COMMUNICATIONS ENGINEERING PART 2dB/NOISE

1. A current change that is equal to twice its original value will correspond to a change of

a. 3 dBb. 9 dBc. 10 dBd. 6 dB

2. What does a power difference of -3 dB mean?

a. a loss of one third of the powerb. a loss of one-half of the powerc. a loss of 3 watts of powerd. no significant change

3. A gain of 60 dB is the same as a gain of

a. 10 volts/voltb. 100 volts/voltc. 1000 volts/voltd. 10,000 volts/volt

4. _____ is mathematically equal to the logarithm to the base ten of the power ratio P1 over P2.

a. Belb. dBc. bel/10d. dB/2

5. Noise that is produced by the active components within the receiver.

a. thermalb. externalc. internald. white

6. Noise due to the random variation in the arrival of charge carriers at the output electrode of an active device.

a. shotb. impulsec. thermald. dynamic

7. A network has a loss of 20 dB. What power ratio corresponds to this loss?

a. 0.01b. 0.1c. 10

d. 100

8. A receiver connected to an antenna whose resistance is 60 ohms has an equivalent noise resistance of 40 ohms. Calculate the receiver’s noise figure in decibels and its equivalent noise temperature.

a. 1.67 & 194Kb. 2.23 & 194 K c. 1.67 & 174Kd. 2.23 & 194K

9. _____ Noise is the most prevalent noise found in urban areas and is normally caused by the arc discharge from automobile or aircraft ignition systems, induction motors, switching gears, high voltage lines and the like.

a. Industrialb. Johnsonc. Flickerd. Mixer

10. The noise figure of a totally noiseless device is

a. unityb. infinityc. zerod. 100

11. Indicate the noise whose source is in a category different from that of the other three.

a. solarb. cosmicc. atmosphericd. galactic

12. The ratio (in dB) of the power of a signal at point to the power of the same signal at the reference point.

a. Transmission Level Pointb. Noise Figurec. S/N Ratiod. Neper

13. A network has a power gain of –3dB. If the input power is 100 watts, the output power is

a. 50 wattsb. 55 watts

c. 60 wattsd. 62 watts

14. Which of the following types of noise becomes of great importance at high frequencies?

a. shot b. randomc. impulsed. transit time

15. The input current of a network is 190 A and the output is 1.3 A. The loss in decibels is

a. 20.2b. 21.6c. 28.6d. 43.3

16. If 4 networks connected in series have gains of –0.5 dB, -0.3 dB, -2 dB and 6.8 dB, the overall gain is

a. 2 dBb. –2 dB

c. 4 dBd. –4dB

17. What is the gain, in dB, if the output to input ratio is 1000.

a. 20b. 30c. 40d. 10

18. The following are characteristics of noise except

a. unwanted energyb. predictable in characterc. present in the channeld. due to any cause

19. The equivalent noise temperature of the amplifier is 25K, what is the noise figure?

a. 10.86b. 1.086c. 0.1086d. 1.86

20. A receiver connected to an antenna whose resistance is 50 ohms has an equivalent noise resistance of 30 ohms. What is the receiver’s noise temperature?

a. 464Kb. 754 Kc. 400 Kd. 174 K

21. A theoretical antenna has a gain of 1 dB. Its gain in nepers is

a. 8.686b. 0.1151c. 6.868d. 0.5111

22. What is the equivalent output of a circuit in dBm, if it has an output of 10 watts?

a. 10 dBmb. 30 dBmc. 20 dBmd. 40 dBm

23. An amplifier with an input resistance of 1000 ohms is operating over a 4 MHz bandwidth. Calculate the rms noise voltage if the amplifier is operating at 27 C.

a. 8.14 nVb. 8.14 Vc. 6.6 nVd. 6.6 V

24. The value of a resistor creating thermal noise is doubled. The noise power generated is therefore

a. halvedb. quadrupledc. doubledd. unchanged

25. One of the following is not a useful quantity for comparing the noise performance of receivers.a. input noise voltageb. equivalent noise resistancec. noise temperatured. noise figure

26. Any unwanted form of energy that tends to interfere with the wanted signals is called

a. noiseb. spectrumc. radiationd. absorption

27. The correct symbol for decibel is

a. DBb. dBc. Dbd. db

28. _____ is the noise created outside the receiver.

a. Internal b. Externalc. Shotd. Industrial

29. _____ is the noise created by man.

a. Solarb. Industrialc. Extraterrestiald. Galactic

30. A voltage change that is equal to twice its original value correspond to a change of

a. 3 dBb. 6 dBc. 9 dBd. 10 dB

31. Indicate the voltage level in dB with reference to one volt. This unit is used in video or TV measurement.

a. dBWb. dBkc. dBmd. dBV

32. Which of the following is not an actual amount of power?

a. dBb. dBmc. dBwd. dBk

33. In noise analysis, the reference temperature is

a. 75 Kb. 250 Kc. 290 Kd. 300 K

34. Noise from distant planets, stars, galaxies and other celestial objects are called

a. cosmicb. extraterrestrialc. galacticd. black body

35. Indicate which one of the following types of noise does not occur in transistors:

a. shot noiseb. flicker noisec. partition noised. resistance noise

36. Which of the following is not a source of space noise?

a. sunb. starc. lightningd. black body

37. Noise that is due to the random and rapid motion of the charge carriers inside a resistive component.

a. Johnsonb. Thermal Agitationc. Whited. all of the above

38. Indicate the false statement. The square of the thermal noise voltage generated by a resistor is proportional to

a. its resistanceb. its temperaturec. Boltzmann’s constantd. The bandwodth over which it is

measured

39. In a communications system, noise is likely to affect the signal

a. at the transmitterb. in the channelc. in the information sourced. at the destination

40. The noise power generated by a resistor is proportional to

a. temperatureb. bandwidthc. a and bd. NOTA

41. Thermal noise is also known as:

a. Gaussian Noiseb. White noisec. Johnson noised. All of the above

42. This type of noise has a power spectrum which decreases with increasing frequency. It is most important at low frequencies (from 0 to about 100 Hz).

a. shot noiseb. flicker noisec. diode noised. BJT noise

43. Industrial noise extends up to what frequency?

a. 500 MHzb. 500 GHzc. 500 THzd. 500 kHz

44. Impulse Noise is

a. a function of currentb. a short duration pulsec. dependent of frequencyd. dependent of temperature

45. When the power ratio of the output to input of a circuit is 200. What is the gain in dB?

a. 23 b. 46c. –23d. –46

46. What is the reference level for random noise measurement, FIA weighted?

a. –82 dBmb. -90 dBmc. –85dBmd. –77dBm

47. A 10 db pad has an output level of –3 dBm. The level at the input is:

a. 13 dBm b. –7 dBmc. 1 dBmd. 7 dBm

48. The sum of three signals of 45dBm each is _____dBm.

a. 45

b. 135c. 20d. 50

49. It is characterized by high amplitude peaks of short duration in the total noise spectrum.

a. intermodulation voiceb. impulse noisec. dropoutd. phase hits

50. Originally was determined by measuring the interfering effect of noise in a Type 144 handset. A tone of 1 kHz, having a power level of 90 dBm was selected as the reference level.

a. noise figureb. S/N ratioc. Signal figured. Figure of merit

51. A power level of 50 W could be expressed as:

a. 1.69 dBmb. –4.3 dBmc. 1 dBmd. –13 dBm

52. If a power of 0.25 mW is launced into a fiber system with an overall loss of 15 dB the output power would be:a. 250 /Wb. 31.6 Wc. 7.9 Wd. 15 dBm

53. A system having an input power of 2 mW an output power of 0.8 mW has a loss of:

a. 2.98 dBmb. 3.98 dBc. 3.98 Wd. 1.98 mW

54. An output of –10 dB means that the power has been

a. halved in valueb. increased by a factor of 10c. reduced by a factor of 10d. doubled

55. Any unwanted form of energy interfering the reception of wanted signal is called

a. noiseb. sidebandc. harmonicsd. modulation

56. Is the reduction of signal amplitude as it passed over the transmission medium.

a. noiseb. distortionc. attenuationd. interference

57. Signal waveform perturbation or deviation caused by imperfect response of the system to the desired signal

a. noiseb. aliasingc. distortiond. interference

58. Signal attenuation can be corrected by

a. filteringb. modulationc. equalizationd. amplification

59. Distortion in a waveform can be corrected by

a. filteringb. modulationc. equalizationd. amplification

60. Signal contamination by extraneous or external sources, such as, other transmitters, power lines, and machinery.

a. noiseb. distortionc. harmonicsd. interference

61. Man-made or industrial noise is also known as

a. noiseb. distortionc. interferenced. thermal noise

62. The noise performance of a receiver or circuit. It is expressed as ratio of the S/N power at the input to the S/N power at the output.

a. Noise figureb. S/N ratioc. Signal figured. Figure of merit

63. Noise that is caused by natural disturbances such as lightning discharge.

a. static noiseb. space noisec. atmospheric noised. A or C

64. Atmospheric or static noise becomes less severe at frequencies

a. below 30 KHzb. between 30 KHz and 300 KHzc. between 300 KHz and 30 MHzd. above 30 MHz

65. Considered as space noise or extraterrestrial noise.

a. solar noiseb. cosmic noisec. black-body noised. all of the above

66. Which statement is not true?

a. Industrial noise is usually of impulse type.

b. Distant stars produce atmospheric noise.

c. Active switches are sources of man-made noise.

d. Static noise is due to lightning discharges and other natural electric disturbances occurring in the atmosphere.

67. Noise performance of microwave system is usually expressed in terms of

a. noise voltage, Vn = 4kTBRb. noise power, Pn = KTBc. noise temperature, Te = (F – 1) 290d. noise figure, F = (S/N)I / (S/N)o

68. Which circuit contributes most to the noise at the receiver?

a. RF amplifierb. Mixerc. Detectord. Local oscillator

69. Which noise figure represents the lowest noise?

a. 1.5 dBb. 2.0 dBc. 3.7 dBd. 4.1 dB

70. Denote the interference of noise in dB above an adjusted reference noise. The adjusted reference noise level was a 1 kHz tone, set at -85 dBm

a. dBab. dBmc. dBa0d. pWp

71. The extent of noise referred to a test tone level of zero dBm.

a. dBab. dBmc. dBa0d. pWp

72. An amplifier operating over a 4-MHz bandwidth has a 100 input resistance and is operating at 300K. Determine the noise power generated.

a. 1.656 x 10-14 Wattsb. 1656 nWc. 1.656 pWd. 1.656 W

73. Generally used when noise readings are measured using the C-message weighting network. The reference level was 1 kHz tone, set at – 90 dBm.

a. dBab. dBmc. dBaOd. dBmC

74. The measurement of noise was made with a C-message filter, and the reading is taken at a test point where the level is zero dBm.

a. dBaOb. dBmCo

c. dBad. dBmC

75.Which of the following is not an important cause of distortion in DC signaling?

a. line resistanceb. line inductancec. line capacitanced. all of the above

76.There are a number of different sources of radio noise, the most important being

a. galactic noiseb. man-made noisec. atmospheric noised. all of the above

77.The amount of noise power is measured using a psophometric weighting network. This unit of measurement is generally used in Europe where the standard reference tone is 800 hertz, 1 picowatt.

a. dBab. dBmc. dBaOd. pWp

78.Noise produced mostly by lightning discharges in thunderstorms.a. white noiseb. industrial noisec. atmospheric noised. extraterrestrial noise

79.Propagation of man-made noise is chiefly bya. transmission over power lines and by

ground waveb. space-wavec. sky-waved. none of these

80.A more precise evaluation of the quality of a receiver as far as noise is concerned.

a. S/Nb. VSWRc. Noise factord. Noise margin

81.NIF stands for

a. non-intrinsic figureb. noise interference figurec. noise improvement factord. narrow intermediate frequency

82.External noise fields are measured in terms of

a. dc values

b. rms valuesc. peak valuesd. average values

83.Form of interference caused by rain, hail, snow or dust storms

a. shot noiseb. galactic noisec. impulse noised. precipitation static

84.Extra-terrestrial noise is observable at frequencies from

a. 0 to 20 KHzb. 8 MHz to 1.43 GHzc. 5 to 8 GHzd. 15 to 160 MHz

85.Industrial noise is observable from

a. 15 to 160 MHzb. 200 to 3,000 MHzc. 0 to 10 KHzd. 8 MHz to 1.43 GHz

86.Noise that becomes significant at VHF range and above

a. atmosphericb. transit-timec. galacticd. white

87.Noise figure for an amplifier with noise is always

a. 0 dBb. infinitec. less than 1d. greater than 1

88.The noise generated by the tube, transistor, or integrated circuit in an amplifier.

a. white noiseb. amplification noisec. active noised. dynamic noise

89.Electrical noise inherent to a particular device, circuit, or system that remains when no other signal is present.

a. shot noiseb. thermal noisec. background noise

d. static noise

90.A wideband form of impulse noise generated by the electric arc in the spark plugs of an internal combustion engine. This noise is a common problem in mobile radio system.

a. thermal noiseb. shot noisec. amplification noised. ignition noise

91. The amount of power in dB referred to one kilowatt.

a. dBWb. dBkc. dBmd. dBV

92.Noise in any form of electromagnetic interference that can be traced to non-natural causes.

a. man-made noiseb. distortionc. external noised. internal noise

93.The frequency range wherein noise is said to be intense.

a. noise equivalent bandwidthb. spectral responsec. cut-off frequencyd. noise cut-off frequency

94.Refers to the temperature that corresponds to the spectral energy distribution of a noise.

a. absolute temperatureb. temperature bandc. noise-equivalent temperatured. critical temperature

95.A passive circuit, usually consisting of capacitance and/or inductance, that is inserted in series with the a-c power cord of an electronic device which will allow the 60-Hz current to pass and suppressed high frequency noise components.

a. noise filterb. noise limiterc. noise floord. noise quieting

96.What do you call the level of background noise, relative to some reference signal.

a. noise figureb. minimum noisec. reference noised. noise floor

97.A circuit often used in radio receivers that prevents externally generated noise from exceeding a certain amplitude. They are also called noise clippers.

a. noise floorb. noise filterc. noise limiterd. noise clamper

98. It is referred to as a short burst of electromagnetic energy.

a. pulseb. noise pulsec. spiked. noise floor

99. The reduction of internal noise level in a frequency-modulated (FM) receiver as a result of an incoming signal.

a. noise quietingb. noise limitingc. noise suppressiond. noise degradation

100. Noise generated within electronic equipment by either passive or active components.

a. shot noiseb. thermal noisec. circuit noised. external noise

COMMUNICATIONS ENGINEERING PART 3MODULATION

1. Having an information signal change some characteristic of a carrier signal is called

a. Multiplexingb. Modulationc. Duplexingd. Linear mixing

2. Which of the following is not true about AM?

a. The carrier amplitude varies.

b. The carrier frequency remains constant.

c. The carrier frequency changes.d. The information signal amplitude

changes the carrier amplitude.

3. The opposite of modulation is

a. Reverse modulationb. Downward modulationc. Unmodulationd. Demodulation

4. The circuit used to produce modulation is called a

a. Modulatorb. Demodulatorc. Variable gain amplifierd. Multiplexer

5. A modulator circuit performs what mathematical operation on its two inputs?

a. Additionb. Multiplicationc. Divisiond. Square root

6. The ratio of the peak modulating signal voltage to the peak carrier voltage is referred to as

a. The voltage ratiob. Decibelsc. The modulation indexd. The mix factor

7. If m is greater than 1, what happens?

a. Normal operationb. Carrier drops to zeroc. Carrier frequency shiftsd. Information signal is distorted

8. For ideal AM, which of the following is true?

a. m = 0b. m = 1c. m < 1d. m > 1

9. The outline of the peaks of a carrier has the shape of the modulating signal and is called the

a. Traceb. Waveshape

c. Enveloped. Carrier variation

10. Overmodulation occurs when

a. Vm > Vc

b. Vm < Vc

c. Vm = Vc

d. Vm = Vc = 0

11. The values of Vmax and Vmin as read from an AM wave on an oscilloscope are 2.8 and 0.3. The percentage of modulation is

a. 10.7 percentb. 41.4 percentc. 80.6 percentd. 93.3 percent

12. The new signals produced by modulation are called

a. Spurious emissionsb. Harmonicsc. Intermodulation productsd. Sidebands

13. A carrier of 880 kHz is modulated by a 3.5-kHz sine wave. The LSB and USB are, respectively,

a. 873 and 887 kHzb. 876.5 and 883.5 kHzc. 883.5 and 876.5 kHzd. 887 and 873 kHz

14. A display of signal amplitude versus frequency is called the

a. Time domainb. Frequency spectrumc. Amplitude spectrumd. Frequency domain

15. Most of the power in an AM signal is in the

a. Carrierb. Upper sidebandc. Lower sidebandd. Modulating signal

16. An AM signal has a carrier power of 5 W. The percentage of modulation is 80 percent. The total sideband power is

a. 0.8 Wb. 1.6 Wc. 2.5 Wd. 4.0 W

17. For 100 percent modulation, what percentage of power is in each sideband?

a. 25 percentb. 33.3 percentc. 50 percentd. 100 percent

18. An AM transmitter has a percentage of modulation of 88. The carrier power is 440 W. The power in one sideband is

a. 85 Wb. 110 Wc. 170 Wd. 610 W

19. An AM transmitter antenna current is measured with no modulation and found to be 2.6 amperes. With modulation, the current rises to 2.9 amperes. The percentage of modulation is

a. 35 percentb. 70 percentc. 42 percentd. 89 percent

20. What is the carrier power in the problem above if the antenna resistance is 75 ohms?

a. 195 Wb. 631 Wc. 507 Wd. 792 W

21. In an AM signal, the transmitted information is contained within the

a. Carrierb. Modulating signalc. Sidebandsd. Envelope

22. An AM signal without the carrier is called a(n)

a. SSBb. Vestigial sidebandc. FM signald. DSB

23. What is the minimum AM signal needed to transmit information?

a. Carrier plus sidebandsb. Carrier only

c. One sidebandd. Both sidebands

24. The main advantage of SSB over standard AM or DSB is

a. Less spectrum space is usedb. Simpler equipment is usedc. Less power is consumedd. A higher modulation percentage

25. In SSB, which sideband is the best to use?

a. Upperb. Lowerc. Neitherd. Depends upon the use

26. The typical audio modulating frequency range used in radio and telephone communications is

a. 50 Hz to 5 kHzb. 50 Hz to 15 kHzc. 100 Hz to 10 kHzd. 300 Hz to 3 kHz

27. An AM signal with a maximum modulating signal frequency of 4.5 kHz has a total bandwidth of

a. 4.5 kHzb. 6.75 kHzc. 9 kHzd. 18 kHz

28. The modulation system used for telegraphy is

a. Frequency-shift keyingb. Two-tone modulationc. Pulse-code modulationd. Single-tone modulation

29. The process of translating a signal, with or without modulation, to a higher or lower frequency for processing is called

a. Frequency multiplicationb. Frequency divisionc. Frequency shiftd. Frequency conversion

30. Frequency translation is carried out by a circuit called a

a. Translatorb. Converter

c. Balanced modulatord. Local oscillator

31. An input signal of 1.8 MHz is mixed with a local oscillator of 5 MHz. A filter selects the difference signal. The output is

a. 1.8 MHzb. 3.2 MHzc. 5 MHzd. 6.8 MHz

32. One type of pulse communications system uses pulse that appear as a group, and which vary in number according to the loudness of the voice. This type of pulse modulation is called

a. Pulse duration modulationb. Pulse amplitude modulationc. Pulse code modulationd. Pulse position modulation

33. An SSB transmitter produces a 400-V peak-to-peak signal across a 52- antenna load. The PEP output is

a. 192.2 Wb. 384.5 Wc. 769.2 Wd. 3077 W

34. The output power of an SSB transmitter is usually expressed in terms of

a. Average powerb. RMS powerc. Peak-to-peak powerd. Peak envelope power

35. The letter-number designation B8E is a form of modulation also known as

a. Pilot-carrier systemb. Independent sideband emissionc. Lincomlexd. Vestigial sideband transmission

36. Amplitude modulation is the same as

a. Linear mixingb. Analog multiplicationc. Signal summationd. Multiplexing

37. In a diode modulator, the negative half of the AM wave is supplied by a(n)

a. Tuned circuitb. Transformerc. Capacitord. Inductor

38. Amplitude modulation can be produced by

a. Having the carrier vary a resistanceb. Having the modulating signal vary a

capacitancec. Varying the carrier frequencyd. Varying the gain of an amplifier

39. Amplitude modulators that vary the carrier amplitude with the modulating signal by passing it through an attenuator work on the principle of

a. Rectificationb. Resonancec. Variable resistanced. Absorption

40. The component used to produce AM at very high frequencies is a

a. Varactorb. Thermistorc. Cavity resonatord. PIN diode

41. Amplitude modulation generated at a very low voltage or power amplitude is known as

a. High-level modulationb. Low-level modulationc. Collector modulationd. Minimum modulation

42. A collector modulator has a supply voltage of 48 V. The peak-to-peak amplitude of the modulating signal for 100 percent modulation is

a. 24 Vb. 48 Vc. 96 Vd. 120 V

43. A collector-modulated transmitter has a supply voltage of 24 V and collector current of 0.5 A. The modulator power for 100 percent modulation is

a. 6 Wb. 12 Wc. 18 Wd. 24 W

44. The circuit that recovers the original modulating information from an AM signal is known as a

a. Modulatorb. Demodulatorc. Mixerd. Crystal set

45. The most commonly used amplitude demodulator is the

a. Diode mixerb. Balanced modulatorc. Envelope detectord. Crystal filter

46. A circuit that generates the upper and lower sidebands but no carrier is called a(n)

a. Amplitude modulatorb. Diode detectorc. Class C amplifierd. Balanced modulator

47. The inputs to a balanced modulator are 1 MHz and a carrier of 1.5 MHz. The outputs are

a. 500 kHzb. 2.5 MHzc. 1.5 MHzd. Both a and b

48. A widely used balanced modulator is called the

a. Diode bridge circuitb. Full-wave bridge rectifierc. Lattice modulatord. Balanced bridge modulator

49. In a diode ring modulator, the diodes act like

a. Variable resistorsb. Switchesc. Rectifiersd. Variable capacitors

50. The output of a balanced modulator is

a. AMb. FMc. SSBd. DSB

51. The principal circuit in the popular 1496/1596 IC balanced modulator is a

a. Differential amplifierb. Rectifierc. Bridged. Constant current source

52. The most commonly used filter in SSB generators uses

a. LC networksb. Mechanical resonatorsc. Crystalsd. RC networks and op amps

53. The equivalent circuit of a quartz crystal is a

a. Series resonant circuitb. Parallel resonant circuitc. Neither a nor bd. Both a and b

54. A crystal lattice filter has crystal frequencies of 27.5 and 27.502 MHz. The bandwidth is approximately

a. 2 kHzb. 3kHzc. 27.501 MHzd. 55.502 MHz

55. An SSB generator has a sideband filter centered at 3.0 MHz. The modulating signal is 3 kHz. To produce both upper and lower sidebands, the following carrier frequencies must be produced:

a. 2.7 and 3.3 MHzb. 3.3 and 3.6 MHzc. 2997 and 3003 kHzd. 3000 and 3003 kHz

56. In the phasing method of SSB generation, one sideband is canceled out due to

a. Phase shiftb. Sharp selectivityc. Carrier suppressiond. Phase inversion

57. A balanced modulator used to demodulate a SSB signal is called a(n)

a. Transponderb. Product detectorc. Converterd. Modulator

58. Frequency translation is done with a circuit called a

a. Summerb. Multiplierc. Filterd. Mixer

59. The inputs to a mixer are fo and fm. In down conversion, which of the following mixer output signals is selected?

a. fo

b. fm

c. fo – fm

d. fo + fm

60. Mixing for frequency conversion is the same as

a. Rectificationb. AMc. Linear summingd. Filtering

61. Which of the following can be used as a mixer?

a. Balanced modulatorb. FETc. Diode modulatord. All the above

62. The desired output from a mixer is usually selected with a

a. Phase-shift circuitb. Crystal filterc. Resonant circuitd. Transformer

63. The amount of frequency deviation from the carrier center frequency in an FM transmitter is proportional to what characteristic of the modulating signal?

a. Amplitudeb. Frequencyc. Phased. Shape

64. Both FM and PM are types of what kind of modulation?

a. Amplitudeb. Phasec. Angled. Duty cycle

65. If the amplitude of the modulating signal decreases, the carrier deviation

a. Increasesb. Decreasesc. Remains constantd. Both b and c

66. In PM, a frequency shift occurs while what characteristic of the modulating signal is changing?

a. Shapeb. Phasec. Frequencyd. Amplitude

67. Maximum frequency deviation of a PM signal occurs at

a. Zero crossing pointsb. Peak positive amplitudec. Peak negative amplituded. Peak positive or negative amplitudes

68. A 100-MHz carrier is deviated 50 kHz by a 4-kHz signal. The modulation index is

a. 5b. 8c. 12.5d. 20

69. The maximum deviation of an FM carrier is 2 kHz by a maximum modulating signal of 400 Hz. The deviation ratio is

a. 0.2b. 5c. 8d. 40

70. According the Nyquist theorem, the sampling rate that can be used in a PCM system is __________ the highest audio frequency.

a. Onceb. Eight timesc. Twiced. Thrice

71. SSB transmission requires only __________ of the bandwidth for a DSBFC

a. ¼b. 2/3c. ½

d. ¾

72. Which of the following pulse modulation systems is analog?

a. Deltab. Differential PCMc. PWMd. PCM

73. Which of the following is not a major benefit of FM over AM?

a. Greater efficiencyb. Noise immunityc. Capture effectd. Lower complexity and cost

74. The primary disadvantage of FM is its

a. Higher cost and complexityb. Excessive use of spectrum spacec. Noise susceptibilityd. Lower efficiency

75. The phenomenon of a strong FM signal dominating a weaker signal on a common frequency is referred to as the

a. Capture effectb. Blot outc. Quieting factord. Domination syndrome

76. If the plate supply voltage for a plate-modulated class C amplifier is E, the maximum plate-cathode voltage could be almost as high as

a. 4Eb. 3Ec. 2Ed. E

77. In a low-level AM system, amplifiers following the modulated stage must be

a. Linear devicesb. Harmonic devicesc. Class C amplifiersd. Nonlinear devices

78. If the carrier of a 100 percent modulated AM wave is suppressed, the percentage power saving will be

a. 50b. 150

c. 100d. 66.66

79. Leak-type bias is used in a plate-modulated class C amplifier to

a. Prevent tuned circuit dampingb. Prevent excessive grid currentc. Prevent overmodulationd. Increase the bandwidth

80. The output stage of a television transmitter is most likely to be a

a. Plate-modulated class C amplifierb. Grid-modulated class C amplifierc. Screen-modulated class C amplifierd. Grid-modulated class A amplifier

81. The modulation index of an AM wave is changed from 0 to 1. The transmitted power is

a. Unchangedb. Halvedc. Doubledd. Increased by 50 percent

82. One of the advantages of base modulation over collector modulation of a transistor class C amplifier is

a. The lower modulating power requiredb. Higher power output per transistorc. Better efficiencyd. Better linearity

83. A carrier is simultaneously modulated by two sine waves with modulation indices of 0.3 and 0.4; the total modulation index

a. 1b. cannot be calculated unless the phase

relations are knownc. 0.5d. 0.7

84. In the stabilized reactance modulator AFC system,

a. The discriminator must have a fast time constant to prevent demodulation

b. The higher the discriminator frequency, the better the oscillator frequency stability

c. The discriminator frequency must not be too low, or the system will fail

d. Phase modulation is converted into FM by the equalizer circuit

85. In the spectrum of a frequency-modulated wave

a. The carrier frequency disappears when the modulation index is large

b. The amplitude of any sideband depends on the modulation index

c. The total number of sidebands depends on the modulation index

d. The carrier frequency cannot disappear

86. The difference between phase and frequency modulation

a. Is purely theoretical because they are the same in practice

b. Is too great to make the two systems compatible

c. Lies in the poorer audio response of phase modulation

d. Lies in the different definitions of the modulation index

87. Indicate the false statement regarding the Armstrong modulation system.

a. The system is basically phase, not frequency, modulation

b. AFC is not needed, as a crystal oscillator is used

c. Frequency multiplication must be usedd. Equalization is unnecessary

88. An FM signal with a modulation index mf is passed through a frequency tripler. The wave in the output of the tripler will have a modulation index of

a. mf/3b. mf

c. 3mf

d. 9mf

89. An FM signal with a deviation is passed through a mixer, and has its frequency reduced fivefold. The deviation in the output of the mixer is

a. 5 b. indeterminatec. /5d.

90. A pre-emphasis circuit provides extra noise immunity by

a. Boosting the bass frequenciesb. Amplifying the higher audio frequenciesc. Pre-amplifying the whole audio bandd. Converting the phase modulation to FM

91. Since noise phase-modulates the FM wave, as the noise sideband frequency approaches the carrier frequency, the noise amplitude

a. Remains constantb. Decreasedc. Increasedd. Equalized

92. When the modulating frequency is doubled, the modulation index is halved, and the modulating voltage remains constant. The modulation system is

a. Amplitude modulationb. Phase modulationc. Frequency modulationd. Any one of the three

93. Indicate which one of the following is not an advantage of FM over AM

a. Better noise immunity is providedb. Lower bandwidth is requiredc. The transmitted power is more usefuld. Less modulating power is required

94. One of the following is an indirect way of generating FM. This is the

a. Reactance FET modulatorb. Varactor diode modulatorc. Armstrong modulatord. Reactance bipolar transistor modulator

95. Indicate the false statement regarding the advantages of SSB over double-sideband, full-carrier AM.

a. More channel space is availableb. Transmitter circuits must be more

stable, giving better receptionc. The signal is more noise-resistantd. Much less power is required for the

same signal strength

96. When the modulation index of an AM wave is doubled, the antenna current is also doubled. The AM system being used is

a. A3Hb. A5Cc. A3Jd. A3

97. Indicate which one of the following advantages of the phase cancellation method of obtaining SSB over the filter method is false

a. Switching from one sideband to the other is simpler

b. It is possible to generate SSB at any frequency

c. SSB with lower audio frequencies present can be generated

d. There are more balanced modulators; therefore the carrier is suppressed better

98. The most commonly used filters in SSB generation are

a. Mechanicalb. RCc. LCd. Low-pass

99. Indicate in which one of the following only one sideband is transmitter

a. A3Hb. A3c. A3Bd. A5C

100.A 3A modulation is sometimes used to

a. Allow the receiver to have a frequency synthesizer

b. Simplify the frequency stability problem in reception

c. Reduce the power that must be transmitted

d. Reduce the bandwidth required for transmission