Teach Yourself Electricity and Electronics, 5th edition

Stan Gibilisco

Explanations for Quiz Answers in Chapter 25

1. Mark and space conditions characterize Morse code signals, also called on/off keying or continuous-wave (CW) mode. Frequency-shift keying (FSK) also has mark and space conditions. However, only FSK maintains the carrier in an "on" state at all times, with mark at one frequency and space at another frequency. The correct choice is therefore (b). The fast-scan television (FSTV) mode and the frequency-modulation (FM) mode have many instantaneous signal states, not only mark and space, so neither (c) nor (d) is relevant here.

2. The CW mode employs a mark component in which the carrier is "full-on" and a space component in which the carrier is absent. The correct answer is (a).

3. We can use a discriminator or a ratio detector to directly demodulate an FM signal; in fact, these two systems are specifically designed for FM detection. Therefore, (a) and (b) are both correct. We can use an envelope detector to extract the information from an FM signal by means of the slope-detection technique, in which the carrier frequency "swings" in and out of the receiver passband, so (c) also constitutes a correct answer. Therefore, we should respond to this question with choice (d), "All of the above."

4. If we want to extract the information from an amplitude-modulated (AM) signal, we'll get the best results with an envelope detector. The correct choice is (c).

5. In spread-spectrum communications, the carrier frequency varies in a controlled manner, with the receiver frequency constantly following the transmitter frequency. We can produce the necessary frequency fluctuations in various ways, one of which is frequency hopping (jumping among several different pre-determined frequencies). The correct choice is (c).

6. Birdies are false signals that superheterodyne receivers generate internally. They can result directly from the local oscillator (LO) signals, from harmonics of LO signals, and from mixing among LO signals in dual-conversion systems. The correct choice is (b). Birdies don't occur in direct-conversion receivers, so choice (a) won't work here. Choices (c) and (d) describe individual circuits, neither of which have oscillators that could produce birdies, so these choices are irrelevant.

7. Engineers quantify the ability of a receiver to maintain constant output and avoid overloading in the presence of incoming signals from extremely weak to extremely strong by conducting experiments to determine the dynamic range. The correct choice is (d). The dynamic range has nothing to do with the modulation mode, the incoming-signal frequency, or the amount of noise, so choices (a), (b), and (c) are all irrelevant.

8. When we break a signal down into "pieces," each of which has a specific time duration, transmit those "pieces" in a rotating sequence, and then reassemble the "pieces" back into the original signal at the receiving end of the circuit, we employ time-division multiplexing (TDM). The correct choice is (b).

9. In general, we can modulate a carrier with information containing frequency components up to about 10% of the carrier frequency. If we have a carrier at 830 kHz, then the highest modulating frequency we can impress upon that carrier, expecting decent results, is approximately 83.0 kHz. The correct choice is (d).

10. We'll observe a dead spot in communications reception if the direct wave (that is, the line-of-sight wave) and the reflected wave arrive 180º out of phase at the receiving antenna. Phase opposition produces the same effect. The correct choice is (b). Other phase relationships produce variable signal strength, with the best results occurring when the direct and reflected waves arrive in phase coincidence. Incidentally, a total dead spot can occur only when the direct and reflected waves have equal amplitude, so that they completely cancel each other when they're 180º out of phase or in phase opposition.

11. Tropospheric bending occurs as a result of the fact that, in general, the earth's atmosphere exhibits a decreasing index of refraction as we get farther and farther above the surface. The correct choice is (c). You might think for a moment that (b) is correct, but troposcatter does not depend on any variation of the atmosphere's index of refraction with increasing altitude. It would occur even if the atmosphere had the same index of refraction from the surface all the way up into outer space!

12. When we amplitude-modulate a carrier with data containing frequency components up to 20 kHz, assuming that the carrier frequency is high enough (in this case 200 kHz or more), we get a total signal bandwidth equal to twice the highest modulating frequency (in this case 40 kHz). However, the question asks us about the bandwidth of a single-sideband (SSB) signal, in which we remove one of the sidebands. That process cuts the bandwidth in half, to 20 kHz, so the correct response to this question is (b).

13. An ideal low-earth-orbit (LEO) satellite system employs a "swarm" or "flock" of satellites that travel in so-called polar orbits, meaning that they pass over, or nearly over, the earth's north and south geographic poles. The correct choice is (d).

14. If we want to generate a double-sideband (DSB) signal with a suppressed carrier, we need a balanced modulator that "cancels" the carrier in the output circuit. The correct choice is (c).

15. The circuit of Fig. 25-18 could serve as either a modulator or a mixer, depending on the frequencies of the signals at the inputs. Because the inputs are clearly labeled as "Carrier input" and "Audio input," we can have confidence that the engineer who drew this schematic intended it to represent a modulator. The correct answer is therefore (b).

16. The components marked X in the modulator circuit of Fig. 25-18 serve to stop DC while allowing AC signals to pass. They're blocking capacitors. The correct choice is (a).

17. Assuming that we've chosen the component values properly and we operate the system in the way its designer intends, we should observe an amplitude-modulated (AM) signal at the output. The correct choice is (d).

18. In the circuit of Fig. 25-19, the gate of the JFET goes to ground through a resistor. This resistor is meant to provide the correct DC bias to the JFET. However, if you look closely at the input circuit, you'll see that the lower portion of the inductor (which comprises a wire coil) "shorts out" the gate to ground for DC, defeating the purpose of the gate resistor! In order to keep the gate properly biased, we must install a component between the gate and the inductor that will pass signals but block DC. A capacitor will perform that function -- a blocking capacitor. The correct choice is (b).

19. In order to ensure optimum performance in the weak-signal amplifier of Fig. 25-19, we should adjust the variable capacitor so that the LC circuit resonates at the frequency of the desired incoming signal. This LC circuit provides selectivity at the receiver's front end. The correct choice is (c). The LC circuit can also prevent strong signals from overloading the system, but only if those signals are far removed in frequency from the desired signal. So, even though choice (b) might tempt us, it's not the best answer here.

20. In the weak-signal amplifier circuit of Fig. 25-19, the resistor marked Y optimizes the bias on the JFET -- provided, of course, that we install the necessary blocking capacitor between the gate and the inductor. The correct choice is (a).