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| 001-1. What are the two primary purposes for modulating a signal? |
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Definition
1. (1) Ease of radiation.
(2) Channel allocation. |
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| 001-2. What must we do first to radiate a signal over long distances? |
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Definition
| 2. The signal must first be converted to analog format. |
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| 002-1. Name the part of the AM signal that does not fluctuate in amplitude by the modulating signal. |
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Definition
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| 002-2. What frequencies do the amplitude modulated waveform contains? |
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Definition
| 2. Carrier frequency, carrier plus the modulating frequency (USB), and carrier minus the modulating frequency (LSB). |
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| 002-3. What part of the modulated carrier wave contains the information carrying component? |
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Definition
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| 002-4. In AM, what is the relation between the bandwidth required to transmit the signal and the frequency of the modulating signal? |
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Definition
| 4. Two times the modulating signal frequency. |
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| 002-5. Define “percent of modulation.” |
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Definition
| 5. It refers to the amount of effect or change that the intelligence has on the carrier. |
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| 002-6. Find the % mod if a 4 Vpk-pk RF signal is modulated by a 2.5 Vpk-pk audio signal. |
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Definition
| 6. The percent of modulation is 62.5 percent—found by dividing 2.5 by 4 and multiplying the result by 100. |
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| 002-7. If the signal in figure 1–3C, has an Emax of 50 mVrms and an Emin of 5 mVrms, what is its % mod? |
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Definition
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| 002-8. What are two results of overmodulation? |
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Definition
8. (1) Severe distortion.
(2) Increases bandwidth of an AM signal. |
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| 003-1. What determines the amount of deviation of a FM carrier? |
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Definition
| 1. The amplitude of the modulating signal. |
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| 003-2. How does the rate of deviation relate to the frequency of the modulating signal? |
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Definition
| 2. They are directly proportional. |
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| 003-3. To prevent interferences with other FM stations, who establishes limits on the maximum amount of deviation in FM? |
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Definition
| 3. The FCC establishes the maximum amount of deviation. |
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| 003-4. What is a significant sideband? |
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Definition
| 4. A sideband that contains at least 1 percent of the total transmitted power. |
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| 003-5. How do you determine the modulation index in FM? |
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Definition
| 5. By dividing the amount of frequency deviation by the frequency of the modulating signal. |
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| 003-6. Where do FM sidebands get their power? |
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Definition
| 6. From the unmodulated carrier. |
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| 003-7. What is the relationship between modulation index and sideband power? |
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Definition
| 7. A higher modulation index means more power in the sidebands. It is even possible to have all the power in the sidebands and none in the carrier. At this point, any further increase in modulation would start taking power from the sidebands and placing it back in the carrier resulting in a redistribution of power. |
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| 004-1. In PM, what effect does the change in carrier frequency have? |
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Definition
| 1. None. The frequency change in PM is incidental. |
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| 004-2. Describe the effect the positive and negative alterations of a modulating signal have on the phase of the carrier in PM. |
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Definition
| 2. During the positive alternation of the modulating signal, the phase of the carrier lags behind the unmodulated carrier. During the negative alternation it leads the unmodulated carrier. |
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| 004-3. When is the carrier at its rest frequency in PM? |
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Definition
| 3. During the constant amplitude part of the modulating frequency. |
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| 004-4. What part of the modulating signal controls the amount of phase shift in PM? |
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Definition
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| 004-5. What part of the modulating signal controls the rate of phase shift? |
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Definition
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| 004-6. What is the advantage for using QPSK over BPSK? |
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Definition
| 6. QPSK has faster data rates, doubling the data carrying capability over BPSK. |
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| 004-7. How can higher levels of PSK be achieved? |
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Definition
| 7. By using smaller phase shifts to allow for more phase shifts and increased capacity as each shift represents more bits in combination. |
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| 005-1. Define the term “digitization.” |
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Definition
| 1. The process of converting analog signals into digital signals. |
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| 005-2. Name the four steps of PCM. |
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Definition
2. (1) Band limiting.
(2) Sampling.
(3) Quantizing.
(4) Encoding. |
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| 005-3. State the main purpose of the band-limiting filter. |
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Definition
| 3. It ensures the input to the sampler never exceeds a maximum frequency. |
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Term
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Definition
| 4. Converting a continuous time signal into a discrete time signal. |
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| 005-5. What part of the pulse train is varied using PAM, PWM, and PPM? |
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Definition
| 5. PAM = amplitude; PWM – width or duration; PPM = position. |
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| 005-6. What is the Nyquist sampling rate? |
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Definition
| 6. Taking samples at twice the highest frequency in the bandlimited signal. |
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| 005-7. Which step of PCM assigns discrete amplitude values to the sampled amplitude values? |
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Definition
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| 005-8. Name the two methods of quantization. |
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Definition
8. (1) Uniform quantizing.
(2) Non-uniform quantizing. |
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| 005-9. Which method assigns amplitude values based on an equal amplitude range? |
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Definition
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| 006-1. With ATM, what is the length of each transmission unit? |
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Definition
| 1. One character in length. |
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| 006-2. What part of an ATM transmission tells the receiving device that a character is coming and that the character has been sent? |
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Definition
| 2. The start and stop bits. |
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| 006-3. How does the receiving device determine whether it has received a correct character? |
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Definition
| 3. By summing the 1 bit. If the character arrives with an even number of 1 bit, the device assumes that it has received a correct character. |
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| 006-4. What is an advantage of synchronous transmission over asynchronous transmission? |
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Definition
| 4. To reduce the overhead costs of data transmission. |
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| 006-5. How does synchronous transmission differ from asynchronous transmission? |
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Definition
| 5. Synchronous transmission blocks many characters together for transmission. |
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| 007-1. What does VRC check each incoming character for? |
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Definition
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| 007-2. In LRC, what transmitted character does the receiver use to determine if a transmission was error-free? |
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Definition
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| 007-3. 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? |
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Definition
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| 007-4. How does the CRC method of error detection determine the dividend when computing the BCC? |
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Definition
| 4. CRC treats the binary ones and zeros in the frame address, control, and information fields as one long binary number. |
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| 007-5. How effective is CRC at detecting errors in most applications? |
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Definition
| 5. 99 percent in most applications. |
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| 007-6. How does ARQ work? |
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Definition
| 6. The receiver automatically sends a retransmittal request to the sender if it finds an error in a received frame. |
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| 007-7. Using FEC, at which end of the transmission link are errors corrected? |
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Definition
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| 007-8. Using FEC, what is the transmitter’s function in error correction? |
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Definition
| 8. To transmit multiple copies of the same message to the distant end. |
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| 007-9. Using FEC, what is the receiver’s function in error correction? |
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Definition
| 9. To compare all copies of transmitted message, and then reconstruct the message using the good portions of the message copies received. |
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Term
| 008-1. What are the three basic components that make up a fiber optics link? |
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Definition
1. Fiber optic communication systems require at least three basic components:
(1) An optical transmitter.
(2) A transmission medium.
(3) A detector or optical receiver. |
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Term
| 008-2. Name the three components that make up the optical transmitter? |
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Definition
2. (1) Normally a transmitter is comprised of a driver.
(2) An optical source.
(3) An FO pigtail. |
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Term
| 008-3. What are the three main types of light sources for optic waveguides? |
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Definition
3. (1) LED.
(2) Semiconductor LD.
(3) Non-semiconductor laser diodes. |
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Term
| 008-4. Give the three main requirements of a light source. |
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Definition
4. (1) Operating speed or rise time must be fast enough to meet the application’s BW requirements.
(2) Must provide enough optical power through the fiber to operate the detector.
(3) It must produce a wavelength that takes advantage of the fiber’s long-loss propagation characteristics. |
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| 008-5. What are the advantages of LEDs over lasers? |
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Definition
| 5. Small size, ruggedness, capability of single and direct modulation, reduced expense, and spectral match with both fiber waveguides and silicon photodetectors. |
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| 008-6. What are the two basic designs of LED? |
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Definition
6. (1) The edge emitter.
(2) The surface emitter. |
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Term
| 008-7. How are lasers different from LEDs? |
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Definition
| 7. LEDs differ from lasers in that their light output is incoherent and the laser’s is coherent. |
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Term
| 008-8. How are lasers’ output power measured? |
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Definition
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Term
| 008-9. What are lasers sensitive to? |
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Definition
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| 009-1. What factors limit a detector’s performance? |
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Definition
| 1. Dispersion or attenuation. |
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| 009-2. What two factors control the light signal that’s received by the detector? |
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Definition
2. (1) Noise floor (noise equivalent power).
(2) The SNR. |
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| 009-3. What terms refer to the relationship between the signal and noise? |
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Definition
| 3. SNR and bit error rate. |
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| 009-4. Upon what does a detector’s responsivity depend? |
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Definition
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| 009-5. What are the two main types of photodetectors? |
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Definition
5. (1) PIN diode.
(2) The APD. |
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| 009-6. For what type of application are APDs ideally suited? |
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Definition
| 6. High-speed, long-distance applications. |
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| 010-1. What are the purposes of repeaters in a FOs communications link? |
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Definition
| 1. To extend the distance and to preserve signal integrity. |
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| 010-2. How do regenerators work? |
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Definition
| 2. 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. |
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| 010-3. Which repeater device does not require converting to and from the electrical domain for amplification? |
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Definition
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| 011-1. Describe the differences between a fiber optic patch cord, pigtail, and breakout cable. |
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Definition
| 1. A fiber optic patch cord is a 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 are 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 are similar to pigtails but they are spliced to multifiber cables. |
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011-2. Match the type of fiber optic connector in column B with their characteristics in column A. Items in column B may be used once, more than once, or not at all.
Column A
____ (1) Have a conical shape.
____ (2) Uses push-pull arrangement for mating.
____ (3) Used in high-vibration environments.
____ (4) Most widely used fiber optic connector for local area networks and premise wiring.
____ (5) Uses threaded coupling nut without a keying device.
____ (6) Use quick-release keyed bayonet couplings
____ (7) First connectors used on single-mode fiber optic cable.
Column B
a. SMA connector.
b. Biconic connector.
c. ST connector.
d. FC connector.
e. SC connector. |
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Definition
2.
(1) b.
(2) e.
(3) d.
(4) c.
(5) a.
(6) c.
(7) b. |
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Term
| 011-3. What are “tunable” fiber optic connectors? |
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Definition
| 3. The keying slot in the connector can be rotated to find the optimal alignment. |
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Term
| 011-4. Why are SC fiber optic connectors better for use in high-density applications than other types of connectors? |
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Definition
| 4. 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. |
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