Digital Communications

1. One of the following is not a binary code:

A. Morse

B. Baudot

C. CCITT-2

D. code 39

2. For a selection of 1 out of 16 equiprobable events the number of bits needed will be:

A. 2

B. log(base10)16

C. 16

D. 4

3. It is not a good idea to use a signaling system in which each letter of the alphabet is represented by a different symbol because:

A. It would be too difficult for an operator to memorize

B. It is redundant

C. Noise would introduce too many errors

D. It used in robotics only

4. What is the Hartley’s Law?

A. The maximum rate of information transmission depends on the channel bandwidth

B. The maximum rate of information transmission depends on the depth of modulation

C. The maximum rate of information transmission depends on the S/N of the system

D. Only binary codes may be used

5. Which is not used in order to combat noise?

A. The channel bandwidth may be increased

B. Redundancy may be used

C. The transmitted power may be increased

D. S/N ratio is increase

6. In telegraphy, what is the most commonly used modulation system?

A. frequency-shift keying

B. two-tone modulation

C. pulse-code modulation

D. CDMA

7. A Pulse-width modulation system can be generated

A. By differentiating pulse-position modulation

B. With a monostable

C. By a bistable multivibrator

D. With a free-running multivibrator

8. Which of the following system is digital?

A. Pulse-position modulation

B. Pulse-code modulation

C. Pulse-width modulation

D. Indirect frequency modulation

9. Which of the following system does quantizing noise occur?

A. pulse-amplitude modulation

B. Frequency division multiplex

C. pulse-code modulation

D. pulse-width modulation

10. Which is the most noise-resistant modulation system in the following?

A. pulse-amplitude modulation

B. Frequency modulation

C. pulse-position modulation

D. pulse-code modulation

11. What is done to reduce quantizing noise?

A. Increase the number of standard amplitudes

B. Send pulses whose sides are more nearly vertical

C. Decrease the sampling frequency

D. Increase the number of samples per second

12. A limit on the_______ is set by Hartley-Shannon theorem.

A. Maximum capacity of a channel with a given sampling frequency

B. Maximum capacity of a channel with a given noise level

C. Maximum number of coding levels in a channel with a given noise level

D. Maximum number of quantizing levels in a channel of a given bandwidth

13. Indicate which of the following pulse modulation systems is analog

A. PCM

B. Differential PCM

C. PWM

D. Delta modulation

14. Companding is used to _____ in PCM.

A. To overcome quantizing noise

B. To allow amplitude limiting in the receivers

C. To protect small signals in PCM from quantizing distortion

D. To overcome impulse noise at the receivers

15. What is the biggest disadvantage of PCM?

A. Its inability to handle analog signals

B. The high error rate which its quantizing noise introduces

C. Its noise superiority

D. The large bandwidths that are required for it

16. What is used to connect the output of a data source to a telephone line?

A. an FMVFT system

B. A modem

C. A lease for the line

D. A router

17. What is the most commonly required connection on- line, real-time data transmission system?

A. duplex

B. simplex

C. semiduplex

D. quadruplex

18. Which is not a reason why an organization wishing to transmit large quantities of data will lease a line, instead of using the public switched network?

A. Because it is cheaper

B. To ensure privacy

C. To ensure ready availability

D. To allow equalization and hence higher data rates

19. Find the dynamic range of a PCM system of 16 bits.

A. 96.32 dB

B. 48.16 dB

C. 24.08 dB

D. 12.04 dB

20. To achieve a dynamic range of 78 dB, how many bits will be used?

A. 16

B. 6

C. 10

D. 13

21. The number of bits required for a binary code to represent 305 different possibilities is:

A. 8 bits

B. 9 bits

C. 2 bits

D. 3 bits

22. An alias signal is:

A. A signal that forms in a PCM system that has failed to meet the Nyquist sampling rate

B. A signal that has a frequency equal to the difference between the applied analog frequency and the sampling frequency

C. Usually filtered out by anti-aliasing filters in PCM systems

D. All of the choices

23. A single LSI integrated circuit that contains the A/D and D/A circuitry in a PCM system is known as:

A. A repeater

B. A codec

C. A quantizer

D. A time-division multiplexer

24. The varying amplitude values which are used in order to code an analog intelligence signal in a PCM system are called:

A. Compression levels

B. Quantizing levels

C. Companding levels

D. Sample levels

25. The type of PCM coding scheme that is popular in optical systems, satellite telemetry links, and magnetic recording systems is:

A. NRZ - non-return to zero

B. RZ - return to zero

C. phase-encoded and delay modulation

D. Multilevel binary

26. Which of the following is an application of PCM?

A. Digital audio recording

B. Telephone systems

C. CD laser disks

D. All of the choices

27. In which of the following is the sample signal voltage held constant between samples?

A. flat-top sampling

B. Natural sampling

C. Sample and hold

D. None of the choices

28. The rule that a signal sampled at a minimum of twice the rate of its highest significant frequency component can be fully reconstructed at the receiver with a high degree of accuracy is known as:

A. The error algorithm

B. Barkenhausen criteria

C. Nyquist criteria

D. the Shannon-Hartley theorem

29. The amount of time it takes the hold capacitor in A/D system to reach its final value.

A. Hold time

B. Sample time

C. Aperture time

D. Acquisition time

30. The time that the sample and hold circuit must hold the sampled voltage.

A. Hold time

B. Sample time

C. Aperture time

D. Acquisition time

31. The phenomenon associated with the generation of an erroneously created signal in the sampling process is called:

A. aliasing

B. Fold over distortion

C. Both aliasing and fold over distortion

D. None of the choices

32. Calculate the minimum data rate needed to transmit audio with a sampling rate of 40 kHz and 14 bits per sample.

A. 560 kb/s

B. 422 kb/s

C. 341 kb/s

D. 209 kb/s

33. Determine the number of bits required for a binary code to represent 110 different possibilities.

A. 5 bits

B. 6 bits

C. 7 bits

D. 8 bits

34. Find the efficiency if 7 bits are used to represent 110 different possibilities.

A. 95%

B. 96%

C. 97%

D. 22.5 %

35. A digital transmission has an error probability of 10^-4 and is 10^9 bits long. Calculate the expected number of error bits.

A. 10^5

B. 10^9

C. 10^4

D. 10^-8

Use the statement below to answer the next 2 questions. A digital system sends 100 bits and finds 10 errors on it. The bit rate is 100bps.

Find the bit error ratio.

A. 10 %

B. 12 %

C. 0.01 %

D. 100 %

38. Find the bit error rate.

A. 10 errors/second

B. 1 error/second

C. 0.1 error/second

D. 100 error/second

39. Determine the baud and minimum bandwidth necessary to pass a 10 kbps binary signal using amplitude shift keying.

A. 500 baud

B. 5,000 baud

C. 10,000 baud

D. 1,000,000 baud

A binary FSK signal with a mark frequency of 49 kHz,

a space frequency of 51 kHz, has an input bit rate of

2 kbps.

Find the peak frequency deviation.

A. 1 kHz

B. 2 kHz

C. 3 kHz

D. 4 kHz

41. Determine the minimum bandwidth.

A. 4 kHz

B. 5 kHz

C. 6 kHz

D. 7 kHz

42. Determine the baud.

A. 1000 baud

B. 2000 baud

C. 3000 baud

D. 4000 baud

A BPSK modulator with a carrier frequency of 70 MHz has an input bit rate of 10 Mbps.

Determine the lower and upper side frequencies.

A. 45 to 55 MHz

B. 55 to 65 MHz

C. 65 to 75 MHz

D. 75 to 85 MHz

44. Determine the minimum Nyquist bandwidth.

A. 5 MHz; 5Mbaud

B. 5 MHz; 10Mbaud

C. 10 MHz; 5Mbaud

D. 10 MHz; 10Mbaud

46. For a single-channel PCM system with a sample rate 6000 samples per second and a seven-bit compressed PCM code, determine the line speed:

A. 42 kbps

B. 32 kbps

C. 52 kbps

D. 12 kbps

47. The North American TDMA digital cell phone standard transmits at 24.3 kilobaud using DQPSK. What is the channel data rate?

A. 12.4 kb/s

B. 33.7 kb/s

C. 48.6 kb/s

D. 55.23 kb/s

A modem uses sixteen different phase angles and four different amplitudes. With a bandwidth of 20 kHz.

48. How many bits does it transmit for each symbol?

A. 16

B. 6

C. 64

D. 36

49. Find the bit rate?

A. 60 kbps

B. 120 kbps

C. 3.33 kbps

D. 20 kbps

50. Find the baud.

A. 60 kbaud

B. 120 kbaud

C. 3.33 kbaud

D. 20 kbaud

An 8PSK system is operating with an information bit

rate of 24 kbps.

Determine baud.

A. 4000 baud

B. 4000 baud

C. 8000 baud

D. 8000 baud

53. Determine minimum bandwidth.

A. 4000 Hz

B. 8000 Hz

C. 4000 Hz

D. 8000 Hz

54. Determine bandwidth efficiency.

A. 3 bits per second per hertz

B. 5 bits per second per hertz

C. 7 bits per second per hertz

D. 1 bits per second per hertz

The input voltage of a compander with a maximum voltage range of 1 V and a μ of 255 is 0.25V.

What is the output voltage?

A. 0.75 V

B. 0.57 V

C. 1.02 V

D. 0.96 V

The input voltage of a compander with a maximum voltage range of 1 V and a μ of 255 is 0.8V.

57. What is the output voltage?

A. 0.75 V

B. 0.57 V

C. 1.02 V

D. 0.96 V

An information signal to be transmitted digitally is a rectangular wave with a period of 71.4 μs. It has been determined that the wave will be adequately passed if the bandwidth includes the fourth harmonic.

59. Find the signal frequency.

A. 12 kHz

B. 14 kHz

C. 20 kHz

D. 56 kHz

60. Find the fourth harmonic

A. 12 kHz

B. 14 kHz

C. 20 kHz

D. 56 kHz

61. Find the minimum Nyquist rate.

A. 28 kHz

B. 112 kHz

C. 56 kHz

D. 14 kHz

A digital system has an SNR of 68 dB, a signal bandwidth of 20 MHz, and a sampling rate of 100 MHz.

62. Find the processing gain.

A. 5 dB

B. 2 dB

C. 4 dB

D. 0 dB

63. Find the total signal to noise.

A. 85 dB

B. 72 dB

C. 94 dB

D. 100 dB

The voltage range of an A/D converter that uses 14- bit numbers is -6 to +6 V.

64. Find the number of discrete levels that are represented.

A. 16,384

B. 16,383

C. 10

D. 256

65. Find the number of voltage increments used to divide the total voltage range.

A. 16,384

B. 16,383

C. 10

D. 256

66. Find the resolution of digitization expressed as the smallest voltage increment.

A. 456.5 uV

B. 554.12 uV

C. 732.5 uV

D. 256.12 uV

67. The dynamic range of a 12-bit converter then is:

A. 72.24 dB dB

B. 36.12

C. 45 dB

D. 123 dB

68. Calculate the SINAD for a 12-bit converter.

A. 74 dB

B. 36 dB

C. 45 dB

D. 123 dB

69. Calculate the ENOB for a converter with a SINAD of 78 dB.

A. 6 bits

B. 12 bits

C. 5 bits

D. 3 bits

A subscriber line has a bandwidth of 4 kHz and a signal-to-noise ratio of 20 dB. A signal is transmitted down this line using a 16-level code.

70. Find the data rate at 16 level coding.

A. 26.63 kb/s

B. 18.27 kb/s

C. 24.63 kb/s

D. 32 kb/s

71. Find the data rate at S/N = 20dB.

A. 26.63 kb/s

B. 18.27 kb/s

C. 24.63 kb/s

D. 32 kb/s

72. What is the maximum data rate the line can accommodate?

A. 26.63 kb/s

B. 18.27 kb/s

C. 24.63 kb/s

D. 32 kb/s

Radio channel has a bandwidth of 10 kHz and a signal- to-noise ratio of 15 dB. It uses a code with four possible states?

73. Solve for the data rate using a code with 4 possible states?

A. 40 kbps

B. 20.5 kbps

C. 50.278 kbps

D. 33 kbps

74. Solve for the data rate using S/N = 15 dB?

A. 40 kbps

B. 20.5 kbps

C. 50.278 kbps

D. 33 kbps

75. What is the maximum data rate that can be transmitted?

A. 40 kb/s

B. 50.3 kb/s

C. 70.012 kb/s

D. 34.45 kb/s

For a QPSK system and the given parameters: C=10^- 12W, fb=60 kbps, N=1.2 ×10^-14W and B=120 kHz.

76. Determine the C/N in dB.

A. 19.2 dB

B. 22.2 dB

C. 70 dB

D. -167.8 dB

77. Determine the Eb⁄No in dB.

A. 32.1 dB

B. 22.2 dB

C. 19.2 dB

D. -167.8 dB

Digital Communications

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