# Group Study - Db Noise Answer Key

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GROUP STUDY - dB Noise

1. 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

2. 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

3. What is the reference level for random noise measurement, FIA weighted? a. – 82 dBm b. – 90 dBm c. – 85 dBm d. – 77 dBm 4. 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

5. A power level of 50 μW could be expressed as:

a. 1.69 dBm b. – 4.3 dBm c. 1 dBm d. – 13 dBm

6. 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

7. Is the reduction of signal amplitude as it passed over the transmission medium? a. Noise b. Distortion c. Attenuation d. Interface

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

a. Noise b. Aliasing c. Distortion d. Interference 9. Signal attenuation can be corrected by

a. Filtering b. Modulation c. Equalization d.

Amplification

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

a. Noise b. Distortion c. Harmonics d. Interference

11. Which noise figure represents the lowest noise?

a. 1.5 dB b. 2.0 dB c. 3.7 dB d. 4.1 dB

12. 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

13. 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

14. Noise figure for an amplifier with noise is always

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15. 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 60-Hz current to pass and suppressed high frequency noise components.

a. Noise filter b. Noise limiter c. Noise floor d. Noise quieting

16. 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 17. 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 18. 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

19. 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

20. 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

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

a. Thermal agitation b. Auto ignition c. The sun d. Fluorescent lights

22. 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

23. If bandwidth is doubled, the signal power is _____

a. Not changed b. Quadrupled c. Tripled d. Doubled

24. 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

25. 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

26. Generally referred to as the primary cause of atmospheric noise a. El Nino phenomenon b. lightningc. rain effect d. thunderstorm 27. Extra terrestrial noise is observable at frequencies from

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a. 5 to 8 GHz B. 0 to 20 kHz c. 1 to 1.43 GHz d. above 2 GHz

28. Noise that is non-continuous, consisting of irregular pulses or noise spikes of short duration with high amplitudes.

a. Shot b. transit time c. impulse d. flicker

29. 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. 754 deg K b. 464 deg K c. 174 deg K d. 293 deg K

30. The noise figure of a totally noiseless device is

a. Unity b. infinity c. zero d. cannot be determined 31. Industrial noise frequency is between

a. 200 and 3000 MHz b. 15 to 160 MHz c. 0 to 10 kHz d. 20 GHz

32. An amplifier operating over a 15 kHz bandwidth has a 500 kohm input resistor. What is the rms noise voltage at the input to this amplifier if the ambient temperature is 23 deg C?

a. 11 uV b. 110 uV c. 0.11 uV d. 1.1 uV

33. It is the interference of a signal from one channel to another channel.

a. Intermodulation distortion b. sideband splatter c. spur d. crosstalk 34. A signal is amplified 100 times in power. The dB gain is

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

35. The random and unpredictable electric signals from natural causes, both internal and external to the system is known as

a. Distortion b. noise c. attenuation d. interference

36. What is the noise power at room temperature when the bandwidth is 1kHz? a. 4 x 10exp-18 w b. 2 x 10exp -19 w c. 3.45 x 10exp -19 w d. 3.45 x

10exp-18w

37. Unit of noise power of psophometer

a. dBa b. pWp c. dBm d. dBm0

38. 25dBrnC F1A weighted equals to

a. -60 dBm b. -63 dBm c. -65 dBm d. -67 dBm

39. If you have available number of power amplifiers with a gain of 100 each, how many such amplifiers do you need to cascade to give an overall gain of 60dB?

a. 2 b. 3 c. 4 d. 5

40. Which noise figure represents the lowest noice? a. 1.6 dB b. 2.1 dB c. 2.7 dB d. 3.4 dB 41. It is also known as burst noise.

a. Johnson b. Popcorn c. Shot d. Mixer 42. What type of noise dominates at lower frequencies?

a. Pink b. Transit Time c. Shot d. Thermal Agitation

43. Two resistors, 5kohms and 20 kohms are at 27degC. Calculate the thermal noise voltage of the series combination for a 10kHz bandwidth.

a. 4.01 uV b. 5 uV c. 2.03 uV d. 3.02 uV

44. Determine the noise current for a diode with a forward bias of 3.5 mA over a 100 kHz bandwidth.

a. 10.58 nA b. 0.335 nA c. 21.2 nA d. 5.6 nA 45. Another name for Johnson Noise

a. Flicker b. White c. IMD d. Shot 46. What is the difference between – 18 and 22 dBm?

a. 40 dB b. 40 dBm c. 4 dB d. 4 dBm

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a. 46 dBm b. 40 dBm c. 80 dBm d. 43 dBm

48. Noise that is very intense and it comes from sources which are points in the sky. a. Flicker b. Industrial c. Thermal Agitation d. Galactic

49. The input power to a loss free cable is 5w. If the reflected power is 7dB down on the incident power, the output power to the load is

a. 4w b. 5w c. 6 w d. 7w

50. How many watts are represented by 40dBw?

a. 40000w b. 1000w c. 40 w d. 10000w

51. One of the following types of noise becomes of great importance at high frequencies. It is the

a. shot noise b. random noise c. impulse noise d. transit-time noise 52. Indicate the false statement.

a. HF mixers are generally noisier than HF amplifiers.

b. Impulse noise voltage is independent of bandwidth.

c. Thermal noise is independent of the frequency at which it is measured. d. Industrial noise is usually of the impulse type.

53. The value of a resistor creating thermal noise is doubleThe noise power generator is therefore

a. halved b. quadrupled c. doubled d. unchanged

54. One of the following is not a useful quantity for comparing the noise performance of receivers:

a. Input noise voltage b. Equivalent noise resistance c. Noise temperature d. Noise figure

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

a. Solar noise b. Cosmic noise c. Atmospheric noise d. Galactic noise

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

a. its resistance b. its temperature

c. Boltzmann’s constant d. the bandwidth over which it is measured 57. Which of broad classifications of noise are most difficult to treat?

a. noise generated in the receiver b. noise generated in the transmitter c. externally generated noise d. internally generated noise 58. Space noise generally covers a wide frequency spectrum, but the strongest

interference occurs

a. between 8 MHz and 1.43 GHz b. below 20 Mhz c. between 20 to 120 MHz d. above 1.5 GHz

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59. When dealing with random noise calculations it must be remembered that a. all calculations are based on peak to peak values

b. calculations are based on peak values c. calculations are based on average values d. calculations are based on RMS values

60. Which of the following is the most reliable measurement for comparing amplifier noise characteristics?

a. signal-to-noise ratio b. noise factor c. shot noise d. thermal noise agitation 61. Which of the following statements is true?

a. Random noise power is inversely proportional to bandwidth b. Flicker is sometimes called demodulation noise

c. Noise is mixers is caused by inadequate image frequency rejection d. A random voltage across a resistance cannot be calculated

62. Voltage gain in decibels is

a. 10 ln (V2/V1) b. 10 log (V2/V1) c. 20 ln (V2/V1) d. 20 log (V2/V1) 63. Which expression indicates a measurement using a 1-W reference

a. dBm(1) b. dB1 c. dBW d. dB(W)

64. An amplifier operating over a 2-Mhz bandwidth has a 75Ω input resistance. If it is operating at 22° C and has a voltage gain of 300, the noise produced at the output of this amplifier would be approximately

a. 128 μV b. 7.33 pV c. 1.56 μV d. 468 μV 65. Which of the following is not an example of external noise

a. Fluorescent light b. Solar emission c. Resistor noise d. Lightning

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

a. 3 dB b. 9 dB c. 10 dB d. 6 dB

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

a. a loss of one third of the power c. a loss of one-half of the power b. a loss of 3 watts of power d. no significant change

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

a. 10 volts/volt b. 100 volts/volt c. 1000 volts/voltd. 10,000 volts/volt

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

a. Bel b. dB c. bel/10 d. dB/2

70. Noise that is produced by the active components within the receiver. a. Thermal b. external c. internal d. white

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

a. Shot b. impulse c. thermal d. dynamic

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

a. 0.01 b. 0.1 c. 10 d. 100

73. 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 & 194K b. 2.23 & 194 K c. 1.67 & 174K d. 2.23 & 174K

74. _____ 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. Industrial b. Johnson c. Flicker d. Mixer 75. The noise figure of a totally noiseless device is

a. Unity b. infinity c. zero d. 100

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

a. Solar b. cosmic c. atmospheric d. galactic

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

a. intermodulation voice b. impulse noise c. dropout d. phase hits

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

a. 50 watts b. 55 watts c. 60 watts d. 62 watts

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

a. shot b. random c. impulse d. transit time

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

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

81. 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 dB b. –2 dB c. 4 dB d. –4dB

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

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

83. The following are characteristics of noise except

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b. present in the channel d. due to any cause

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

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

85. 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. 464K b. 754 K c. 400 K d. 174 K

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

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

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

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

88. 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 nV b. 8.14 V c. 6.6 nV d. 6.6 V

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

a. Halved b. quadrupled c. doubled d. unchanged

90. 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 W d. 1.98 mW

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

a. Noise b. spectrum c. radiation d. absorption 92. The correct symbol for decibel is

a. DB b. dB c. Db d. db

93. _____ is the noise created outside the receiver.

a. Internal b. External c. Shot d. Industrial

94. _____ is the noise created by man.

a. Solar b. Industrial c. Extraterrestial d. Galactic 95. A 10 db pad has an output level of –3 dBm. The level at the input is:

a. 13 dBm b. –7 dBm c. 1 dBm d. 7 dBm

96. 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

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

a. dB b. dBm c. dBw d. dBk

98. In noise analysis, the reference temperature is

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99. Noise from distant planets, stars, galaxies and other celestial objects are called a. Cosmic b. extraterrestrial c. galactic d. black body

100. Indicate which one of the following types of noise does not occur in transistors: a. shot noise b. flicker noise c. partition noise d. resistance noise

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