principles of electronic communication systems n.
Skip this Video
Loading SlideShow in 5 Seconds..
Principles of Electronic Communication Systems PowerPoint Presentation
Download Presentation
Principles of Electronic Communication Systems

Principles of Electronic Communication Systems

687 Vues Download Presentation
Télécharger la présentation

Principles of Electronic Communication Systems

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Principles of ElectronicCommunication Systems Third Edition Louis E. Frenzel, Jr.

  2. Transmission-Line Basics • Transmission lines in communication carry: • Telephone signals, • Computer data in LANs, • TV signals in cable TV systems, • Signals from a transmitter to an antenna or from an antenna to a receiver. • Transmission lines are also circuits.

  3. Transmission-Line Basics • The two primary requirements of a transmission line are: • The line should introduce minimum attenuation to the signal. • The line should not radiate any of the signal as radio energy.

  4. Transmission-Line Basics Velocity Factor • The speed of the signal in the transmission line is slower than the speed of a signal in free space. • The velocity of propagation of a signal in a cable is less than the velocity of propagation of light in free space by a fraction called the velocity factor (VF). VF = VC/VL

  5. Transmission-Line Basics Time Delay • Because the velocity of propagation of a transmission line is less than the velocity of propagation in free space, any line will slow down or delay any signal applied to it. • A signal applied at one end of a line appears some time later at the other end of the line. • This is called the time delay or transit time. • A transmission line used specifically for the purpose of achieving delay is called a delay line.

  6. Transmission-Line Basics The effect of the time delay of a transmission line on signals. (a) Sine wave delay causes a lagging phase shift. (b) Pulse delay.

  7. Transmission-Line Basics Attenuation versus length for RG-58A/U coaxial cable. Note that both scales on the graph are logarithmic.

  8. Standing Waves • If the load on the line is an antenna, the signal is converted into electromagnetic energy and radiated into space. • If the load at the end of the line is an open or a short circuit or has an impedance other than the characteristic impedance of the line, the signal is not fully absorbed by the load.

  9. Standing Waves • When a line is not terminated properly, some of the energy is reflected and moves back up the line, toward the generator. • This reflected voltage adds to the forward or incident generator voltage and forms a composite voltage that is distributed along the line. • The pattern of voltage and its related current constitute what is called a standing wave. • Standing waves are not desirable.

  10. Standing Waves Matched Lines • A matched transmission line is one terminated in a load that has a resistive impedance equal to the characteristic impedance of the line. • Alternating voltage (or current) at any point on a matched line is a constant value. A correctly terminated transmission line is said to be flat. • The power sent down the line toward the load is called forward or incident power. • Power not absorbed by the load is reflected power.

  11. Standing Waves A transmission line must be terminated in its characteristic impedance for proper operation.

  12. Standing Waves Calculating the Standing Wave Ratio • The magnitude of the standing waves on a transmission line is determined by • the ratio of the maximum current to the minimum current, • or the ratio of the maximum voltage to the minimum voltage, along the line. • These ratios are referred to as the standing wave ratio (SWR).

  13. The Smith Chart • The mathematics required to design and analyze transmission lines is complex, whether the line is a physical cable connecting a transceiver to an antenna or is being used as a filter or impedance-matching network. • This is because the impedances involved are complex ones, involving both resistive and reactive elements. • The impedances are in the familiar rectangular form, R + jX.

  14. The Smith Chart • The Smith Chart is a sophisticated graph that permits visual solutions to transmission line calculations. • Despite the availability of the computing options today, this format provides a more or less standardized way of viewing and solving transmission-line and related problems. ZO ZIN ZL

  15. The Smith Chart • The horizontal axis is the pure resistance or zero-reactance line. • The point at the far left end of the line represents zero resistance, and the point at the far right represents infinite resistance. The resistance circles are centered on and pass through this pure resistance line. • The circles are all tangent to one another at the infinite resistance point, and the centers of all the circles fall on the resistance line.

  16. The Smith Chart • Any point on the outer circle represents a resistance of 0 Ω. • The R = 1 circle passes through the exact center of the resistance line and is known as the prime center. • Values of pure resistance and the characteristic impedance of transmission line are plotted on this line. • The linear scales printed at the bottom of Smith charts are used to find the SWR, dB loss, and reflection coefficient.

  17. The Smith Chart The Smith chart.

  18. Optical Communication

  19. Optical Principles • Optical communication systems use light to transmit information from one place to another. • Light is a type of electromagnetic radiation like radio waves. • Today, infrared light is being used increasingly as the carrier for information in communication systems. • The transmission medium is either free space or a light-carrying cable called a fiber-optic cable. • Because the frequency of light is extremely high, it can accommodate very high rates of data transmission with excellent reliability.

  20. Optical Principles Physical Optics: Reflection • The simplest way of manipulating light is to reflect it. • When light rays strike a reflective surface, the light waves are thrown back or reflected. • By using mirrors, the direction of a light beam can be changed.

  21. Optical Principles Physical Optics: Reflection • The law of reflection states that if the light ray strikes a mirror at some angle A from the normal, the reflected light ray will leave the mirror at the same angle B to the normal. • In other words, the angle of incidence is equal to the angle of reflection. • A light ray from the light source is called an incident ray.

  22. Optical Principles n=c/v Sin A/Sin C=(n2/n1) Illustrating reflection and refraction at the interface of two optical materials.

  23. Optical Principles Physical Optics: Refraction • The direction of the light ray can also be changed by refraction, which is the bending of a light ray that occurs when the light rays pass from one medium to another. • Refraction occurs when light passes through transparent material such as air, water, and glass. • Refraction takes place at the point where two different substances come together. • Refraction occurs because light travels at different speeds in different materials.

  24. Optical Principles Physical Optics: Refraction • The amount of refraction of the light of a material is usually expressed in terms of the index of refraction n. • This is the ratio of the speed of light in air to the speed of light in the substance. • It is also a function of the light wavelength.

  25. Optical Communication Systems Light Wave Communication in Free Space • An optical communication system consists of: • A light source modulated by the signal to be transmitted. • A photodetector to pick up the light and convert it back into an electrical signal. • An amplifier. • A demodulator to recover the original information signal.

  26. Optical Communication Systems Free-space optical communication system.

  27. Optical Communication Systems Fiber-Optic Communication System • Fiber-optic cables many miles long can be constructed and interconnected for the purpose of transmitting information. • Fiber-optic cables have immense information-carrying capacity (wide bandwidth). • Many thousands of signals can be carried on a light beam through a fiber-optic cable.

  28. Optical Communication Systems Fiber-Optic Communication System • The information signal to be transmitted may be voice, video, or computer data. • Information must be first converted to a form compatible with the communication medium, usually by converting analog signals to digital pulses. • These digital pulses are then used to flash a light source off and on very rapidly. • The light beam pulses are then fed into a fiber-optic cable, which can transmit them over long distances.

  29. Optical Communication Systems Basic elements of a fiber-optic communication system.

  30. Benefits of fiber-optic cables over conventional electrical cables. Optical Communication Systems

  31. Fiber-Optic Cables Fiber-Optic Cable Specifications • The most important specifications of a fiber-optic cable are: • Size • Attenuation • Bandwidth

  32. Principles of ElectronicCommunication Systems Third Edition Louis E. Frenzel, Jr. Modified by Sunantha Sodsee

  33. Telephones The telephone system The largest and most complex electronic communication system in the world. The primary purpose Provide voice communication. Widely used for Facsimile transmission Computer data transmission.

  34. Telephones The telephone system Full-duplex analog communication of voice signals. Telephone can connect with any other telephone in the world. Identification code Telephone number Country code + Subscriber numbers : +66 XXXX XXXX Trunk prefix + Subscriber numbers: 02 XXX XXXX Subscriber numbers: area code, local number

  35. Telephones The Local Loop Single central office 10,000 telephone lines can be connected The two-wire, twisted-pair connection Telephone and central office local loop or subscriber loop.

  36. Telephones Telephone Set Analog baseband transceiver Handset: a microphone and a speaker, transmitterand receiver. Ringer and a dialing mechanism ringer: bell or an electronic oscillator connected to a speaker. A switch hook a double-pole mechanical switch Dialing circuits : dual-tone multifrequency (DTMF) system. Hybrid circuit special transformer used to convert signals from the four wires from the transmitter and receiver into a signal suitable for a single two-line pair to the local loop.

  37. Telephones Basic telephone set.

  38. Telephones • DTMF

  39. Telephones Standard Telephone and Local Loop Telephone wires: color coded: tip wire is green and usually connected to ground, and the ring wire is red.

  40. Telephones Subscriber interface.

  41. Telephone System Telephone Hierarchy a telephone call, your voice is connected through your local exchange to the telephone system. Several other facilities may provide switching, multiplexing, and other services required to transmit your voice. The telephone system is referred to as the public switched telephone network (PSTN).

  42. Telephone System

  43. Telephone System Trunk: A communications path between two switching systems Organization of the telephone system in the United States.

  44. Telephone System Private Telephone System Telephone service among the telephones in a company or organization The two basic types : Key systems Private branch exchanges

  45. Telephone System Private Telephone System: Key Systems serve 2–50 user telephones within an organization. individual telephone units called stations, all of which are connected to a central answering station. The central answering station is connected to one or more local loop lines, or trunks,back to the local exchange. The telephone sets in a key system typically have a group of pushbuttons that allow each phone to select two or more outgoing trunking lines.

  46. Telephone System Private Telephone System: Private Branch Exchange For larger organizations: thousands of individual telephones within an organization. private automatic branch exchanges (PABXs) computer branch exchanges (CPXs). Advantages of efficiency and cost reduction when many telephones are required. An alternative to PBX is Centrex. This service performs the function of a PBX but uses special equipment and special trunk lines.

  47. Telephone System A PBX.

  48. Circuit Switch

  49. Circuit-Switching PSTN is a circuit-switched network Circuit establishment Transfer of information point-to-point from endpoints to node internal switching/multiplexing among nodes Circuit disconnect Circuit switching is well suited for analog voice communications as in the telephone network. in-efficient for data networks due to its resource allocation nature data traffic is BAD

  50. Setting up a Path Before any data can be sent, the path between the caller and callee must be established. It can easily take 10 seconds to set up the path (more if its an international call). During this time interval, the switching equipment is searching for a ‘copper’ path through the network.