous measurements or physical simulations. Channel models may be continuous channel models in that there is no limit to how precisely their values may be defined. Communication channels are also studied in a discrete-alphabet setting. This corresponds to abstracting a real world communication system in which the analog-> digital and digital-> analog blocks are out of the control of the designer. The mathematical model consists of a transition probability that specifies an output distribution for each possible sequence of channel inputs. In information theory, it is common to start with memoryless channels in which the output probability distribution only depends on the current channel input. A channel model may either be digital (quantified, e.g. binary) or analog. Do the following tasks: 1) Read and translate the text. 2) Write a summary. 3) Speak on the forms, which channels of communication can take.
Transmitter and modulation Each system contains a transmitter. This consists of a source of electrical energy, producing alternating current of a desired frequency of oscillation. The transmitter contains a system to modulate (change) some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties. The transmitter sends the modulated electrical energy to a tuned resonant antenna; this structure converts the rapidly changing alternating current into an electromagnetic wave that can move through free space (sometimes with a particular polarization).
An audio signal (top) may be carried by an AM or FM radio wave. Amplitude modulation of a carrier wave works by varying the strength of the transmitted signal in proportion to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light intensity of television pixels. It was the method used for the first audio radio transmissions, and remains in use today. " AM" is often used to refer to the medium wave broadcast band. Frequency modulation varies the frequency of the carrier. The instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency-shift keying. FM is commonly used at VHF radio frequencies for high fidelity broadcasts of music and speech. Normal (analog) TV sound is also broadcast using FM. Angle modulation alters the instantaneous phase of the carrier wave to transmit a signal. It is another term for phase modulation.
Antenna An antenna (or aerial) is an electrical device, which converts electrical currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter applies an oscillating radio frequency electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals that is applied to a receiver to be amplified. An antenna can be used for both transmitting and receiving. Propagation Once generated, electromagnetic waves either travel through space directly, or have their path altered by reflection, refraction or diffraction. The intensity of the waves diminishes due to geometric dispersion (the inverse-square law); some energy may also be absorbed by the intervening medium in some cases. Noise will generally alter the desired signal; this electromagnetic interference comes from natural sources, as well as from artificial sources such as other transmitters and accidental radiators. Noise is also produced at every step due to the inherent properties of the devices used. If the magnitude of the noise is large enough, the desired signal will no longer be discernible; this is the fundamental limit to the range of radio communications. Resonance Electrical resonance of tuned circuits in radios allow individual stations to be selected. A resonant circuit will respond strongly to a particular frequency and much less so to differing frequencies. This allows the radio receiver to discriminate between multiple signals differing in frequency. Receiver and demodulation
A crystal receiver consists of an antenna, rheostat, coil, crystal rectifier, capacitor, headphones and ground connection. The electromagnetic wave is intercepted by a tuned receiving antenna; this structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents. At the receiver, these currents are demodulated, which is conversion to a usable signal form by a detector sub-system. The receiver is " tuned" to respond preferentially to the desired signals, and reject undesired signals. Early radio systems relied entirely on the energy collected by an antenna to produce signals for the operator. Radio became more useful after the invention of electronic devices such as the vacuum tube and later the transistor, which made it possible to amplify weak signals. Today radio systems are used for applications from walkie-talkie children's toys to the control of space vehicles, as well as for broadcasting, and many other applications. A radio receiver receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna. It amplifies the signal to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc.
Telephony Mobile phones transmit to a local cell site (transmitter/receiver) that ultimately connects to the public switched telephone network (PSTN) through an optic fiber or microwave radio and other network elements. When the mobile phone nears the edge of the cell site's radio coverage area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital modulation schemes. Recent developments in Sweden (such as DROPme) allow for the instant downloading of digital material from a radio broadcast (such as a song) to a mobile phone. Satellite phones use satellites rather than cell towers to communicate. Video Television sends the picture as AM and the sound as AM or FM, with the sound carrier a fixed frequency (4.5 MHz in the NTSC system) away from the video carrier. Analog television also uses a vestigial sideband on the video carrier to reduce the bandwidth required. Digital television uses 8VSB modulation in North America (under the ATSC digital television standard), and COFDM modulation elsewhere in the world (using the DVB-T standard). A Reed-Solomon error correction code adds redundant correction codes and allows reliable reception during moderate data loss. Although many current and future codecs can be sent in the MPEG transport stream container format, as of 2006 most systems use a standard-definition format almost identical to DVD: MPEG-2 video in anamorphic widescreen and MPEG layer 2 (MP2) audio. High-definition television is possible simply by using a higher-resolution picture, but H.264/AVC is being considered as a replacement video codec in some regions for its improved compression. With the compression and improved modulation involved, a single " channel" can contain a high-definition program and several standard-definition programs.
Navigation All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math. Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases, they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators. LORAN systems also used time-of-flight radio signals, but from radio stations on the ground. Very High Frequency omnidirectional Range (VOR), systems (used by aircraft), have an antenna array that transmits two signals simultaneously. A directional signal rotates like a lighthouse at a fixed rate. When the directional signal is facing north, an omnidirectional signal pulses. By measuring the difference in phase of these two signals, an aircraft can determine its bearing or radial from the station, thus establishing a line of position. An aircraft can get readings from two VORs and locate its position at the intersection of the two radials, known as a " fix". When the VOR station is collocated with DME (Distance Measuring Equipment), the aircraft can determine its bearing and range from the station, thus providing a fix from only one ground station. Such stations are called VOR/DMEs. The military operates a similar system of navaids, called TACANs, which are often built into VOR stations. Such stations are called VORTACs. Because TACANs include distance measuring equipment, VOR/DME and VORTAC stations are identical in navigation potential to civil aircraft. Radar Radar (Radio Detection And Ranging) detects objects at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface. Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft. General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position. Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the Doppler effect to separate moving vehicles from clutter. Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more. Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the Doppler effect to measure wind speeds.
Radio systems Most new radio systems are digital, including Digital TV, satellite radio, and Digital Audio Broadcasting. The oldest form of digital broadcast was spark gap telegraphy, used by pioneers such as Marconi. By pressing the key, the operator could send messages in Morse code by energizing a rotating commutating spark gap. The rotating commutator produced a tone in the receiver, where a simple spark gap would produce a hiss, indistinguishable from static. Spark-gap transmitters are now illegal, because their transmissions span several hundred megahertz. This is very wasteful of both radio frequencies and power. The next advance was continuous wave telegraphy, or CW (Continuous Wave), in which a pure radio frequency, produced by a vacuum tube electronic oscillator was switched on and off by a key. A receiver with a local oscillator would " heterodyne" with the pure radio frequency, creating a whistle-like audio tone. CW uses less than 100 Hz of bandwidth. CW is still used, these days primarily by amateur radio operators (hams). Strictly, on-off keying of a carrier should be known as " Interrupted Continuous Wave», ICW, or on-off keying (OOK). Radio teletype equipment usually operates on short-wave (HF) and is much loved by the military because they create written information without a skilled operator. They send a bit as one of two tones using frequency-shift keying. Groups of five or seven bits become a character printed by a tele printer. From about 1925 to 1975, radio teletype was how most commercial messages were sent to less developed countries. These are still used by the military and weather services. Aircraft use a 1200-Baud radio teletype service over VHF to send their ID, altitude and position, and get gate and connecting-flight data. Microwave dishes on satellites, telephone exchanges and TV stations usually use quadrature amplitude modulation (QAM). QAM sends data by changing both the phase and the amplitude of the radio signal. Engineers like QAM because it packs the most bits into a radio signal when given an exclusive (non-shared) fixed narrowband frequency range. Usually the bits are sent in " frames" that repeat. A special bit pattern is used to locate the beginning of a frame.
Modern GPS receivers. Communication systems that limit themselves to a fixed narrowband frequency range are vulnerable to jamming. A variety of jamming-resistant spread spectrum techniques were initially developed for military use, most famously for Global Positioning System satellite transmissions. Commercial use of spread spectrum began in the 1980s. Bluetooth, most cell phones, and the 802.11b version of Wi-Fi each use various forms of spread spectrum. Systems that need reliability, or that share their frequency with other services, may use " coded orthogonal frequency-division multiplexing" or COFDM. COFDM breaks a digital signal into as many as several hundred slower sub channels. The digital signal is often sent as QAM on the sub channels. Modern COFDM systems use a small computer to make and decode the signal with digital signal processing, which is more flexible and far less expensive than older systems that implemented separate electronic channels. COFDM resists fading and ghosting because the narrow-channel QAM signals can be sent slowly. An adaptive system or one that sends error-correction codes can also resist interference, because most interference can affect only a few of the QAM channels. COFDM is used for Wi-Fi, some cell phones, Digital Radio Mondiale, Eureka 147, and many other local area network, digital TV and radio standards. Do the following task: 1) Read the texts and try to speak on the information presented in these texts. References 1. Eric H. Gkendinning, John McEwan, “Oxford English for Electronics”. - Oxford University Press, 2007. 2. Rod Revell, Jeremy Comfort and others, “English for the Telecommunications Industry”. - Oxford University Press, 2011. 3. V.A. Radovel, “English for Technical Universities”, - Moscow, 2010. 4. World Wide Web. Contents Unit 1. Radio communications systems………………………………………3 Unit 2. Radio transmitters and receivers…………………………………….15 Unit 3. Basic principles of television ………………………………………21 Unit 4. Supplementary texts…………………………………………………28
Unit 1 Radio engineering systems 1. Memorize the words: To travel – распространяться transmitting range – дальность передачи a receiver – приемник a transmitter – передатчик a high-frequency oscillator – высокочастотный генератор колебаний an oscillatory circuit – колебательный контур a capacitor – конденсатор an amplifier – усилитель a detector – детектор, следящий механизм a rectifier – выпрямитель, детонатор the audio frequency – звуковая частота to couple together – соединять, спаривать by means of a switch – с помощью переключателя (коммутатора) means of communication – средства связи telegraph sending key – телеграфный ключ dots and dashes – точки и тире the mirror galvanometer – зеркальный гальванометр powdered carbon – порошковый углерод a far sensitive receiver – гораздо более чувствительный приемник Популярное: |
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