Objective: Determine the modern uses of FM radio today.
Procedures:
1. Perform Internet searches on the terms FM, FM radio applications, or similar terms.
2. Look for the major uses of FM. What services use FM?
3. Answer the following questions. Keep searching until all questions are answered.
Questions:
1. Name at least i ve places where FM is still used.
2. What is the main benei t of FM over AM?
3. What is the main disadvantage of FM?
4. In what frequency range is FM normally used?
5. What is the digital version of FM called?
Questions
1. What is the general name given to both FM and PM?
2. State the effect on the amplitude of the carrier during FM or PM.
3. What are the name and mathematical expressions for the amount that the carrier varies from its unmodulated center frequency during modulation?
4. State how the frequency of a carrier varies in an FM system when the modulating signal amplitude and frequency change.
5. State how the frequency of a carrier varies in a PM system when the modulating signal amplitude and frequency change.
6. When does maximum frequency deviation occur in an FM signal? A PM signal?
7. State the conditions that must exist for a phase modula- tor to produce FM.
8. What do you call FM produced by PM techniques?
9. State the nature of the output of a phase modulator during the time when the modulating signal voltage is constant.
10. What is the name given to the process of frequency modulation of a carrier by binary data?
11. What is the name given to the process of phase modula- tion of a carrier by binary data?
12. How must the nature of the modulating signal be modii ed to produce FM by PM techniques?
13. What is the difference between the modulation index and the deviation ratio?
14. Dei ne narrowband FM. What criterion is used to indi- cate NBFM?
15. What is the name of the mathematical equation used to solve for the number and amplitude of sidebands in an FM signal?
16. What is the meaning of a negative sign on the sideband value in Fig. 5-8?
17. Name two ways that noise affects an FM signal.
18. How is the noise on an FM signal minimized at the receiver?
19. What is the primary advantage of FM over AM?
20. List two additional advantages of FM over AM.
21. What is the nature of the noise that usually accompa- nies a radio signal?
22. In what ways is an FM transmitter more efi cient than a low-level AM transmitter? Explain.
23. What is the main disadvantage of FM over AM? State two ways in which this disadvantage can be overcome.
24. What type of power amplii er is used to amplify FM signals? Low-level AM signals?
25. What is the name of the receiver circuit that eliminates noise?
26. What is the capture effect and what causes it?
27. What is the nature of the modulating signals that are most negatively affected by noise on an FM signal?
28. Describe the process of preemphasis. How does it improve communication performance in the presence of noise?
Where is it performed, at the transmitter or receiver?
29. What is the basic circuit used to produce preemphasis?
30. Describe the process of deemphasis. Where is it performed, at the transmitter or receiver?
31. What type of circuit is used to accomplish deemphasis?
32. What is the cutoff frequency of preemphasis and deem- phasis circuits?
33. List four major applications for FM.
1. A 162-MHz carrier is deviated by 12 kHz by a 2-kHz modulating signal. What is the modulation index? ◆
2. The maximum deviation of an FM carrier with a 2.5-kHz signal is 4 kHz. What is the deviation ratio?
3. For Problems 1 and 2, compute the bandwidth occu- pied by the signal, by using the conventional method and Carson’s rule. Sketch the spectrum of each signal, showing all signii cant sidebands and their exact amplitudes.
4. For a single-frequency sine wave modulating signal of 3 kHz with a carrier frequency of 36 MHz, what is the spacing between sidebands?
5. What are the relative amplitudes of the fourth pair of sidebands for an FM signal with a deviation ratio of 8? ◆
6. At approximately what modulation index does the am- plitude of the i rst pair of sidebands go to zero? Use Fig. 5-8 or 5-9 to i nd the lowest modulation index that gives this result.
7. An available channel for FM transmission is 30 kHz wide. The maximum allowable modulating signal fre- quency is 3.5 kHz. What deviation ratio should be used? ◆
8. The signal-to-noise ratio in an FM system is 4:1. The maximum allowed deviation is 4 kHz. How much fre- quency deviation is introduced by the phase shift caused by the noise when the modulating frequency is 650 Hz? What is the real signal-to-noise ratio?
9. A deemphasis circuit has a capacitor value of 0.02 µF.
What value of resistor is needed? Give the closest stan- dard EIA value. ◆
10. Use Carson’s rule to determine the bandwidth of an FM channel when the maximum deviation allowed is 5 kHz at frequencies up to 3.333 kHz. Sketch the spectrum, showing carrier and sideband values.
◆ Answers to Selected Problems follow Chap. 22.
Problems
1. The AM broadcast band consists of 107 channels for stations 10 kHz wide. The maximum permitted modu- lating frequency is 5 kHz. Could FM be used on this band? If so, explain what would be necessary to make it happen.
2. A carrier of 49 MHz is frequency-modulated by a 1.5-kHz square wave. The modulation index is 0.25.
Sketch the spectrum of the resulting signal. (Assume that only harmonics less than the sixth are passed by the system.)
3. The FM radio broadcast band is allocated the frequency spectrum from 88 to 108 MHz. There are 100 channels spaced 200 kHz apart. The i rst channel center fre- quency is 88.1 MHz; the last, or 100th, channel center
Critical Thinking
frequency is 107.9 MHz. Each 200-kHz channel has a 150-kHz modulation bandwidth with 25-kHz “guard bands” on either side of it to minimize the effects of overmodulation (overdeviation). The FM broadcast band permits a maximum deviation of 675 kHz and a maximum modulating frequency of 15 kHz.
a. Draw the frequency spectrum of the channel centered on 99.9 MHz, showing all relevant frequencies.
b. Draw the frequency spectrum of the FM band, showing details of the three lowest-frequency chan- nels and the three highest-frequency channels.
c. Determine the bandwidth of the FM signal by using the deviation ratio and the Bessel table.
d. Determine the bandwidth of the FM signal by using Carson’s rule.
e. Which of the above bandwidth calculations best i ts the available channel bandwidth?
4. A 450-MHz radio transmitter uses FM with a maxi- mum allowed deviation of 6 kHz and a maximum mod- ulating frequency of 3.5 kHz. What is the minimum bandwidth required? Use Fig. 5-14 to determine the approximate amplitudes of the carrier and the i rst three signii cant sidebands.
5. Assume that you could transmit digital data over the FM broadcast band radio station. The maximum allowed bandwidth is 200 kHz. The maximum devia- tion is 75 kHz, and the deviation ratio is 5. Assuming that you wished to preserve up to the third harmonic, what is the highest- frequency square wave you could transmit?
Figure 5-14
Value of Jn
⫹1.0
⫹0.9
⫹0.8
⫹0.7
⫹0.6
⫹0.5
⫹0.4
⫹0.3
⫹0.2
⫹0.1
0
⫺0.1
⫺0.2
⫺0.3
⫺0.4
12 Modulation index
0 1 2 3 4 5 6 7 8 9 10 11
J0 J1
J3
J8 J7 J6 J5 J4 J2
chapter