Teach Yourself Electricity and Electronics, 5th edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 32

1. We can increase or maximize the transmitter range in a line-of-sight modulated-light communications system by using a paraboloidal, concave reflecting mirror to collimate the light rays (make them essentially parallel). The correct choice is (b). A convex lens or convex-equivalent Fresnel lens will also work, although this option does not appear as a choice here.

2. With respect to security, the term "biometric" refers to the measurement or analysis of a biological parameter such as a hand print, iris pattern, voice pattern, or other human body characteristic. The correct answer is (b); iris-pattern recognition is the only biological parameter mentioned among the four choices.

3. We can connect a computer to a cable system for Internet access using a modem (that's an acronym for modulator/demodulator). This device is a "hybrid" digital-to-analog (D/A) and analog-to-digital (A/D) converter. The correct choice is (a).

4. In any multimode optical fiber, the cladding, which surrounds the core, always has a lower index of refraction than any part of the core. Therefore, the core's refractive index exceeds the cladding's refractive index. The correct choice is (c).

5. In a well-designed low-earth-orbit (LEO) satellite system, every individual user can always "see" at least one satellite, ensuring continuous communications capability. The correct choice is (b).

6. In a line-of-sight optical communications receiver, aperture is a fancy term for the light-gathering area. We can increase this parameter by increasing the diameter of a Fresnel lens or reflector that collects the light from the transmitted signal. The correct choice is (d).

7. When we link two or more computers together using wireless links within the confines of a building, we have a local-area network (LAN). The correct choice is (a).

8. In a line-of-sight modulated-light communications system that uses a photodiode to pick up the light, we must avoid all three of the adverse situations mentioned in choices (a), (b), and (c). The correct answer, therefore, is (d), "All of the above."

9. If we employ end-to-end encryption in a communications system, the most secure means of obtaining the links involves the use of fiber-optic connections. The correct choice is (a). In fact, for any particular level or form of encryption, choice (a) represents the most secure mode of any alternative given in this question. Optical fibers are nigh impossible to "tap."

10. In a cellular telephone system, the region covered by each base station (repeater) is known as a cell. The correct choice is (b).

11. The intensity of the light that we see from a point source, assuming that the source emits a constant amount of light power and we remain at the center of the beam, varies inversely in proportion to the square of the distance. If we increase our distance by a factor of 16, we'll witness a decrease in the intensity by a factor of 16 squared, or 256. The correct choice is (c).

12. The shortwave radio band covers frequencies from 3 to 30 MHz, also known as the high-frequency (HF) band. The correct choice is (a). The term "shortwave" was coined around 1920, when signals at these frequencies had shorter wavelengths (100 m down to 10 m) than the signals commonly used by broadcasters (longer than 100 m, and in some cases several kilometers long). In the early years of radio, some people believed that signals above 3 MHz were so high, and the wavelengths so short, that they'd never work for any practical purpose except short-range or line-of-sight communications! Amateur radio operators proved the naysayers wrong, and "the rest is history."

13. We define latency as the delay produced when signals must travel great distances between the communicating stations. This delay, if more than a few hundredths of a second, makes rapid-response, two-way signaling impossible to carry out effectively. Internet-based computer games require short latency times, but geostationary satellite links have relatively long latency times. The correct choice is (c).

14. We can use a photodiode to detect the visible EM rays in a simple modulated-light receiver. Neither a light-emitting diode (LED) nor an ordinary rectifier diode will work. The correct choice is (a).

15. A medium-earth-orbit (MEO) satellite travels fastest at, or near, its point of perigee, when it's closest to the earth. None of the choices given here mention perigee, so the correct choice is (d), "None of the above."

16. In a communications system where part or all of the path involves wireless links, level-2 and level-3 security require end-to-end encryption; level-1 security requires nothing more than wireless-only encryption. The correct choice is (c).

17. In full-duplex communication, either user can interrupt the other user at any time. The correct choice is (d).

18. All of the computers have similar computing power, speed, and storage capacity in a peer-to-peer LAN. The correct choice is (c).

19. We can modulate a single visible-light or IR beam with thousands of different RF signals. The correct choice is (c). In some cases, millions of individual RF signals can "fit" onto a visible-light or IR beam, but not infinitely many, so choice (d) goes too far.

20. A packet communications system can involve the use of the Internet along with fixed amateur radio stations. The correct choice is (a).