Comms 1 - Modulation Answers

MITRC REFRESHER COURSE AM & FM 1.

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ANSWER KEY

A portion in a communications system, which processes the information so that it will become suitable to the characteristic of the transmission medium a. Encoder b. Modulation c. Transmitter d. Multiplexer Signal whose physical quantity varies continuously with time a. Digital b. Analog c. Discrete d. Information Digital information is processed with a specified degree of a. Fidelity b. Accuracy c. Sensitivity d. Correctness Indicate the voltage level in dB with reference to one volt. This unit is used in video or TV measurement a. dBW b. dBk c. dBm d. dBV When the power ratio of the output to input of a circuit is 200. What is the gain in dB? a. 23 b. 46 c. – 23 d. – 46 What is the reference level for random noise measurement, FIA weighted? a. – 82 dBm b. – 90 dBm c. – 85 dBm d. – 77 dBm A 10 db pad has an output level of -3dBm. The level at the input is: a. 13 dBm b. – 7 dBm c. 1 dBm d. 7 dBm A power level of 50 μW could be expressed as: a. 1.69 dBm b. – 4.3 dBm c. 1 dBm d. – 13 dBm A system having an input power of 2 mW an output power of 0.8 mW has a loss of: a. 2.98 dBm b. 3.98 dB c. 3.98 μW d. 1.98 mW

10. Is the reduction of signal amplitude as it passed over the transmission medium? a. Noise b. Distortion c. Attenuation d. Interface 11. Signal waveform perturbation or deviation caused by imperfect response of the system to the desired signal a. Noise b. Aliasing c. Distortion d. Interference 12. Signal attenuation can be corrected by a. Filtering b. Modulation c. Equalization d. Amplification 13. Signal contamination by extraneous or external sources, such as, other transmitters, power lines and machinery. a. Noise

dB Noise, b. c. d.

Distortion Harmonics Interference

14. Which noise figure represents the lowest noise? a. 1.5 dB b. 2.0 dB c. 3.7 dB d. 4.1 dB 15. 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. dBa b. dBm c. dBa0 d. pWp 16. A more precise evaluation of the quality of a receiver as far as noise is concerned. a. S/N b. VSWR c. Noise factor d. Noise margin 17. Noise figure for an amplifier with noise is always a. 0 dB b. Infinite c. Less than 1 d. Greater than 1

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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 allow the 60Hz current to pass and suppressed high frequency noise components. a. Noise filter b. Noise limiter c. Noise floor d. Noise quieting

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A circuit often used in radio receivers that prevents externally generated noise from exceeding certain amplitude. They are also called noise clippers. a. Noise floor b. Noise filter c. Noise limiter d. Noise clamper

20. Extra-terrestrial noise is observable at frequencies from a. 0 to 20 KHz b. 8 Mhz to 1.43 GHz c. 5 to 8 GHz d. 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 the carrier amplitude. 22. Modulator circuit performs what mathematical operation on its two inputs? a. Addition b. Multiplication c. Division d. Square root 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

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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

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

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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

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

c. d.

12.5 20

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 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

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

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 d. 66.66

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

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

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

44. One of the following transmits only one sideband a. H3E b. R3E c. J3E d. B8E

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 33. What is produced by over modulation in AM? a. Sidebands b. Splatter c. Envelope d. Deviation 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

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A widely used balanced modulator is called the a. Diode bridge circuit b. Full-wave bridge rectifier c. Lattice modulator d. Balanced bridge modulator

37. What is the third character in the emission designation for telephony? a. F b. B c. D d. E 38. A 100-MHz carrier is deviated 50 kHz by a 4-kHz signal a. 5 b. 8

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 46. AM a. b. c. d.

transmission power increases with _____ Frequency Source Load Modulation

47. What type of emission is frequency modulation? a. F3E b. G3E c. A3E d. B3E 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 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 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

MITRC REFRESHER COURSE AM & FM

ANSWER KEY

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

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The function which tends to silence the receiver in the absence of transmitted carrier a. Squelch b. Muting c. AGC d. AFC

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 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 55. In a narrow-band FM system, the deviation ratio is commonly one and the highest audio frequency is generally limited to a. 300 Hz b. 10,000 Hz c. 3,000 Hz d. 7,500 Hz

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Which of the following contains de-emphasis circuit? a. FM transmitter b. FM receiver c. VHF transmitter d. VHF receiver

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 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

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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 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

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 62. Which of the following is not a source of external noise?

dB Noise, a. b. c. d.

Thermal agitation Auto ignition The sun Fluorescent lights

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 64. If bandwidth is doubled, the signal power is _____ a. Not changed b. Quadrupled c. Tripled d. Doubled 65. In an FM transmitter, what is the result of over modulation? a. Lower frequency b. Distortion c. Higher power d. Excessive bandwidth

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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

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

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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

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 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? a. 1035 MHz b. 1.035 MHz c. 3.51 MHz d. 351 kHz 74. What circuit accompanies a mixer? a. RF Amplifier b. LO c. IF Amplifier d. Detector

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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

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

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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

78. In an FM receiver, which circuit removes amplitude variations? a. Exciter b. Mixer c. Discriminator d. Limiter 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 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