3D1X3 Self Test Questions Vol. 1 Unit 1

Front 1

What are the two primary purposes for modulating a signal?

Back 1

(1) Ease of radiation.
(2) Channel allocation.

Front 2

What must we do first to radiate a signal over long distances?

Back 2

The signal must first be converted to analog format.

Front 3

Name the part of the AM signal that does not fluctuate in amplitude by the modulating signal.

Back 3

The carrier.

Front 4

What frequencies do the amplitude modulated waveform contains?

Back 4

Carrier frequency, carrier plus the modulating frequency (USB), and carrier minus the modulating
frequency (LSB).

Front 5

What part of the modulated carrier wave contains the information carrying component?

Back 5

The sidebands

Front 6

In AM, what is the relation between the bandwidth required to transmit the signal and the
frequency of the modulating signal?

Back 6

Two times the modulating signal frequency.

Front 7

Define “percent of modulation.”

Back 7

It refers to the amount of effect or change that the intelligence has on the carrier.

Front 8

Find the % mod if a 4 Vpk-pk RF signal is modulated by a 2.5 Vpk-pk audio signal.

Back 8

The percent of modulation is 62.5 percent—found by dividing 2.5 by 4 and multiplying the result by 100.

Front 9

If the signal in figure 1–3C, has an Emax of 50 mVrms and an Emin of 5 mVrms, what is its %
mod?

Back 9

81.8 percent

Front 10

What are two results of overmodulation?

Back 10

(1) Severe distortion.
(2) Increases bandwidth of an AM signal.

Front 11

What determines the amount of deviation of a FM carrier?

Back 11

The amplitude of the modulating signal.

Front 12

How does the rate of deviation relate to the frequency of the modulating signal?

Back 12

They are directly proportional.

Front 13

To prevent interferences with other FM stations, who establishes limits on the maximum amount
of deviation in FM?

Back 13

The FCC establishes the maximum amount of deviation.

Front 14

What is a significant sideband?

Back 14

A sideband that contains at least 1 percent of the total transmitted power.

Front 15

How do you determine the modulation index in FM?

Back 15

By dividing the amount of frequency deviation by the frequency of the modulating signal.

Front 16

Where do FM sidebands get their power?

Back 16

From the unmodulated carrier.

Front 17

What is the relationship between modulation index and sideband power?

Back 17

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.

Front 18

In PM, what effect does the change in carrier frequency have?

Back 18

None. The frequency change in PM is incidental.

Front 19

Describe the effect the positive and negative alterations of a modulating signal have on the phase
of the carrier in PM.

Back 19

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.

Front 20

When is the carrier at its rest frequency in PM?

Back 20

During the constant amplitude part of the modulating frequency.

Front 21

What part of the modulating signal controls the amount of phase shift in PM?

Back 21

Amplitude.

Front 22

What part of the modulating signal controls the rate of phase shift?

Back 22

Frequency.

Front 23

What is the advantage for using QPSK over BPSK?

Back 23

QPSK has faster data rates, doubling the data carrying capability over BPSK.

Front 24

How can higher levels of PSK be achieved?

Back 24

By using smaller phase shifts to allow for more phase shifts and increased capacity as each shift represents
more bits in combination.

Front 25

Define the term “digitization.”

Back 25

The process of converting analog signals into digital signals.

Front 26

Name the four steps of PCM.

Back 26

(1) Band limiting.
(2) Sampling.
(3) Quantizing.
(4) Encoding.

Front 27

State the main purpose of the band-limiting filter.

Back 27

It ensures the input to the sampler never exceeds a maximum frequency.

Front 28

Define sampling.

Back 28

Converting a continuous time signal into a discrete time signal.

Front 29

What part of the pulse train is varied using PAM, PWM, and PPM?

Back 29

PAM = amplitude; PWM – width or duration; PPM = position.

Front 30

What is the Nyquist sampling rate?

Back 30

Taking samples at twice the highest frequency in the bandlimited signal.

Front 31

Which step of PCM assigns discrete amplitude values to the sampled amplitude values?

Back 31

Quantization.

Front 32

Name the two methods of quantization.

Back 32

(1) Uniform quantizing.
(2) Non-uniform quantizing.

Front 33

Which method assigns amplitude values based on an equal amplitude range?

Back 33

Uniform quantizing.

Front 34

With ATM, what is the length of each transmission unit?

Back 34

One character in length.

Front 35

What part of an ATM transmission tells the receiving device that a character is coming and that
the character has been sent?

Back 35

The start and stop bits.

Front 36

How does the receiving device determine whether it has received a correct character?

Back 36

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.

Front 37

What is an advantage of synchronous transmission over asynchronous transmission?

Back 37

To reduce the overhead costs of data transmission.

Front 38

How does synchronous transmission differ from asynchronous transmission?

Back 38

Synchronous transmission blocks many characters together for transmission.

Front 39

What does VRC check each incoming character for?

Back 39

Odd or even parity.

Front 40

In LRC, what transmitted character does the receiver use to determine if a transmission was errorfree?

Back 40

BCC.

Front 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?

Back 41

255.

Front 42

How does the CRC method of error detection determine the dividend when computing the BCC?

Back 42

CRC treats the binary ones and zeros in the frame address, control, and information fields as one long
binary number.

Front 43

How effective is CRC at detecting errors in most applications?

Back 43

99 percent in most applications.

Front 44

How does ARQ work?

Back 44

The receiver automatically sends a retransmittal request to the sender if it finds an error in a received frame.

Front 45

Using FEC, at which end of the transmission link are errors corrected?

Back 45

Receiving end.

Front 46

Using FEC, what is the transmitter’s function in error correction?

Back 46

To transmit multiple copies of the same message to the distant end.

Front 47

Using FEC, what is the receiver’s function in error correction?

Back 47

To compare all copies of transmitted message, and then reconstruct the message using the good portions of
the message copies received.

Front 48

What are the three basic components that make up a fiber optics link?

Back 48

Fiber optic communication systems require at least three basic components:
(1) An optical transmitter.
(2) A transmission medium.
(3) A detector or optical receiver.

Front 49

Name the three components that make up the optical transmitter?

Back 49

(1) Normally a transmitter is comprised of a driver.
(2) An optical source.
(3) An FO pigtail.

Front 50

What are the three main types of light sources for optic waveguides?

Back 50

(1) LED.
(2) Semiconductor LD.
(3) Non-semiconductor laser diodes.

Front 51

Give the three main requirements of a light source.

Back 51

(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.

Front 52

What are the advantages of LEDs over lasers?

Back 52

Small size, ruggedness, capability of single and direct modulation, reduced expense, and spectral match
with both fiber waveguides and silicon photodetectors.

Front 53

What are the two basic designs of LED?

Back 53

(1) The edgeemitter.
(2) The surface emitter.

Front 54

How are lasers different from LEDs?

Back 54

LEDs differ from lasers in that their light output is incoherent and the laser’s is coherent.

Front 55

How are lasers’ output power measured?

Back 55

Milliwatt.

Front 56

What are lasers sensitive to?

Back 56

Temperature.

Front 57

What factors limit a detector’s performance?

Back 57

Dispersion or attenuation.

Front 58

What two factors control the light signal that’s received by the detector?

Back 58

(1) Noise floor (noise equivalent power).
(2) The SNR.

Front 59

What terms refer to the relationship between the signal and noise?

Back 59

SNR and bit error rate.

Front 60

Upon what does a detector’s responsivity depend?

Back 60

Wavelength of light.

Front 61

What are the two main types of photodetectors?

Back 61

(1) PIN diode.
(2) The APD.

Front 62

For what type of application are APDs ideally suited?

Back 62

High-speed, long-distance applications.

Front 63

What are the purposes of repeaters in a FOs communications link?

Back 63

To extend the distance and to preserve signal integrity.

Front 64

How do regenerators work?

Back 64

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.

Front 65

Which repeater device does not require converting to and from the electrical domain for
amplification?

Back 65

Optical amplifier.

Front 66

Describe the differences between a fiber optic patch cord, pigtail, and breakout cable.

Back 66

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.

Front 67

What are “tunable” fiber optic connectors?

Back 67

The keying slot in the connector can be rotated to find the optimal alignment.

Front 68

Why are SC fiber optic connectors better for use in high-density applications than other types of
connectors?

Back 68

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.