# Volume 1 Unit 1

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1. What are the two primary purposes for modulating a signal?
Ease of radiation and channel allocation.
2. What must we do first to radiate a signal over long distances?
Modulate and change to analog format.
3. Name the part of the AM signal that does not fluctuate in amplitude by the modulating signal.
Carrier.
4. What frequencies does the amplitude modulated waveform contain?
USB, Carrier, LSB
5. What part of the modulated carrier wave contains the information carrying component?
Sidebands.
6. In AM, what is the relation between the bandwidth required to transmit the signal and the frequency of the modulating signal?
Bandwidth required = 2X Modulating signal frequency.
7. Define "Percent of modulation"
• It refers to the amount of effect or change that the intelligence has on the carrier.
8. Find the %MOD if a 4Vpk-pk RF signal is modulated by a 2.5Vpk-pk audio signal.
9. If the signal has an Emax of 50mVrms and an Emin of 5mVrms, what is its %MOD?
10. What are two results of overmodulation?
Severe distortion and increases bandwidth of an AM signal.
11. What determines the amount of deviation of a FM carrier?
The amplitude of the modulated signal.
12. How does the rate of deviation relate to the frequency of the modulating signal?
They are directly proportional.
13. To prevent interferences with other FM stations, who establishes limits on the maximum amount of deviation in FM?
FCC Federal Communications Commission
14. What is a significant sideband?
Sidebands containing at least 1% of the total transmitted power.
15. How do you determine the modulation index in FM?
By dividing the amount of frequency deviation by the frequency of the modulating signal.
16. Where do FM sidebands get their power?
From the unmodulated carrier.
17. what is the relationship between modulation index and sideband power?
A higher modulation index means more power in the sidebands.
18. In PM, what effect does the change in carrier frequency have?
NONE. The frequency change in PM is incidental.
19. Describe the effect the positive and negative alternations of a modulating signal have on the phase of the carrier in PM.
Positive alternation- Lags behind unmodulated carrier.

20. When is the carrier at its rest frequency in PM?
During the constant amplitude part of the modulating frequency.
21. What part of the modulating signal controls the amount of phase shift in PM?
Amplitude.
22. What part of the modulating signal controls the rate of phase shift?
Frequency.
23. What is the advantage for using QPSK over BPSK?
Faster data rates, doubling the data carrying capability over BPSK.
24. How can higher levels of PSK be achieved?
Smaller phase shifts to allow for more phase shifts and increased capacity as each shift represents more bits in combination.
25. Define the term digitization.
The process of converting an analog signal into a digital signal.
26. Name the four steps of PCM.
• Band limiting
• Sampling
• Quantizing
• Encoding
27. State the main purpose of the band-limiting filter.
To ensure the input to the sampler never exceeds the max frequency.
28. Define sampling.
Converting a continuous time signal into a discrete time signal.
29. What part of the pulse train is varied using PAM,PWM, and PPM?
• PAM- Amplitude
• PWM- Width/Duration
• PPM- Position
30. What s the Nyquist sampling rate?
Taking samples at twice the highest frequency in the bandlimited signal.
31. Which step of PCM assigns discrete amplitude values to the sampled amplitude values?
Quantization.
32. Name two methods of quantization.
• Uniform
• Non-Uniform
33. Which method of quantization assigns amplitude values based on an equal amplitude range?
Uniform.
34. With ATM, what is the length of each transmission unit?
One character.
35. What part of an ATM transmission tells the receiving device that a character is coming and that the character has sent?
Start and stop bits.
36. How does the receiving device determine whether it has received a correct character?
By summing the 1 bits.  If the character arrives with an even number of 1 bits, the device assumes that it has received a correct character.
37. What is an advantage of synchronous transmission over asynchronous transmission?
• Reduce the overhead cost of data transmission.
• Not prone to distortion.
38. How does synchronous transmission differ from asynchronous transmission?
Synchronous blocks many characters together for transmission.
39. What does VRC check each incoming character for?
Odd or even parity.
40. In LRC, what transmission character does the receiver use to determine if a transmission was error-free?
BCC Block Character Count
41. When using the checksum method of error detection, what binary number is used to divide the sum of all the characters in order to derive the checksum?
255
42. How does the CRC method of error detection determine the dividend when computing the BCC?
The CRC treats the binary ones and zeros in the frame address, control, and information fields as a single long binary number.
43. How effective is CRC at detecting errors in most applications?
99%
44. How does ARQ work?
The receiver automatically sends a re-transmittal request to the sender if it finds an error in a receiving frame.
45. Using forward error control, at which end of the transmission link are errors corrected?
Receiving end.
46. Using forward error control, what is the transmitter's function in error detection?
To send multiple copies of the same message to the distant end.
47. Using forward error control, what is the receiver's function in error detection?
To compare all the copies and then reconstruct a message by using the good portions of the message copies received.
48. What are the three basic components that make up a fiber optics link?
• Optical transmitter
• Transmission medium
49. Name the three components that make up the optical transmitter?
• Driver
• Light source
• FO Pigtail
50. What are the three main types of light sources for optic waveguides?
• LED
• Semiconductor laser diodes LD
• Non-semiconductor laser diodes LD
51. Give the three main requirements of a light source.
• Operating speed or rise time must be fast enough to meet the application's bandwidth requirements.
• Must provide enough optical power through the fiber to operate the detector.
• It must produce a wavelength that takes advantage of the fiber's long-loss propagation characteristics.
52. What are the advantages of LEDs over lasers?
Small size, ruggedness, capability of single and direct modulation, reduced expense, and spectral match with both fiber waveguides and silicone photodetectors.
53. What are the two basic designs of LED?
• Edge emitter
• Surface emitter
54. How are lasers different from LEDs?
• LED's light output is incoherent
• Laser's light output is coherent
55. How are lasers' output power measured?
Milliwatt
56. What are lasers sensitive to?
Temperature.
57. What factors limit a detector's performance?
Dispersion or attenuation.
58. What two factors control the light signal that's received by the detector?
• Noise floor (Noise equivalent power)
• Signal-to-noise ratio SNR
59. What terms refer to the relationship between the signal and noise?
• Signal-to-noise ratio SNR
• Bit error rate BER
60. Upon what does a detector's responsivity depend?
Wavelength of light.
61. What are the two main types of photodetectors?
Positive intrinsic negative (PIN) diode and the avalanche photodiode (APD)
62. For what type of application are APDs ideally suited?
High-speed, long-distance applications.
63. What are the purposes of repeaters in a fiber optics communications link?
To extend the distance and to preserve signal integrity.
64. How do regenerators work?
Regenerators receive incoming low-level, dispersed (distorted) digital pulses.  After conversion to the electrical domain, the pulses are regenerated (the leading and falling edges are sharpened and the pulses are amplified).  Finally the regenerated pulses are retransmitted optically.
65. Which repeater device does not require converting to and from the electrical domain for signal amplification?
Optical amplifier.
66. Describe the differences between a fiber optic patch cord, pigtail and breakout cable.
• Fiber optic patch cord- short section of single fiber cable with a connector on each end.  Each end is connected to a patch panel to connect circuits together.
• Pigtails- similar to patch cords but they only have a connector on one end- the other end is spliced to the fiber optic cable.
• Breakout cables- similar to pigtails but they are spliced to multifiber cables.
67. _____ connectors have conical shape.
Biconic
68. _____ connectors use push-pull arrangement for mating.
SC Subscriber Connector
69. _____ connectors are used in high-vibration environments.
FC Field Connector
70. _____ connectors are the most widely used fiber optic connectors for local area networks and premise wiring.
ST Straight Tip
71. The _____ connector uses threaded coupling nut without a keying device.
SMA Sub-Miniature, Type A
72. _____ connector uses quick-release keyed bayonet couplings.
ST Straight Tip
73. The _____ connectors were the first connectors used on single-mode fiber optic cable.
Biconic
74. What are "tunable" fiber optic connectors?
It means the keying slot in the connector can be rotated to find the optimal alignment.
75. Why are SC fiber optic connectors better for use in high-density applications than other types of connectors?
The other types of connectors require room to twist them for alignment.  SC connectors use a push-pull arrangement and do not need this extra room.
76. What is one reason for modulation that involves modulating low frequency signals for transmission over long distances?
a. Spectrum conservation
b. Channel allocation
d. Companding
77. What are the three general categories used to produce modulation in RF transmission today?
• Amplitude
• Frequency
• Phase
78. When the modulating signal and carrier signal are combined within a modulator, the output signal contains
• USB
• Carrier
• LSB
79. If a carrier frequency of 1MHz and a modulating tone of 10kHz goes to the modulator, the output signal will include
1.01MHz, 1MHz, and 0.99MHz
80. The bandwidth of an amplitude modulated signal is
two times the modulating signal.
81. If an oscillloscope displays a modulation envelope having an Emax of 200 volts and an Emin of 20 volts, what is the modulation percentage?
82. Which statement concerning bandwidth is true?
a. Overmodulating increases bandwidth because the distortion produces harmonics.
b.Undermodulating increases bandwidth because the distortion produces harmonics.
c. Overmodulating increases bandwidth because the output's increased amplitude.
d. Undermodulating increases bandwidth because the output's decreased amplitude.
a. Overmodulating increases bandwidth because the distortion produces harmonics.
83. The output of the oscillator in a FM modulator increases in frequency with each
positive half cycle of the carrier.
84. In FM, a significant sideband contains at least what percentage of the total transmitted power?
1%
85. What is the formula to find the modulating index?
86. A 5 kHz modulating signal has enough peak voltage to cause a deviation of 15 kHz.  What is the modulation index?
3
87. What is it called when a narrow frequency band between adjacent stations prevent frequency modulated sidebands from overlapping?
Guard band
88. In PM, what part of the carrier signal is varied?
Phase
89. In phase modulation, the carrier's
a. phase is shifted at the rate of the modulating signal.
b. phase shifted with the phase of the modulating signal.
c. amplitude is shifted at the rate of the modulating signal.
d. amplitude is shifted with the phase of the modulating signal.
a. phase is shifted at the rate of the modulating signal.
90. What is the first step in the pulse code modulation process?
The analog signal is band limited.
91. In asynchronous transmissions, what bit is used by the receiving device to verify that the transmission was received correctly?
Parity
92. If an error should occur, what transmission is lost in a synchronous transmission?
a. One character
b. Block of data
c. A parity bit
d. A stop bit
b. Block of data
93. When using vertical redundancy check, what significance does the amount of ones have in a data bit pattern?
Determines parity
94. What error detection method is used for checking a data block greater than 512 and is about 99% effective in most applications?
Cyclic Redundancy Check  CRC
95. Light sources that are applicable to fiber optic waveguide are light-emitting diodes and
Semiconductor laser diodes
96. What type of light is emitted from a laser?
Coherent
97. What photo detector converts one photon to one electron?
Positive intrinsic negative diode
98. Which repeater amplifies optical signal without converting to and from the electrical domain?
Optical amplifiers
99. A short section of single fiber cable that has a connector at each end is called a
patch cord
100. Which fiber-optic connector uses quick-release, keyed bayonet couplings that are preferred in situations where severe vibrations are not expected?
ST Straight Tip
 Author: nachotrull ID: 283034 Card Set: Volume 1 Unit 1 Updated: 2014-09-14 22:45:24 Tags: study Folders: Description: Self Test and Unit Review Show Answers: