Unit 2. Communications Principles

Question 1

6. Amplitude modulation

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

Answer 1

The carrier.

Question 2

2. What frequencies does the amplitude modulated waveform contain?

Answer 2

Unmodulated carrier frequency, USB, and LSB.

Question 3

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

Answer 3

The sidebands.

Question 4

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

Answer 4

The bandwidth required is two times the modulating signal frequency.

Question 5

5. Define “percent of modulation.”

Answer 5

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

Question 6

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

Answer 6

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

Question 7

7. If the signal in figure 2–3, C, has an Emax of 50 mVrms and an Emin of 5 mVrms, what is its %mod?

Answer 7

81.8 percent.

Question 8

8. What are two results of over modulation?

Answer 8

Severe distortion and increases bandwidth of an AM signal.

Question 9

7. Frequency modulation

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

Answer 9

They are directly proportional.

Question 10

2. What determines the amount of deviation of a FM carrier?

Answer 10

The amplitude of the modulating signal.

Question 11

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

Answer 11

The FCC establishes the maximum amount of deviation.

Question 12

4. What is a significant sideband?

Answer 12

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

Question 13

5. How do you determine the modulation index in FM?

Answer 13

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

Question 14

6. Where do FM sidebands get their power?

Answer 14

From the unmodulated carrier.

Question 15

7. What is the relationship between modulation index and sideband power?

Answer 15

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.

Question 16

8. Phase modulation

1. In PM, what effect does the change in carrier frequency have on modulated waveshape?

Answer 16

None. The frequency change in PM is incidental.

Question 17

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

Answer 17

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.

Question 18

3. When is the carrier at its rest frequency in PM?

Answer 18

During the constant amplitude part of the modulating frequency.

Question 19

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

Answer 19

Amplitude.

Question 20

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

Answer 20

Frequency.

Question 21

2–2. Digital Transmission Techniques

9. Pulse code modulation
10. Name the four steps of PCM.

Answer 21

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

Question 22

2. State the main purpose of the band-limiting filter.

Answer 22

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

Question 23

3. Define sampling.

Answer 23

Converting a continuous time signal into a discrete time signal.

Question 24

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

Answer 24

PAM = amplitude, PWM = width or duration, and PPM = position.

Question 25

010. Asynchronous and synchronous transmission | 1. With ATM, what is the length of each transmission unit?

Answer 25

One character in length.

Question 26

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

Answer 26

The start and stop bits.

Question 27

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

Answer 27

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.

Question 28

4. How do networks reduce the overhead cost of data transmission?

Answer 28

By sending messages using synchronous transmissions.

Question 29

5. How does synchronous transmission differ from asynchronous transmission?

Answer 29

Synchronous transmission blocks many characters together for transmission.

Question 30

011. Error detection and correction | 1. What does VRC check for on each incoming character?

Answer 30

Odd or even parity.

Question 31

2. In LRC, what transmitted character does the receiver use to determine if a transmission was error- free?

Answer 31

BCC.

Question 32

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?

Answer 32

255.

Question 33

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

Answer 33

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

Question 34

5. How effective is CRC at detecting errors in most applications?

Answer 34

99 percent in most applications.

Question 35

6. How does ARQ work?

Answer 35

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

Question 36

7. Using forward error control, at which end of the transmission link are errors corrected?

Answer 36

Receiving end.

Question 37

8. Using forward error control, what is the transmitter’s function in error correction?

Answer 37

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

Question 38

9. Using forward error control, what is the receiver’s function in error correction?

Answer 38

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