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Abbreviation VLF LF MF HF VHF UHF Frequency range 9 kHz and below 30 kHz 300 kHz 300 kHz 3 MHz 3 MHz 30 MHz 30 MHz 300 MHz 300 MHz 3 GHz 3 GHz and above
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At very high frequencies (VHF) and above, many communications circuits use satellites in geostationary orbits around the earth. If a satellite is directly over the equator at an altitude of 22,300 miles and orbits from west to east, it will follow the earth s rotation, thereby staying in the same spot in the sky as seen from the surface. A single geostationary satellite is on a line of sight with about 40 percent of the earth s surface. Three such satellites, placed at 120-degree (1/3-circle) intervals around the planet, allow coverage of all populated regions. A dish antenna can be aimed at a geostationary satellite, and once the antenna is in place, it needn t be turned or adjusted. Perhaps you have a satellite TV system.
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Beams of infrared or visible light can be modulated just as can radio-frequency carriers. The frequencies of infrared and visible light are extremely high, allowing modulation by data at rates into the gigahertz range. A simple modulated-light transmitter is diagrammed schematically in Fig. 26-17. The output of the light-emitting diode (LED) is modulated by audio from the transistor. The light is guided into an optical fiber, made from a special mixture of very clear glass. In recent years, fiberoptics has begun to replace conventional cable networks on a massive scale. Fiberoptics has several advantages over wire cables. A fiberoptic cable is cheap, and it s light in weight. It is totally immune to interference from outside electromagnetic
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Quiz 495
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26-17 A simple circuit for voice modulating a light beam.
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fields. A fiberoptic cable will not corrode as metallic wires do. Fiberoptic cables are inexpensive to maintain and easy to repair. An optical fiber can carry far more signals than a cable, because the carrier frequency is practically infinite. The whole radio spectrum, from VLF through microwaves, can be imprinted on a beam of light and sent through a single glass fiber no thicker than a strand of your hair.
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Refer to the text in this chapter if necessary. A good score is at least 18 correct. Answers are in the back of the book. 1. A radio wave has a frequency of 1.55 MHz. The highest modulating frequency that can be used effectively is about: A. 1.55 kHz. B. 15.5 kHz. C. 155 kHz. D. 1.55 MHz. 2. Morse code is a form of: A. Digital modulation. B. Analog modulation. C. Phase modulation. D. dc modulation. 3. An advantage of FSK over simple on-off keying for RTTY is: A. Better frequency stability. B. Higher speed capability.
496 Data transmission C. Reduced number of misprints. D. On-off keying is just as good as FSK. 4. The maximum AM percentage possible without distortion is: A. 33 percent. B. 67 percent. C. 100 percent. D. 150 percent. 5. If an AM signal is modulated with audio having frequencies up to 5 kHz, then the complete signal bandwidth will be: A. 10 kHz. B. 6 kHz. C. 5 kHz. D. 3 kHz. 6. An AM transmitter using a class-C PA should employ: A. Carrier suppression. B. High-level modulation. C. Lower sideband. D. Single sideband. 7. Which of the following modulation methods is used to send teleprinter data over the phone lines A. CW. B. SSB. C. AM. D. AFSK. 8. An advantage of SSB over AM is: A. Higher data transmission rate. B. More effective use of transmitter power. C. Greater bandwidth. D. Enhanced carrier wave level. 9. An SSB suppressed carrier is at 14.335 MHz. The voice data is contained in a band from 14.335-14.338 MHz. The mode is: A. AM. B. LSB. C. USB. D. FSK.
Quiz 497 10. A spectrum analyzer displays: A. Time as a function of frequency. B. Frequency as a function of time. C. Signal strength as a function of time. D. Signal strength as a function of frequency. 11. The deviation for voice FM signals is usually: A. Plus-or-minus 3 kHz. B. Plus-or-minus 5 kHz C. Plus-or-minus 6 kHz. D. Plus-or-minus 10 kHz. 12. Wideband FM is preferable to narrowband FM for music transmission because: A. Lower frequencies are heard better. B. Spectrum space is conserved. C. The fidelity is better. D. No! Narrowband FM is better for music. 13. In which mode of PM does the pulse level vary A. PAM. B. PDM. C. PWM. D. PFM. 14. In which PM mode do pulses last for varying times A. PAM. B. PWM. C. PFM. D. PCM. 15. How many states are commonly used for the transmission of digitized voice signals A. Two. B. Four. C. Six. D. Eight. 16. In an SSTV signal, the frame time is: A. 1/525 second. B. 1/30 second. C. 1/8 second. D. 8 seconds.
498 Data transmission 17. The bandwidth of a fax signal is kept narrow by: A. Sending the data at a slow rate of speed. B. Limiting the image resolution. C. Limiting the range of shades sent. D. Using pulse modulation. 18. What is the wavelength of a 21.3-MHz signal A. 46.2 m. B. 14.1 m. C. 21.0 km. D. 6.39 km. 19. A coaxial cable: A. Keeps the signal confined. B. Radiates efficiently. C. Works well as a transmitting antenna. D. Can pick up signals from outside. 20. An advantage of fiberoptics over cable communications is: A. More sensitivity to noise. B. Improved antenna efficiency. C. Higher RF output. D. Simpler and easier maintenance.
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