Chapter 5 Problems

1. Chapter 5 Problems ECET 214 Prof. Park NJIT

2. Problem 1 The modulation index is: a. top envelope divided by center frequency b. deviation divided by intelligence frequency c. VCO voltage divided by center frequency d. all of the above

3. Problem 2 Noise is usually clipped by: a. amplifiers b. phase detectors c. limiter circuits d. ARC circuits

4. Problem 3 If the S/N of the input signal is 4 and the intelligence signal is 10 kHz, determine the deviation. a. 145 kHz b. 160 kHz c. 200 kHz d. 100 kHz

5. Problem 4 • The standard time constant used for pre-emphasis in a non-Dolby system is: • a. 75 μs. • b. 25 μs. • c. 50 μs . • d. 175 μs.

6. Problem 5 • Which of the following is indirect FM generation? • a. varactor diode • b. reactance modulator • c. Crosby modulator • d. Armstrong modulator

7. Problem 6 • The two types of angle modulation are: • a. amplitude and frequency modulation. • b. phase and frequency modulation. • c. pulse and frequency modulation. • d. phase and amplitude modulation

8. Problem 7 • Phase modulation is an indirect form of: • a. amplitude modulation. • b. pulse modulation. • c. frequency modulation. • d. angle modulation.

9. Problem 8 • The first working FM system was developed by: • a. Marconi. • b. Armstrong. • c. Sarnoff. • d. Hartley.

10. Problem 9 • In an FM modulator, the amplitude of the intelligence signal determines the: • a. rate of the carrier frequency deviation. • b. power level of the FM signal. • c. phase angle of the carrier frequency component. • d. amount of carrier frequency deviation.

11. Problem 10 • In an FM modulator, the intelligence frequency determines the: • a. rate of carrier frequency deviation. • b. power level of the FM signal. • c. phase angle of the carrier frequency component. • d. amount of carrier frequency deviation.

12. Problem 11 • An FM signal has a center frequency of 154.5 MHz, but is swinging between 154.45 MHz • and 154.55 MHz at a rate of 500 times per second. Its input intelligence frequency is: • a. 100 kHz. • b. 50 kHz. • c. 154.5 MHz. • d. 500 Hz.

13. Problem 12 • An FM signal has a center frequency of 154.5 MHz but is swinging between 154.45 MHz • and 154.55 MHz at a frequency of 500 times per second. Its input carrier frequency is: • a. 100 kHz. • b. 50 kHz. • c. 154.5 MHz. • d. 500 Hz.

14. Problem 13 • An FM signal has a center frequency of 154.5 MHz but is swinging between 154.45 • MHz and 154.55 MHz at a rate of 500 times per second. Its index of modulation is: • a. 50,000. • b. 100. • c. 500 • d. 100,000.

15. Problem 14 • The amount of frequency deviation is proportional to the amplitude of the intelligence • signal in: • a. an FM signal. • b. a PM signal. • c. both FM and PM signals. • d. neither FM nor PM signals.

16. Problem 15 • To solve for the frequency components of an FM signal, what high-level mathematical • tool is used? • a. Laplace transforms • b. Calculus • c. Bessel functions • d. Fourier transforms

17. Problem 16 • An FM signal has an intelligence frequency of 5 kHz and a maximum deviation of 30 kHz. • Its index of modulation is: • a. 60. • b. 35. • c. 150. • d. 6.

18. Problem 17 • An FM signal has an intelligence frequency of 5 kHz and a maximum deviation of 30 kHz. • Its bandwidth, using the Bessel chart of Figure 5-1, is: • a. 6 kHz. • b. 60 kHz. • c. 90 kHz. • d. 45 kHz.

19. Problem 18 • An FM signal has an intelligence frequency of 5 kHz and a maximum deviation of 30 kHz. How • many frequency components are there in the output spectra? (Use the Bessel Chart of Figure 5-1.) • a. 9 • b. 18 • c. 19 • d. 6

20. Problem 19 • An FM signal has an intelligence frequency of 2 kHz and a maximum deviation of 10 kHz. • If its carrier frequency is set at 162.4 MHz, what is the frequency of its highest frequency • component within its bandwidth? (Use the Bessel Chart of Figure 5-1.) • a. 162.401 MHz • b. 162.400 MHz • c. 162.408 MHz • d. 162.404 MHz • e. 162.416 MHz

21. Problem 20 • An FM signal has an intelligence frequency of 2 kHz and a maximum deviation of 10 kHz. • If its carrier frequency is set at 162.4 MHz, what is its bandwidth? (Use the Bessel Chart of • Figure 5-1.) • a. 32 kHz. • b. 20 kHz. • c. 16 kHz. • d. 10 kHz.

22. Problem 21 • Carson's rule is: • a. a quick approximation method to find the bandwidth of an FM signal. • b. a quick approximation method to find the index of modulation of an FM signal. • c. a quick approximation method to find the bandwidth of a PM signal. • d. a quick approximation method to find the index of modulation of a PM signal.

23. Problem 22 • The FM signal given in Figure 5-2 is applied to a 50 ohm load. Its carrier frequency is: • a. 15 kHz. • b. 300 MHz. • c. 150 kHz. • d. 150 MHz.

24. Problem 23 • The FM signal given in Figure 5-2 is applied to a 50 ohm load. Its output power level is • approximately: • a. 450 watts. • b. 2025 watts. • c. 2863 watts. • d. 4050 watts.

25. Problem 24 • The FM signal given in Figure 5-2 is applied to a 50 ohm load. Its index of modulation is: • a. 4 • b. 450 • c. 9.42. • d. 112.5.

26. Problem 25 • The FM signal given in Figure 5-2 is applied to a 50 ohm load. It has an intelligence frequency of: • a. 30 kHz. • b. 150 MHz. • c. 15 kHz. • d. 10 kHz.

27. Problem 26 • The FM signal given in Figure 5-2 is applied to a 50 ohm load. It has a frequency deviation of: • a. 60 kHz. • b. 15 kHz. • c. 30 kHz. • d. 3.75 kHz.

28. Problem 27 • The FM signal given in Figure 5-2 is applied to a 50 ohm load. Using Carson's • rule, it has a bandwidth of: • a. 120 kHz. • b. 150 kHz. • c. 60 kHz. • d. 75 kHz.

29. Problem 28 • Viewing an FM signal on a spectrum analyzer, if the carrier frequency component is zero and • there are four or five sidebands on either side of the carrier frequency, the index of modulation is: • a. 0. • b. 2.2. • c. 5.5 • d. 8.65.

30. Problem 29 • Standard FM broadcast stations use a maximum intelligence frequency of: • a. 5 kHz. • b. 15 kHz. • c. 75 kHz. • d. 150 kHz.

31. Problem 30 • Standard FM broadcast stations use a maximum frequency deviation of: • a. 5 kHz. • b. 200 kHz. • c. 75 kHz. • d. 150 kHz.

32. Problem 31 • Narrowband FM stations: • a. use a maximum deviation of 10 kHz. • b. use intelligence frequencies ranging from 100 Hz to 3 kHz. • c. are found in police, aircraft, taxicabs, weather service, and industrial applications. • d. all of the above.

33. Problem 32 • The modulation index that occurs when the deviation and intelligence frequencies are • maximum is called: • a. the maximum bandwidth. • b. the maximum modulation index. • c. the deviation ratio. • d. the maximum side frequency component.

34. Problem 33 • The most important advantage of FM over AM is: • a. its limited bandwidth. • b. its conservation of energy. • c. its superior noise characteristics. • d. its frequency stability.

35. Problem 34 • The noise characteristics of an FM communication system are mainly due to: • a. its modulator stage. • b. its narrow bandwidth characteristics. • c. its low level of modulation index. • d. its limiter and detector stages.

36. Problem 35 • The worst case signal-to-noise ratio at the output of an FM detector occurs when: • a. the desired signal is 90 degrees out of phase with the noise signal. • b. the desired signal is 90 degrees out of phase with the resultant signal of • adding the signal to the noise. • c. the noise signal is 90 degrees out of phase with the resultant signal of adding the signal to • the noise. • d. the desired signal is 90 degrees out of phase with the intelligence signal.

37. Problem 36 • Determine the worst case output signal-to-noise ratio for a broadcast FM receiver with a • maximum deviation of 75 kHz and a maximum intelligence frequency of 15 kHz if the input • signal-to-noise ratio is 2:1. • a. 5:1 • b. 10:1 • c. 14.7:1 • d. 3:1

38. Problem 37 • Determine the worst case output signal-to-noise ratio for a broadcast FM receiver with a • maximum deviation of 75 kHz and a maximum intelligence frequency of 10 kHz if the input • signal-to-noise ratio is 2:1. • a. 10:1 • b. 15:1 • c. 30:1 • d. 2:1

39. Problem 38 • Determine the worst case output signal-to-noise ratio for a broadcast FM receiver with a • maximum deviation of 75 kHz and a maximum intelligence frequency of 15 kHz if the input • signal-to-noise ratio is 3:1. • a. 5:1 • b. 10:1 • c. 14.7:1 • d. 3:1

40. Problem 39 • Pre-emphasis is: • a. increasing the relative strength of low-frequency components before being fed into the • modulator of an FM transmitter. • b. decreasing the relative strength of low-frequency components of the output signal of an • FM detector in an FM receiver. • c. decreasing the relative strength of high-frequency components at the output signal of an • FM detector in an FM receiver. • d. increasing the relative strength of high-frequency components before being fed into the • modulator of an FM transmitter.

41. Problem 40 • De-emphasis is: • a. increasing the relative strength of low-frequency components before being fed into the • modulator of an FM transmitter. • b. decreasing the relative strength of low-frequency components of the output signal of an FM • detector of an FM receiver. • c. decreasing the relative strength of high-frequency components of the output signal of an FM • detector in an FM receiver. • d. increasing the relative strength of high frequency components before being fed into the • modulator of an FM transmitter.

42. Problem 41 • A de-emphasis network has: • a. an upper cutoff frequency of 2.120 kHz. • b. a time constant of 75 microseconds. • c. a high-frequency roll-off rate of –20 db per decade. • d. all of the above.

43. Problem 42 • The main purpose of pre-emphasis and de-emphasis networks in FM communication systems is to: • a. provide a near constant noise reduction capability between low and high • frequency intelligence signals. • b. allow for a reduction in bandwidth of the FM communication channel. • c. allow for stereo broadcasts to be received by a monaural receiver. • d. filter out noise produced by the FM transmitter's modulator stage.

44. Problem 43 • Which of the following is not an example of a direct FM modulator? • a. varactor diode modulator. • b reactance modulator. • c. 566 VCO modulator. • d. Armstrong modulator.

45. Problem 44 • Which best describes how a varactor diode modulator creates FM? • a. The intelligence signal creates mixing action in the nonlinear varactor diode to • create an FM signal. • b. The intelligence signal alters the amount of forward bias of the varactor diode to create • an FM signal. • c. The intelligence signal alters the capacitance of the diode to shift the resonant frequency • of a tank circuit. • d. The intelligence signal causes the diode to create phase shift which indirectly creates FM.

46. Problem 45 • In this modulator, FM is created by having the intelligence signal cause a change in the • transconductance of a JFET, which causes a change in the JFET amplifier's input capacitance, • which shifts the resonant frequency of a tank circuit. • a. varactor diode modulator • b. reactance modulator • c. VCO modulator • d. PLL modulator

47. Problem 46 • The main disadvantage of direct FM modulators is: • a. they have very limited frequency stability. • b. they have insufficient frequency deviation. • c. they can only work at low radio frequencies. • d. they work reliably only with low-level intelligence signals.