COMMS 1 - Modulation

7/27/2019 COMMS 1 - Modulation

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MITRC REFRESHER COURSE ANSWER KEY dB Noise,AM & FM

1. A portion in a communications system, whichprocesses the information so that it will becomesuitable to the characteristic of the transmissionmediuma. Encoder b. Modulation

c. Transmitterd. Multiplexer

2. Signal whose physical quantity varies continuouslywith timea. Digitalb. Analogc. Discreted. Information

3. Digital information is processed with a specifieddegree ofa. Fidelityb. Accuracyc. Sensitivityd. Correctness

4. Indicate the voltage level in dB with reference toone volt. This unit is used in video or TVmeasurementa. dBWb. dBkc. dBmd. dBV

5. When the power ratio of the output to input of acircuit is 200. What is the gain in dB?

a. 23b. 46c. 23d. 46

6. What is the reference level for random noisemeasurement, FIA weighted?a. 82 dBmb. 90 dBmc. 85 dBmd. 77 dBm

7. A 10 db pad has an output level of -3dBm. The

level at the input is:a. 13 dBmb. 7 dBmc. 1 dBmd. 7 dBm

8. A power level of 50 W could be expressed as:a. 1.69 dBmb. 4.3 dBmc. 1 dBmd. 13 dBm

9. 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

10. Is the reduction of signal amplitude as it passedover the transmission medium?a. Noiseb. Distortionc. Attenuation

d. Interface

11. Signal waveform perturbation or deviation causedby imperfect response of the system to the desiredsignala. Noiseb. Aliasing

c. Distortiond. Interference

12. Signal attenuation can be corrected bya. Filteringb. Modulationc. Equalizationd. Amplification

13. Signal contamination by extraneous or externalsources, such as, other transmitters, power linesand machinery.a. Noiseb. Distortionc. Harmonicsd. Interference

14. Which noise figure represents the lowest noise?a. 1.5 dBb. 2.0 dBc. 3.7 dBd. 4.1 dB

15. Denote the interference of noise in dB above anadjusted reference noise. The adjusted referencenoise level was a 1 kHz tone, set at 85 dBma. dBa

b. dBmc. dBa0d. pWp

16. A more precise evaluation of the quality of areceiver as far as noise is concerned.a. S/Nb. VSWRc. Noise factord. Noise margin

17. Noise figure for an amplifier with noise is alwaysa. 0 dB

b. Infinitec. Less than 1d. Greater than 1

18. A passive circuit, usually consisting of capacitanceand/or inductance, that is inserted in series withthe a-c power cord of an electronic device whichallow the 60-Hz current to pass and suppressedhigh frequency noise components.a. Noise filterb. Noise limiterc. Noise floord. Noise quieting

19. A circuit often used in radio receivers that preventsexternally generated noise from exceeding certainamplitude. They are also called noise clippers.a. Noise floorb. Noise filterc. Noise limiterd. Noise clamper

20. Extra-terrestrial noise is observable at frequenciesfrom


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a. 0 to 20 KHzb. 8 Mhz to 1.43 GHzc. 5 to 8 GHzd. 15 to 60 MHz

21. 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 thecarrier amplitude.

22. Modulator circuit performs what mathematicaloperation on its two inputs?a. Additionb. Multiplicationc. Divisiond. Square root

23. If m is greater than 1, what happens?a. Normal operationb. Carrier drops to zeroc. Carrier frequency shiftsd. Information signal is distorted

24. The outline of the peaks of a carrier has the shapeof the modulating signal and is called thea. Traceb. Waveshapec. Enveloped. Carrier variation

25. The values of Vmax and Vmin as read from an AMwave on an oscilloscope are 2.8 and 0.3. Thepercentage of the modulation isa. 10.7 percentb. 41.4 percent

c. 80.6 percentd. 93.3 percent

26. A carrier of 880 kHz is modulated by a 3.5-kHzsine wave. The LSB and USB are respectively,a. 873 and 887kHzb. 876.5 and 883.5 kHzc. 883.5 and 876.5 kHzd. 887 and 873 kHz

27. For 100 percent modulation, what percentage ofcarrier power is in each sideband?a. 25 percent

b. 33.3 percentc. 50 percentd. 100 percent

28. An AM signal has a carrier power of 5W. Thepercentage of modulation is 80 percent. The totalsideband power isa. 0.8 Wb. 1.6 Wc. 2.5 Wd. 4.0 W

29. An AM transmitter antenna current is measured

with no modulation and found to be 2.6 amperes.The percentage of modulation isa. 35 percentb. 70 percentc. 42 percentd. 89 percent

30. The typical audio modulating frequency rangeused in radio and telephone communications isa. 50 Hz to 5 kHzb. 50 Hz to 15 kHz

c. 100 Hz t 10 kHzd. 300 Hz to 3 kHz

31. An AM signal with a maximum modulating signalfrequency of 4.5 kHz as a total bandwidth ofa. 4.5 kHzb. 6.75 kHzc. 9 kHzd. 18 kHz

32. An input signal of 1.8 MHz mixed with a localoscillator of 5 MHz. A filter selects the differencesignal. The output isa. 1.8 MHzb. 3.2 MHzc. 5 MHzd. 6.8 MHz

33. What is produced by over modulation in AM?a. Sidebandsb. Splatterc. Enveloped. Deviation

34. The letter-number designation B8E is a form ofmodulation also known asa. Pilot-carrier systemb. Independent sideband emissionc. Lincompexd. Vestigial sideband transmission

35. The inputs to a balanced modulator are 1 MHzand a carrier of 1.5 MHz. The outputs area. 500 kHzb. 2.5 Mhzc. 1.5 MHz

d. Both a and b

36. A widely used balanced modulator is called thea. Diode bridge circuitb. Full-wave bridge rectifierc. Lattice modulatord. Balanced bridge modulator

37. What is the third character in the emissiondesignation for telephony?a. Fb. Bc. D

d. E

38. A 100-MHz carrier is deviated 50 kHz by a 4-kHzsignala. 5b. 8c. 12.5d. 20

39. The maximum deviation of an FM carrier is 2 kHzby a maximum modulating signal of 400 Hz. Thedeviation ratio isa. 0.2

b. 5c. 8d. 40

40. The phenomenon of a strong FM signaldominating a weaker signal on a commonfrequency is referred to as thea. Capture effectb. Blot outc. Quieting factord. Dominating syndrome


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41. In a low-level AM system, amplifier following themodulated stage must bea. Linear devicesb. Harmonic devicesc. Class C amplifierd. Nonlinear devices

42. If the carrier of a 100 percent modulated AM waveis suppressed, the percentage power saving willbea. 50b. 150c. 100d. 66.66

43. A pre-emphasis circuit provides extra noiseimmunity bya. Boosting the bass frequenciesb. Amplifying the higher audio frequenciesc. Pre-amplifying the whole audio bandd. Converting the phase modulation to FM

44. One of the following transmits only one sidebanda. H3Eb. R3Ec. J3Ed. B8E

45. _____ is an electronic instrument used to showboth the carrier and the sidebands of a modulatedsignal in the frequency domaina. Spectrum analyzerb. Oscilloscopec. Digital counter

d. Frequency counter

46. AM transmission power increases with _____a. Frequencyb. Sourcec. Loadd. Modulation

47. What type of emission is frequency modulation?a. F3Eb. G3Ec. A3Ed. B3E

48. What is the carrier swing of an FM transmitterwhen modulated by 75%?a. 53.2 kHzb. 48 kHzc. 56.25 kHzd. 112.5 kHz

49. An increase in transmitter power from 25 W to 30W will cause the antenna current to increase from700 mA toa. 800 mAb. 750 mA

c. 767 mAd. 840 mA

50. _____ is the function which tends to maintain thesound volume level of a voice receiver nearlyconstant for a large signal strength rangea. Squelchb. Mutingc. AGCd. AFC

51. An FM receiver with an I-F of 10.7 MHz is tuned to98.7 MHz. What is the numerical value of theimage frequency?a. 77.3 MHzb. 88.0 MHzc. 109.4 MHzd. 120.1 MHz

52. The function which tends to silence the receiver inthe absence of transmitted carriera. Squelchb. Mutingc. AGCd. AFC

53. The term used to refer to the condition where thesignals from a very strong station aresuperimposed on other signals being receiveda. Cross-modulation interferenceb. Intermodulation interferencec. Receiver quietingd. Capture effect

54. Stages that are common to both AM and FMreceiversa. Tuner, local oscillator, detector, AF amplifierb. RF amplifier, mixer, IF amplifier, AF

amplifierc. Local oscillator, RF amplifier, frequency

discriminator, detectord. Tuner, IF amplifier, detector, AF amplifier

55. In a narrow-band FM system, the deviation ratio iscommonly one and the highest audio frequency is

generally limited toa. 300 Hzb. 10,000 Hzc. 3,000 Hzd. 7,500 Hz

56. Which of the following contains de-emphasiscircuit?a. FM transmitterb. FM receiverc. VHF transmitterd. VHF receiver

57. The term used to refer to the reduction of receivergain caused by the signal of a nearby stationtransmitter in the same frequency band?a. Quietingb. Cross-modulation interferencec. Squelch gain rollbackd. Desensitizing

58. What is the approximate bandwidth of an FM witha modulation factor of 12.5 and a modulatingfrequency of 10 kHz?a. 20 kHzb. 270 kHz

c. 250 kHzd. 45 kHz

59. An amplifier operating over the frequency range of455 to 460 kHz has a 200 k input resistor. Whatis the RMS noise voltage at the input to thisamplifier if the ambient temperature is 17C?a. 40 Vb. 4.0 Vc. 400 Vd. 4.0 mV


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60. Calculate the noise figure of the amplifier whoseReq equals 2518 (RT = 600 ) if its driven by agenerator whose output impedance is 50a. 39.4b. 3.94c. 394d. 0.394

61. A receiver connected to an antenna whoseresistance is 50 has an equivalent noiseresistance of 30 . Calculate its equivalent noisetemperature if the noise figure is equal to 1.6a. 17.4 Kb. 174 Kc. 1.74 Kd. 17 K

62. Which of the following is not a source of externalnoise?a. Thermal agitationb. Auto ignitionc. The sund. Fluorescent lights

63. What is the noise voltage across a 300 inputresistance to a TV set with a 6 MHz bandwidthand temperature of 30C?a. 2.3 Vb. 3.8 Vc. 5.5 Vd. 6.4 V

64. If bandwidth is doubled, the signal power is _____a. Not changedb. Quadrupled

c. Tripledd. Doubled

65. In an FM transmitter, what is the result of overmodulation?a. Lower frequencyb. Distortionc. Higher powerd. Excessive bandwidth

66. What are the basic elements of communicationssystem?a. Source, transmission channel, transmitter

b. Transmitter, receiver, transmissionchannel

c. Information, transmission channel, receiverd. Sender and receiver

67. A network that has an input of 75 dB and anoutput of 35dB. The loss of the network is _____a. -40 dBb. 40 dBc. -40 dBmd. 40 dBm

68. If an amplifier has equal input and output

impedances, what voltage ratio does the gain of50 dB represent?a. 316.2b. 325.2c. 320.1d. 315.0

69. An interfering signal with a frequency equal to thereceived signal plus twice the IF is calleda. Image frequencyb. Center frequency

c. Rest frequencyd. Interference frequency

70. Subcarriers that are arranged so that the channelsoccupying adjacent frequency band with somefrequency space between them is known asa. Guard bandsb. AM bandsc. Band gap

d. Void band

71. What is the lowest number of sections required bycommunications receiver?a. 1b. 2c. 3d. 4

72. What determines the selectivity of a receiver?a. The bandwidth of the tuned circuitsb. The gain of the amplifierc. The power handling capabilityd. The frequency stability

73. For an IF frequency of 455 kHz, what must be theLO frequency when receiving a 580 kHztransmission?a. 1035 MHzb. 1.035 MHzc. 3.51 MHzd. 351 kHz

74. What circuit accompanies a mixer?a. RF Amplifierb. LOc. IF Amplifier

d. Detector

75. The superheterodyne circuit uses a local oscillatorto _____ with the RF signal of the station andconverts the carrier to the intermediate frequencya. Beatb. Mixc. Heterodyned. All of the above

76. The local FM stereo rock station is at 96.5 MHz.What must be the local oscillator frequency?a. 93.9 MHz

b. 107.2 MHzc. 112.5 MHzd. 105.9 MHz

77. A superheterodyne is tuned to 2738 kHz. The IF is475 kHz. What is the image frequency?a. 3.09 MHzb. 4.5 kHzc. 4385 kHzd. 3688 kHz

78. In an FM receiver, which circuit removesamplitude variations?

a. Exciter b. Mixer c. Discriminatord. Limiter

79. In an FM receiver, the circuit that keeps thereceiver tuned exactly to the desired station is

_____a. AFCb. AGCc. Limiter


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d. Discriminator

80. What connects the front-end circuit of a VHF TVsuperheterodyne receiver?a. Mixer, RF amplifier and AFCb. RF amplifier, Band pass filter and mixerc. Local oscillator, mixer and RF amplifier

d. Local oscillator, AGC and antenna

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