MITRC REFRESHER COURSE
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ANSWER KEY
dB Noise, AM & FM
b. c. d.
A portion portion in a communicati communications ons system system,, which which proces processes ses the information so that it will become suitable to the characteristic of the transmission medium a. Encoder b. Modulation c. Transmitter d. Multiplexer
12. Signal attenuati attenuation on can be corrected corrected by a. Filtering b. Modulation c. Equalization d. Ampli mplifi fica cati tion on
Signal whose physical physical quanti quantity ty varies varies continuou continuously sly with with time time a. Digital b. Analog c. Discrete d. Information
13. Signal contami contaminatio nationn by extraneous extraneous or external external sources, sources, such as, other transmitters, power lines and machinery. a. Noise b. Distortion c. Harmonics d. Interference
Digital Digital informat information ion is process processed ed with with a specified specified degree degree of a. Fidelity b. Accuracy c. Sensitivity d. Correctness
14. Which noise noise figure figure represen represents ts the lowest lowest noise? noise? a. 1.5 dB b. 2.0 dB c. 3.7 dB d. 4.1 dB
Indicate Indicate the the voltage voltage level level in dB with with referen reference ce to one one volt. volt. This This unit is used in video or TV measurement a. dBW b. dBk c. dBm d. dBV
15. Denote Denote the interfer interference ence of noise noise in dB above above an adjusted adjusted reference noise. The adjusted reference noise level was a 1 kHz tone, set at – 85 dBm a. dBa b. dBm c. dBa0 d. pWp
When the the power power ratio ratio of the outpu outputt to input input of of a circuit circuit is 200. 200. What What is the gain in dB? a . 23 b . 46 c. – 23 d. – 46
16. A more precise precise evaluatio evaluationn of the quality quality of a receiver receiver as far far as noise is concerned. a. S/N b. VSWR c. Noise fa factor d. Noise ma margin
What is the reference reference level for rando random m noise noise measur measurement ement,, FIA weighted? a. – 82 dBm b. – 90 dBm c. – 85 dBm d. – 77 dBm
17. Noise figure figure for for an amplifier amplifier with with noise noise is always always a. 0 dB b. Infinite c. Less than 1 d. Greater th than 1
A 10 db pad has has an output output level of -3dBm. -3dBm. The The level at the the input input is: a. 13 dBm b. – 7 dBm c. 1 dBm d. 7 dBm
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A power power leve levell of 50 50 μW could could be be expre expresse ssedd as: as: a. 1.69 dBm b. – 4.3 dBm c. 1 dBm d. – 13 dBm
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A system system having having an input input power power of 2 mW an an output output power power of 0.8 0.8 mW has a loss of: a. 2.98 dBm b. 3.98 dB c. 3.98 μW d. 1.98 mW
Aliasing Distortion Interference
18. A passive circuit, circuit, usually usually consistin consistingg of capacitanc capacitancee and/or and/or inductance, that is inserted in series with the a-c power cord of an electronic device which allow the 60-Hz current to pass and suppressed high frequency noise components. a. Noise fifilter b. Noise lim limiter c. Noise flfloor d. Noise quiet ieting 19. A circuit often often used in radio receivers receivers that that prevents prevents externally externally generated noise from exceeding certain amplitude. They are also called noise clippers. a. Noise floor b. Noise fifilter c. Noise limiter d. Noise cla clamper
10. Is the reductio reductionn of signal signal amplitude amplitude as it passed passed over over the transmission medium? a. Noise b. Distortion c. Attenuation d. Interface
20. Extra-terres Extra-terrestrial trial noise noise is observable observable at frequenci frequencies es from a. 0 to 20 KHz b. 8 Mhz Mhz to 1. 1.43 GHz c. 5 to 8 GHz d. 15 to 60 MHz 21. Which of of the followi following ng is not not true true about about AM? a. The The car carrie rierr ampl amplititud udee vari varies es.. b. The carrie carrierr frequ frequenc encyy remai remains ns cons constan tant. t. c. The The carr carrie ierr freq freque uenc ncyy chan change ges. s.
11. Signal wavefor waveform m perturbatio perturbationn or deviation deviation caused caused by imperfect imperfect response of the system to the desired signal a. Noise 1
MITRC REFRESHER COURSE
d.
dB Noise, AM & FM
The information signal amplitude changes the carrier amplitude.
32. An input signal of 1.8 MHz mixed with a local oscillator of 5 MHz. A filter selects the difference signal. The output is a. 1.8 MHz b. 3.2 MHz c. 5 MHz d. 6.8 MHz
22. Modulator circuit performs what mathematical operation on its two inputs? a. Addition b. Multiplication c. Division d. Square root
33. What is produced by over modulation in AM? a. Sidebands b. Splatter c. Envelope d. Deviation
23. If m is greater than 1, what happens? a. Normal operation b. Carrier drops to zero c. Carrier frequency shifts d. Information signal is distorted 24. The outline of the peaks of a carrier has the shape of the modulating signal and is called the a. Trace b. Waveshape c. Envelope d. Carrier variation
34. The letter-number designation B8E is a form of modulation also known as a. Pilot-carrier system b. Independent sideband emission c. Lincompex d. Vestigial sideband transmission 35. The inputs to a balanced modulator are 1 MHz and a carrier of 1.5 MHz. The outputs are a. 500 kHz b. 2.5 Mhz c. 1.5 MHz d. Both a and b
25. The values of Vmax and Vmin as read from an AM wave on an oscilloscope are 2.8 and 0.3. The percentage of the modulation is a. 10.7 percent b. 41.4 percent c. 80.6 percent d. 93.3 percent
36. A widely used balanced modulator is called the a. Diode bridge circuit b. Full-wave bridge rectifier c. Lattice modulator d. Balanced bridge modulator
26. A carrier of 880 kHz is modulated by a 3.5-kHz sine wave. The LSB and USB are respectively, a. 873 and 887kHz b. 876.5 and 883.5 kHz c. 883.5 and 876.5 kHz d. 887 and 873 kHz
37. What is the third character in the emission designation for telephony? a. F b. B c. D d. E
27. For 100 percent modulation, what percentage of carrier power is in each sideband? a. 25 percent b. 33.3 percent c. 50 percent d. 100 percent
38. A 100-MHz carrier is deviated 50 kHz by a 4-kHz signal a. 5 b. 8 c. 12.5 d. 20
28. An AM signal has a carrier power of 5W. The percentage of modulation is 80 percent. The total sideband power is a. 0.8 W b. 1.6 W c. 2.5 W d. 4.0 W
39. The maximum deviation of an FM carrier is 2 kHz by a maximum modulating signal of 400 Hz. The deviation ratio is a. 0.2 b. 5 c. 8 d. 40
29. An AM transmitter antenna current is measured with no modulation and found to be 2.6 amperes. The percentage of modulation is a. 35 percent b. 70 percent c. 42 percent d. 89 percent
40. The phenomenon of a strong FM signal dominating a weaker signal on a common frequency is referred to as the a. Capture effect b. Blot out c. Quieting factor d. Dominating syndrome 41. In a low-level AM system, amplifier following the modulated stage must be a. Linear devices b. Harmonic devices c. Class C amplifier d. Nonlinear devices
30. The typical audio modulating frequency range used in radio and telephone communications is a. 50 Hz to 5 kHz b. 50 Hz to 15 kHz c. 100 Hz t 10 kHz d. 300 Hz to 3 kHz 31. An AM signal with a maximum modulating signal frequency of 4.5 kHz as a total bandwidth of a. 4.5 kHz b. 6.75 kHz c. 9 kHz d. 18 kHz
42. If the carrier of a 100 percent modulated AM wave is suppressed, the percentage power saving will be a. 50 b. 150 c. 100 2
MITRC REFRESHER COURSE
d.
ANSWER KEY
dB Noise, AM & FM
66.66
53. The term used to refer to the condition where the signals from a very strong station are superimposed on other signals being received a. Cross-modulation interference b. Intermodulation interference c. Receiver quieting d. Capture effect
43. A pre-emphasis circuit provides extra noise immunity by a. Boosting the bass frequencies b. Amplifying the higher audio frequencies c. Pre-amplifying the whole audio band d. Converting the phase modulation to FM 44. One of the following transmits only one sideband a. H3E b. R3E c. J3E d. B8E
54. Stages that are common to both AM and FM receivers a. Tuner, local oscillator, detector, AF amplifier b. RF amplifier, mixer, IF amplifier, AF amplifier c. Local oscillator, RF amplifier, frequency discriminator, detector d. Tuner, IF amplifier, detector, AF amplifier
45. _____ is an electronic instrument used to show both the carrier and the sidebands of a modulated signal in the frequency domain a. Spectrum analyzer b. Oscilloscope c. Digital counter d. Frequency counter
55. In a narrow-band FM system, the deviation ratio is commonly one and the highest audio frequency is generally li mited to a. 300 Hz b. 10,000 Hz c. 3,000 Hz d. 7,500 Hz
46. AM transmission power increases with _____ a. Frequency b. Source c. Load d. Modulation
56. Which of the following contains de-emphasis circuit? a. FM transmitter b. FM receiver c. VHF transmitter d. VHF receiver
47. What type of emission is frequency modulation? a. F3E b. G3E c. A3E d. B3E
57. The term used to refer to the reduction of receiver gain caused by the signal of a nearby station transmitter in the same frequency band? a. Quieting b. Cross-modulation interference c. Squelch gain rollback d. Desensitizing
48. What is the carrier swing of an FM transmitter when modulated by 75%? a. 53.2 kHz b. 48 kHz c. 56.25 kHz d. 112.5 kHz
58. What is the approximate bandwidth of an FM with a modulation factor of 12.5 and a modulating frequency of 10 kHz? a. 20 kHz b. 270 kHz c. 250 kHz d. 45 kHz
49. An increase in transmitter power from 25 W to 30 W will cause the antenna current to increase from 700 mA to a. 800 mA b. 750 mA c. 767 mA d. 840 mA
59. An amplifier operating over the frequency range of 455 to 460 kHz has a 200 kΩ input resistor. What is the RMS noise voltage at the input to this amplifier if the ambient temperature is 17°C? a. 40 μV b. 4.0 μV c. 400 μV d. 4.0 mV
50. _____ is the function which tends to maintain the sound volume level of a voice receiver nearly constant for a large signal strength range a. Squelch b. Muting c. AGC d. AFC
60. Calculate the noise figure of the amplifier whose Req equals 2518 Ω (RT = 600 Ω) if it’s driven by a generator whose output impedance is 50Ω a. 39.4 b. 3.94 c. 394 d. 0.394
51. An FM receiver with an I-F of 10.7 MHz is tuned to 98.7 MHz. What is the numerical value of the image frequency? a. 77.3 MHz b. 88.0 MHz c. 109.4 MHz d. 120.1 MHz
61. A receiver connected to an antenna whose resistance is 50 Ω has an equivalent noise resistance of 30 Ω. Calculate its equivalent noise temperature if the noise figure is equal to 1.6 a. 17.4 K b. 174 K c. 1.74 K d. 17 K
52. The function which tends to silence the receiver in the absence of transmitted carrier a. Squelch b. Muting c. AGC d. AFC
62. Which of the following is not a source of external noise? a. Thermal agitation b. Auto ignition c. The sun 3
MITRC REFRESHER COURSE
dB Noise, AM & FM
d. Fluorescent lights
a. b. c. d.
63. What is the noise voltage across a 300 Ω input resistance to a TV set with a 6 MHz bandwidth and temperature of 30°C? a. 2.3 μV b. 3.8 μV c. 5.5 μV d. 6.4 μV
1035 MHz 1.035 MHz 3.51 MHz 351 kHz
74. What circuit accompanies a mixer? a. RF Amplifier b. LO c. IF Amplifier d. Detector
64. If bandwidth is doubled, the signal power is _____ a. Not changed b. Quadrupled c. Tripled d. Doubled
75. The superheterodyne circuit uses a local oscillator to _____ with the RF signal of the station and converts the carrier to the intermediate frequency a. Beat b. Mix c. Heterodyne d. All of the above
65. In an FM transmitter, what is the result of over modulation? a. Lower frequency b. Distortion c. Higher power d. Excessive bandwidth
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 MHz c. 112.5 MHz d. 105.9 MHz
66. What are the basic elements of communications system? a. Source, transmission channel, transmitter b. Transmitter, receiver, transmission channel c. Information, transmission channel, receiver d. Sender and receiver
77. A superheterodyne is tuned to 2738 kHz. The IF is 475 kHz. What is the image frequency? a. 3.09 MHz b. 4.5 kHz c. 4385 kHz d. 3688 kHz
67. A network that has an input of 75 dB and an output of 35dB. The loss of the network is _____ a. -40 dB b. 40 dB c. -40 dBm d. 40 dBm
78. In an FM receiver, which circuit removes amplitude variations? a. Exciter b. Mixer c. Discriminator d. Limiter
68. If an amplifier has equal input and output impedances, what voltage ratio does the gain of 50 dB represent? a. 316.2 b. 325.2 c. 320.1 d. 315.0
79. In an FM receiver, the circuit that keeps the receiver tuned exactly to the desired station is _____ a. AFC b. AGC c. Limiter d. Discriminator
69. An interfering signal with a frequency equal to the received signal plus twice the IF is called a. Image frequency b. Center frequency c. Rest frequency d. Interference frequency
80. What connects the front-end circuit of a VHF TV superheterodyne receiver? a. Mixer, RF amplifier and AFC b. RF amplifier, Band pass filter and mixer c. Local oscillator, mixer and RF amplifier d. Local oscillator, AGC and antenna
70. Subcarriers that are arranged so that the channels occupying adjacent frequency band with some frequency space between them is known as a. Guard bands b. AM bands c. Band gap d. Void band 71. What is the lowest number of sections required by communications receiver? a. 1 b. 2 c. 3 d. 4 72. What determines the selectivity of a receiver? a. The bandwidth of the tuned circuits b. The gain of the amplifier c. The power handling capability d. The frequency stability 73. For an IF frequency of 455 kHz, what must be the LO frequency when receiving a 580 kHz transmission? 4
MITRC REFRESHER COURSE
ANSWER KEY
dB Noise, AM & FM
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