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Chapter 22

Chapter 22. AM and FM Radio Communications. Objectives. Discuss the history of radio wave receivers. Describe the different types of waves. Convert back and forth between frequency and wavelength. Describe several different types of microphones. Objectives.

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Chapter 22

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  1. Chapter 22 AM and FM Radio Communications

  2. Objectives • Discuss the history of radio wave receivers. • Describe the different types of waves. • Convert back and forth between frequency and wavelength. • Describe several different types of microphones. Permission granted to reproduce for educational use only.

  3. Objectives • Discuss amplitude modulation and frequency modulation. • Calculate the percent of modulation. • List the components and explain the operation of an AM receiver and transmitter. Permission granted to reproduce for educational use only.

  4. Objectives • List the components and explain the operation of an FM receiver and transmitter. • List the components and explain the operation of a superheterodyne receiver. • Describe the use of transducers in radio wave receivers. Permission granted to reproduce for educational use only.

  5. Simple Radio Receivers • Consist of: • Antenna • Ground • Tank circuit • Diode • Filter • Speaker or set of headphones • Radio signal consists of a carrier wave and an audio signal Permission granted to reproduce for educational use only.

  6. Crystal Radio Receivers • Radio waves of all stations connect with radio receiver antenna • Radio signals are converted to alternating current • Alternating current is conducted up and down the antenna • Antenna and tank circuit are coupled by mutual induction Permission granted to reproduce for educational use only.

  7. Crystal Radio Receivers (Cont.) • Tank circuit has parallel connection of inductor and variable capacitor • Variable capacitor is used to vary frequency for different stations • A detector rectifies radio signal to a pulsing dc signal Permission granted to reproduce for educational use only.

  8. Radio Waves • Electromagnetic radiation surrounds transmitting antenna • Magnetic fields surround current-carrying conductors • Electrostatic radiation of energy an oscillator circuit produces Permission granted to reproduce for educational use only.

  9. Radio Waves (Cont.) • Electrostatic field and electromagnetic field are perpendicular to each other • Polarization occurs when radio waves radiate Permission granted to reproduce for educational use only.

  10. Radio Waves (Cont.) • A moving electrostatic field produces a moving electromagnetic field • A moving electromagnetic field produces a moving electrostatic field Permission granted to reproduce for educational use only.

  11. Ground Waves • Parts of a ground wave • Surface wave • Direct wave • Ground reflected wave • Space waves Permission granted to reproduce for educational use only.

  12. Ground Waves (Cont.) • Broadcast stations use surface waves for reliable communications • Currents are induced in the earth’s surface as surface waves travel • Waves become weaker as they travel because currents use the energy Permission granted to reproduce for educational use only.

  13. Ground Waves (Cont.) • Salt water conducts surface waves better than land • Transmitters near coastlines cause overseas communication to be more reliable • Water allows stations to use higher power but lower frequencies Permission granted to reproduce for educational use only.

  14. Sky Waves • Earth’s ionosphere is used for transmission of waves • Waves travel 40 to 300 miles above earth’s surface • Some of the wave is absorbed by ionosphere, but some bounces back to the earth Permission granted to reproduce for educational use only.

  15. Review What is a radio wave? Electromagnetic radiation produced from current alternating through an antenna Permission granted to reproduce for educational use only.

  16. Review What is the direction in which radio waves radiate, in respect to the earth? Polarization Permission granted to reproduce for educational use only.

  17. Review What are the three parts of a ground wave? Surface wave, direct wave, and ground reflected wave Permission granted to reproduce for educational use only.

  18. Review Where in the earth’s atmosphere is a sky wave transmitted? Ionosphere Permission granted to reproduce for educational use only.

  19. Radio Waves • Similar to waves in a pond • Travel 186,000 miles per second Permission granted to reproduce for educational use only.

  20. Frequency and Wavelength • As frequency increases, wavelength decreases • Symbol representing wavelength is  Permission granted to reproduce for educational use only.

  21. Relationship of Frequency and Wavelength •  is wavelength in miles • V is wave’s velocity in miles per second • f is frequency in hertz Permission granted to reproduce for educational use only.

  22. Frequency Spectrum • System of regulating use of transmitters • Stops frequencies from interfering with each other • Usage of frequencies is controlled because of number of users of these systems • Federal Communications Commission (FCC) controls U.S. frequency assignments Permission granted to reproduce for educational use only.

  23. Frequency Spectrum Assignments Permission granted to reproduce for educational use only.

  24. Frequency Ranges • Waves are grouped according to frequency • AM stations use MF range • Television uses VHF and UHF ranges • Residential electricity operates at audio frequency Permission granted to reproduce for educational use only.

  25. Review What is the representation of a wavelength? Lambda, or  Permission granted to reproduce for educational use only.

  26. Review What is the system that ensures an orderly use of air waves? Frequency spectrum Permission granted to reproduce for educational use only.

  27. Review Why must the use of frequencies be strictly controlled? Because of the many users of the systems Permission granted to reproduce for educational use only.

  28. Review What government organization controls frequency assignments in the United States? Federal Communications Commission (FCC) Permission granted to reproduce for educational use only.

  29. Radio Transmitters • Continuous wave (CW) transmitters • Microphones • Modulation Permission granted to reproduce for educational use only.

  30. CW Transmitters • Conventional crystal oscillator • Power amplifier • Power supply Permission granted to reproduce for educational use only.

  31. CW Transmitters (Cont.) • Radio frequency (RF) power amplifier amplifies carrier wave to desired wattage • Power supply must provide voltages and current for oscillator and power amplifier Permission granted to reproduce for educational use only.

  32. CW Transmitters (Cont.) • Have only two states of on or off • No audio or video message • Adding a switch allows CW transmitters to follow a code Permission granted to reproduce for educational use only.

  33. CW Transmitters (Cont.) • Buffer amplifier can be added between oscillator and RF amplifier • Frequency multipliers • Fundamental frequency • Harmonic frequency Permission granted to reproduce for educational use only.

  34. Microphones • Sometimes called transducers • Convert sound waves of vocal vibrations to electrical energy Permission granted to reproduce for educational use only.

  35. Types of Microphones • Carbon microphones • Crystal microphones • Dynamic microphones • Condenser microphones • Velocity microphones Permission granted to reproduce for educational use only.

  36. Carbon Microphones • Granules of carbon, transformer, and small battery joined in series • Diaphragm attached • Nondirectional, picking up sound from all directions Permission granted to reproduce for educational use only.

  37. Crystal Microphones • Use piezoelectric effect of crystals • Mechanical pressure transferred to crystal when sound waves hit the diaphragm • Sensitive to shock and vibration Permission granted to reproduce for educational use only.

  38. Dynamic Microphones • Voice coil moves as sound waves hit the diaphragm • Voltage is induced in the coil as it moves • Directional • Require no outside voltage Permission granted to reproduce for educational use only.

  39. Condenser Microphones • Operate on capacitance principle • Two plates separated by air • Distance between plates varies, causing audio signal • Sensitive, compared to other types of microphones Permission granted to reproduce for educational use only.

  40. Velocity Microphones • High quality • Metal ribbon in magnetic field • Ribbon vibrates when hit by sound waves • Voltage is induced • “Booming effect” occurs when not used properly Permission granted to reproduce for educational use only.

  41. Modulation • Audio wave superimposed on carrier wave • Types • Amplitude modulation (AM) • Frequency modulation (FM) Permission granted to reproduce for educational use only.

  42. Review What is the basic frequency an oscillator produces? Fundamental frequency Permission granted to reproduce for educational use only.

  43. Review What is a microphone? A transducer that converts sound waves to electrical audio waves of the same frequency and relative amplitude Permission granted to reproduce for educational use only.

  44. Review What kind of microphone uses the piezoelectric effect? Crystal microphone Permission granted to reproduce for educational use only.

  45. Review What is the process of combining an audio wave with a carrier wave called? Modulation Permission granted to reproduce for educational use only.

  46. AM • Audio wave combined with a carrier wave • Wave is then modulated Permission granted to reproduce for educational use only.

  47. Sideband Frequencies • Upper sideband • Lower sideband Permission granted to reproduce for educational use only.

  48. Amplitude Modulated Waves • Sum of carrier wave and sidebands • Broadcast band for AM radio is from 535 kHz to 1605 kHz • Vestigial sideband filter improves quality of music Permission granted to reproduce for educational use only.

  49. Modulation Patterns • 100 percent modulation • Overmodulation • Causes splatter Permission granted to reproduce for educational use only.

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