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Lec microwave

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Lec microwave

  1. 1. 1Microwave Communication
  2. 2. 2 Microwave Concepts Microwaves are the ultrahigh, superhigh, and extremely high frequencies directly above the lower frequency ranges where most radio communication now takes place and below the optical frequencies that cover infrared, visible, and ultraviolet light. © 2008 The McGraw-Hill Companies
  3. 3. 3 Microwave ConceptsMicrowave Frequencies and Bands  The practical microwave region is generally considered to extend from 1 to 30 GHz, although frequencies could include up to 300 GHz.  Microwave signals in the 1- to 30-GHz have wavelengths of 30 cm to 1 cm.  The microwave frequency spectrum is divided up into groups of frequencies, or bands.  Frequencies above 40 GHz are referred to as millimeter (mm) waves and those above 300 GHz are in the submillimeter band. © 2008 The McGraw-Hill Companies
  4. 4. 4 Microwave Concepts: Microwave frequency bands. © 2008 The McGraw-Hill Companies
  5. 5. 5 Microwave ConceptsBenefits of Microwaves  Moving into higher frequency ranges has helped to solve the problem of spectrum crowding.  Today, most new communication services are assigned to the microwave region.  At higher frequencies there is a greater bandwidth available for the transmission of information.  Wide bandwidths make it possible to use various multiplexing techniques to transmit more information.  Transmission of high-speed binary information requires wide bandwidths and these are easily transmitted on microwave frequencies. © 2008 The McGraw-Hill Companies
  6. 6. 6 Microwave ConceptsDisadvantages of Microwaves  The higher the frequency, the more difficult it becomes to analyze electronic circuits.  At microwave frequencies, conventional components become difficult to implement.  Microwave signals, like light waves, travel in perfectly straight lines. Therefore, communication distance is limited to line-of-sight range.  Microwave signals penetrate the ionosphere, so multiple-hop communication is not possible. © 2008 The McGraw-Hill Companies
  7. 7. 7 Microwave ConceptsMicrowave Communication Systems: Transmission Lines  Coaxial cable, most commonly used in lower-frequency communication has very high attenuation at microwave frequencies and conventional cable is unsuitable for carrying microwave signals.  Special microwave coaxial cable that can be used on bands L, S, and C is made of hard tubing. This low-loss coaxial cable is known as hard line cable.  At higher microwave frequencies, a special hollow rectangular or circular pipe called waveguide is used for the transmission line. © 2008 The McGraw-Hill Companies
  8. 8. 8 Microwave Lines and Devices Although vacuum and microwave tubes like the klystron and magnetron are still used, most microwave systems use transistor amplifiers. Special geometries are used to make bipolar transistors that provide voltage and power gain at frequencies up to 10 GHz. Microwave FET transistors have also been created. Monolithic microwave integrated circuits (MMICs) are widely used. © 2008 The McGraw-Hill Companies
  9. 9. 9 Microwave Lines and DevicesMicrowave Transistors  The primary differences between standard lower- frequency transistors and microwave types are internal geometry and packaging.  To reduce internal inductances and capacitances of transistor elements, special chip configurations known as geometries are used.  Geometries permit the transistor to operate at higher power levels and at the same time minimize distributed and stray inductances and capacitances. © 2008 The McGraw-Hill Companies
  10. 10. 10 Microwave Lines and DevicesMicrowave Transistors The GaAs MESFET metal semiconductor field effect transistor, a type of JFET using a Schottky barrier junction, can operate at frequencies above 5 GHz. A high electron mobility transistor (HEMT) is a variant of the MESFET and extends the range beyond 20 GHz by adding an extra layer of semiconductor material such as AlGaAs. A popular device known as a heterojunction bipolar transistor (HBT) is making even higher-frequency amplification possible in discrete form and in integrated circuits. © 2008 The McGraw-Hill Companies
  11. 11. 11 Microwave Lines and DevicesMicrowave transistors. (a) and (b) Low-power small signal. (c) FET power. (d) NPNbipolar power. © 2008 The McGraw-Hill Companies
  12. 12. 12 Microwave Lines and DevicesSmall-Signal Amplifiers: Transistor Amplifiers  A low-noise transistor with a gain of about 10 to 25 dB is typically used as a microwave amplifier.  Most microwave amplifiers are designed to have input and output impedances of 50 Ω.  The transistor is biased into the linear region for class A operation.  RFCs are used in the supply leads to keep the RF out of the supply and to prevent feedback paths that can cause oscillation and instability in multistage circuits.  Ferrite beads (FB) are used in the collector supply lead for further decoupling. © 2008 The McGraw-Hill Companies
  13. 13. 13 Microwave Lines and DevicesSmall-Signal Amplifiers: MMIC Amplifiers  A common monolithic microwave integrated circuit (MMIC) amplifier is one that incorporates two or more stages of FET or bipolar transistors made on a common chip to form a multistage amplifier.  The chip also incorporates resistors for biasing and small bypass capacitors.  Physically, these devices look like transistors. © 2008 The McGraw-Hill Companies
  14. 14. 14 Microwave Lines and DevicesSmall-Signal Amplifiers: Power Amplifiers  A typical class A microwave power amplifier is designed with microstrip lines used for impedance matching and tuning.  Input and output impedances are 50 Ω.  Typical power-supply voltages are 12, 24, and 28 volts.  Most power amplifiers obtain their bias from constant- current sources.  A single-stage FET power amplifier can achieve a power output of 100 W in the high UHF and low microwave region. © 2008 The McGraw-Hill Companies
  15. 15. 15 WaveguidesWaveguides  Most microwave energy transmission above 6 GHz is handled by waveguides.  Waveguides are hollow metal conducting pipes designed to carry and constrain the electromagnetic waves of a microwave signal.  Most waveguides are rectangular.  Waveguides are made from copper, aluminum or brass.  Often the insides of waveguides are plated with silver to reduce resistance and transmission losses. © 2008 The McGraw-Hill Companies
  16. 16. 16© 2008 The McGraw-Hill Companies
  17. 17. 17 WaveguidesWave paths in a waveguide at various frequencies. (a) High frequency. (b) Medium frequency. (c) Low frequency. (d) Cutoff frequency. © 2008 The McGraw-Hill Companies
  18. 18. 18 WaveguidesWaveguide Hardware and Accessories  Waveguides have a variety of special parts, such as couplers, turns, joints, rotary connections, and terminations.  Most waveguides and their fittings are precision-made so that the dimensions match perfectly.  A choke joint is used to connect two sections of waveguide. It consists of two flanges connected to the waveguide at the center.  A T section or T junction is used to split or combine two or more sources of microwave power. © 2008 The McGraw-Hill Companies
  19. 19. 19 WaveguidesA choke joint permits sections of waveguide to be interconnected withminimum loss and radiation. © 2008 The McGraw-Hill Companies
  20. 20. 20 Microwave Semiconductor DiodesSmall Signal Diodes  Diodes used for signal detection and mixing are the most common microwave semiconductor devices.  Two types of widely used microwave diodes are:  Point-contact diode  Schottky barrier or hot-carrier diode © 2008 The McGraw-Hill Companies
  21. 21. 21 Microwave Semiconductor DiodesSmall Signal Diodes: Point-Contact Diode  The oldest microwave semiconductor device is the point- contact diode, also called a crystal diode.  A point-contact diode is a piece of semiconductor material and a fine wire that makes contact with the semiconductor material.  Point-contact diodes are ideal for small-signal applications.  They are widely used in microwave mixers and detectors and in microwave power measurement equipment. © 2008 The McGraw-Hill Companies
  22. 22. 22 Microwave Semiconductor DiodesSmall Signal Diodes: Hot Carrier Diodes  For the most part, point-contact diodes have been replaced by Schottky diodes, sometimes referred to as hot carrier diodes.  Like the point-contact diode, the Schottky diode is extremely small and has a tiny junction capacitance.  Schottky diodes are widely used in balanced modulators and mixers.  They are also used as fast switches at microwave frequencies. © 2008 The McGraw-Hill Companies
  23. 23. 23 Microwave Semiconductor DiodesHot carrier or Schottky diode. © 2008 The McGraw-Hill Companies
  24. 24. 24 Microwave Semiconductor DiodesOscillator Diodes  Three types of diodes other than the tunnel diode that can oscillate due to negative resistance characteristics are:  Gunn diode  IMPATT diode  TRAPATT diode © 2008 The McGraw-Hill Companies
  25. 25. 25 Microwave Semiconductor DiodesOscillator Diodes: Gunn Diodes  Gunn diodes, also called transferred-electron devices (TEDs), are not diodes in the usual sense because they do not have junctions.  A Gunn diode is a thin piece of N-type gallium arsenide (GaAs) or indium phosphide (InP) semiconductor which forms a special resistor when voltage is applied to it.  The Gunn diode exhibits a negative-resistance characteristic.  Gunn diodes oscillate at frequencies up to 150 GHz. © 2008 The McGraw-Hill Companies
  26. 26. 26 Microwave Semiconductor DiodesOscillator Diodes: IMPATT and TRAPATT Diodes  Two microwave diodes widely used as oscillators are the IMPATT and TRAPATT diodes.  Both are PN-junction diodes made of silicon, GaAs, or InP.  They are designed to operate with a high reverse bias that causes them to avalanche or break down.  IMPATT diodes are available with power ratings up to 25 W to frequencies as high as 300 GHz.  IMPATT are preferred over Gunn diodes if higher power is required. © 2008 The McGraw-Hill Companies
  27. 27. 27 Microwave Semiconductor DiodesPIN Diodes  A PIN diode is a special PN-junction diode with an I (intrinsic) layer between the P and the N sections.  The P and N layers are usually silicon, although GaAs is sometimes used and the I layer is a very lightly doped N-type semiconductor.  PIN diodes are used as switches in microwave circuits.  PIN diodes are widely used to switch sections of quarter- or half-wavelength transmission lines to provide varying phase shifts in a circuit. © 2008 The McGraw-Hill Companies
  28. 28. 28 Microwave AntennasHorn Antenna  Microwave antennas must be some extension of or compatible with a waveguide.  Waveguide are not good radiators because they provide a poor impedance match with free space. This results in standing waves and reflected power.  This mismatch can be offset by flaring the end of the waveguide to create a horn antenna.  Horn antennas have excellent gain and directivity.  The gain and directivity of a horn are a direct function of its dimensions; the most important dimensions are length, aperture area, and flare angle. © 2008 The McGraw-Hill Companies
  29. 29. 29 Microwave AntennasBasic horn antenna. © 2008 The McGraw-Hill Companies
  30. 30. 30© 2008 The McGraw-Hill Companies
  31. 31. 31 Microwave AntennasParabolic Antennas  A parabolic reflector is a large dish-shaped structure made of metal or screen mesh.  The energy radiated by the horn is pointed at the reflector, which focuses the radiated energy into a narrow beam and reflects it toward its destination.  Beam widths of only a few degrees are typical with parabolic reflectors.  Narrow beam widths also represent extremely high gains. © 2008 The McGraw-Hill Companies
  32. 32. 32 Microwave AntennasCross-sectional view of a parabolic dish antenna. © 2008 The McGraw-Hill Companies
  33. 33. 33 Microwave AntennasParabolic Antennas: Feed Methods  A popular method of feeding a parabolic antenna is an arrangement known as a Cassegrain feed.  The horn antenna is positioned at the center of the parabolic reflector.  At the focal point is another small reflector with either a parabolic or a hyperbolic shape.  The electromagnetic radiation from the horn strikes the small reflector, which then reflects the energy toward the large dish which radiates the signal in parallel beams. © 2008 The McGraw-Hill Companies
  34. 34. 34 Microwave AntennasCassegrain feed. © 2008 The McGraw-Hill Companies
  35. 35. 35 Microwave AntennasHelical Antennas  A helical antenna, as its name suggests, is a wire helix.  A center insulating support is used to hold heavy wire or tubing formed into a circular coil or helix.  The diameter of the helix is typically one-third wavelength, and the spacing between turns is approximately one-quarter wavelength.  The gain of a helical antenna is typically in the 12- to 20-dB range and beam widths vary from approximately 12 to 45 .  Helical antennas are favored in many applications because of their simplicity and low cost. © 2008 The McGraw-Hill Companies
  36. 36. 36 Microwave AntennasThe helical antenna. © 2008 The McGraw-Hill Companies
  37. 37. 37© 2008 The McGraw-Hill Companies
  38. 38. 38 Microwave AntennasBicone Antennas  One of the most widely used omnidirectional microwave antennas is the bicone.  The signals are fed into bicone antennas through a circular waveguide ending in a flared cone.  The upper cone acts as a reflector, causing the signal to be radiated equally in all directions with a very narrow vertical beam width. © 2008 The McGraw-Hill Companies
  39. 39. 39 Microwave AntennasThe omnidirectional bicone antenna. © 2008 The McGraw-Hill Companies
  40. 40. 40 Microwave AntennasDielectric (Lens) Antennas  Dielectric or lens antennas use a special dielectric material to collimate or focus the microwaves from a source into a narrow beam.  Lens antennas are usually made of polystyrene or some other plastic, although other types of dielectric can be used.  Their main use is in the millimeter range above 40 GHz. © 2008 The McGraw-Hill Companies
  41. 41. 41 Microwave AntennasLens antenna operations. (a) Dielectric lens. (b) Zoned lens. © 2008 The McGraw-Hill Companies
  42. 42. 42 Microwave AntennasPatch Antennas  Patch antennas are made with microstrip on PCBs.  The antenna is a circular or rectangular area of copper separated from the ground plane on the bottom of the board by the PCB’s insulating material.  Patch antennas are small, inexpensive, and easy to construct. © 2008 The McGraw-Hill Companies
  43. 43. 43© 2008 The McGraw-Hill Companies
  44. 44. 44 Microwave AntennasIntelligent Antenna Technology  Intelligent antennas or smart antennas are antennas that work in conjunction with electronic decision-making circuits to modify antenna performance to fit changing situations.  They adapt to the signals being received and the environment in which they transmit. © 2008 The McGraw-Hill Companies
  45. 45. 45TV Smart Antenna Multi-Directional HDTV Multiple–radio smart antenna platform the Smart BRO antenna. © 2008 The McGraw-Hill Companies
  46. 46. 46 Microwave AntennasIntelligent Antenna Technology  Also called adaptive antennas, these new designs greatly improve transmission and reception in multipath environments and can also multiply the number of users of a wireless system.  Some popular adaptive antennas today use diversity, multiple-input multiple-output, and automatic beam forming. © 2008 The McGraw-Hill Companies
  47. 47. 47 Microwave AntennasAdaptive Beam Forming  Adaptive antennas are systems that automatically adjust their characteristics to the environment.  They use beam-forming and beam-pointing techniques to zero in on signals to be received and to ensure transmission under noisy conditions.  Beam-forming antennas use multiple antennas such as phase arrays. © 2008 The McGraw-Hill Companies
  48. 48. 48 Microwave AntennasAdaptive Beam Forming  There are two kinds of adaptive antennas: switched beam arrays and adaptive arrays.  Both switched beam arrays and adaptive arrays are being employed in some cell phone systems and in newer wireless LANs.  They are particularly beneficial to cell phone systems because they can boost the system capacity. © 2008 The McGraw-Hill Companies
  49. 49. 49 Microwave ApplicationsMajor applications of microwave radio. © 2008 The McGraw-Hill Companies
  50. 50. 50 Microwave ApplicationsRadar  The electronic communication system known as radar (radio detection and ranging) is based on the principle that high-frequency RF signals are reflected by conductive targets.  In a radar system, a signal is transmitted toward the target and the reflected signal is picked up by a receiver in the radar unit.  The radar unit can determine the distance to a target (range), its direction (azimuth), and in some cases, its elevation (distance above the horizon). © 2008 The McGraw-Hill Companies
  51. 51. 51 16-7: Microwave ApplicationsRadar  There are two basic types of radar systems: pulsed and continuous-wave (CW).  The pulsed type is the most commonly used radar system.  Signals are transmitted in short bursts or pulses.  The time between transmitted pulses is known as the pulse repetition time (PRT).  In continuous-wave (CW) radar, a constant-amplitude continuous microwave sine wave is transmitted. © 2008 The McGraw-Hill Companies
  52. 52. 52 Microwave ApplicationsRadar: UWB  The newest form of radar is called ultrawideband (UWB) radar.  It is a form of pulsed radar that radiates a stream of very short pulses several hundred picoseconds long.  The very narrow pulses give this radar extreme precision and resolution of small objects and details.  The low power used restricts operation to short distances. © 2008 The McGraw-Hill Companies

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