Chapter 2 Problems ECET 214 Prof. Park NJIT

Problem 1 Which of the following is NOT produced when two sine waves are combined through a nonlinear device? a. Components of each of the two original frequencies b. Components at the sum and difference frequencies c. Harmonics of the two original frequencies d. ac level

Problem 2 Which of the following is considered a nonlinear device? a. Resistor b. Capacitor c. Potentiometer d. Transistor

Problem 3 Which of the following modulation techniques is the most economical? a. High-level b. Low-level c. Medium-level d. Ultra low-level

Problem 4 If the % modulation of an AM transmitter is 60% and the unmodulated antenna current is 10 A, what is the modulated current? a. 11 A b. 14 A c. 5 A d. 13 A

Problem 5 If the carrier transmits 12 kW, what is the modulated power from Problem 4? a. 12 kW b. 10 kW c. 14 kW d. 16 kW

Problem 6 The reason modulation is used in electronic communication is: Since all intelligence signals occur at approximately the same frequency, there would be catastrophic interference problems if these frequencies were used. Audio frequency radio waves do not propagate long distances very reliably. Efficient transmission and reception of radio waves are not possible unless extremely large antennas are used. All of the above.

Problem 7 Which of the following is not created by nonlinear mixing? a. The original two frequencies b. Harmonics of the sum and difference frequencies c. The sum and difference of the two original frequencies d. dc (0 Hz)

Problem 8 A 2 kHz sinewave is mixed with a 1.5 MHz carrier sinewave through a nonlinear device. Which frequency is not present in the output signal? a. 3 MHz b. 1.502 MHz c. 3.004 MHz d. 1.498 MHz

Problem 9 A 2.5 MHz carrier is modulated by a music signal that has frequency components ranging from 100 Hz to 5 kHz. What is the range of frequencies generated for the upper sideband? a. 2.5 MHz to 2.505 MHz b. 2.495 MHz to 2.505 MHz c. 2.5001 MHz to 2.505 MHz d. 2.495 MHz to 2.499 MHz

Problem 10 Overmodulation: a. results when the modulation index exceeds unity. b. is undesirable because it produces sideband splatter. c. causes the AM signal to become distorted so that the receiver cannot produce a clean replica of the original intelligence signal. d. all of the above.

Problem 11 In Figure 2-1, A is 220 mVp-p and B is 350 mVp-p. The percent modulation of the AM signal is: a. 62.9% b. 1.59% c. 22.8% d. 4.38%

Problem 12 In Figure 2-1, Ei is 530 mV peak and Ec is 780 mV peak. The percent modulation of the AM signal is: a. 67.9% b. 32.1% c. 19.1% d. 14.7%

Problem 13 In Figure 2-1, A is 1.5Vp-p and B is 3.3Vp-p. If the carrier signal is set at 1.2V peak, the intelligence signal is: a. 3.2 V peak b. 0.55 V peak c. 0.9 V peak d. 0.45 V peak

Problem 14 The total output power of an AM transmitter that is being operated at 50% modulation is measured to be 1800 watts. What is the carrier power? a. 1440 watts b. 1600 watts c. 900 watts d. 2025 watts

Problem 15 A 250W carrier is to be modulated at an 85% modulation level. What is the total transmitted power? a. 340.3 watts b. 183.7 watts c. 430.6 watts d. 356.3 watts

Problem 16 An AM broadcast station operates at its maximum allowed output power of 80W at a percent modulation of 60%. What is the upper sideband power? a. 6.1 watts b. 18.47 watts c. 9.23 watts d. 12.2 watts

Problem 17 The antenna current of an AM transmitter is 5A when it is not modulated. It increases to 6A when it is modulated. Its modulation index expressed as a percentage is: a. 83.3% b. 63.2% c. 69.4% d. 93.8%

Problem 18 An intelligence signal is amplified by a 65% efficient amplifier before being combined with a 250W carrier to generate an AM signal. If it is desired to operate at 100% modulation, what must be the dc input power to the final intelligence signal amplifier? a. 384.6W b. 192.3W c. 162.5W d. 83.3W

Problem 19 High-level modulation is used: a. when the intelligence signal is added to the carrier at the last possible point before the transmitting antenna. b. in high-power applications such as standard radio broadcasting. c. when the transmitter must be made as power efficient as possible. d. all of the above.

Problem 20 The process of neutralization is: a. placing a negative feedback capacitor in an RF amplifier to reduce the tendency for self-oscillation. b. a technique for filtering out all of the undesired frequencies produced by mixing action in a nonlinear amplifier except for the carrier, sum, and difference frequencies. c. the process of adjusting the tank circuit so that the transmitter produces the proper output frequency. d. the process of adjusting the percent modulation to its desired level in a modulator stage.

Problem 21 The transmitter scheme in Figure 2-2 is set up for: a. high-level modulation. b. low-level modulation. c. medium-level modulation.

Problem 22 In Figure 2-2, the audio amplifier and modulator stages: a. are biased Class A or B for low distortion. b. are biased Class A or B for high power efficiency. c. are biased Class C for good mixing action. d. are biased Class C for high power efficiency.

Problem 23 In Figure 2-2, which stages contain "tuned" amplifiers? a. stages b and c b. stages e and f c. stages c and f d. stages b and f

Problem 24 In Figure 2-2, the AM waveform is created in: a. stage b b. stage c c. stage e d. stage f

Problem 25 The transmitter scheme in Figure 2-3 is set up for: a. high-level modulation. b. low-level modulation. c. medium-level modulation.

Problem 26 In Figure 2-3, which stages contain tuned amplifiers? a. stages b, c, and d b. stages b and f c. stages c and f d. stages e and f

Problem 27 In Figure 2-3, the AM waveform is created in: a. stage b. b. stage c. c. stage d. d. stage f.

Problem 28 In Figure 2-3, which stages use linear amplification? a. stages b, c, and d b. stages c, d, and f c. stages b and f d. stages b and c

Problem 29 The O.T.A. is: a. a special type of op amp used to create an AM signal. b. an operational transconductance amplifier. c. a linear integrated circuit that creates AM with an absolute minimum of design considerations. d. all of the above.

Problem 30 The last stage of intelligence amplification before mixing with the carrier occurs in: a. the modulator. b. the modulated amplifier. c. the buffer. d. the RF linear amplifier.

Problem 31 In Figure 2-4a, the trapezoidal display indicates: a. improper bias or low carrier signal power. b. proper in-phase trapezoidal pattern for typical AM signal. c. poor linearity of the modulator. d. lack of an intelligence signal.

Problem 32 In Figure 2-4b, the trapezoidal display indicates: a. improper bias or low carrier signal power. b. proper in-phase trapezoidal pattern for typical AM signal. c. poor linearity of the modulator. d. lack of an intelligence signal.

Problem 33 In Figure 2-4c, the trapezoidal display indicates: a. improper bias or low carrier signal power. b. proper in-phase trapezoidal pattern for typical AM signal. c. poor linearity of the modulator. d. lack of an intelligence signal.

Problem 34 In Figure 2-4d, the trapezoidal display indicates: a. improper bias or low carrier signal power. b. proper in-phase trapezoidal pattern for typical AM signal. c. poor linearity of the modulator d. lack of an intelligence signal.

Problem 35 In Figure 2-5, the carrier frequency is shown as being 50.003 MHz. The frequencies of the spurs on either side of the carrier are: a. approximately 24 kHz away from the carrier. b. 48.0034 MHz and 52.0034 MHz. c. approximately 15 dB below the carrier level. d. approximately 14 dB above the noise floor. e. none of the above.

Problem 36 The equation defining the AM envelope is a. e =(Ec +Ei sin ωit) sin ωit b. e = Ec sin ωct c. e = Ei sin ωit + sin ωct d. e = Ec sin ωct × e = Ei sin ωit e. none of the above

Problem 37 The result of the trigonometric identity (sin x)( sin y) is a. cos(x-y) – cos(x+y) b. –0.5 cos(x-y) + cos(x+y) c. –0.5 cos(x-y) + 0.5cos(x+y) d. 0.5 cos(x-y) – 0.5 cos(x+y) e. none of the above

Problem 38 Determine the side frequency voltage if the modulation index is 70% and the carrier amplitude is 50V. a. 25 b. 17.5 c. 35 d. 50 e. none of the above

Problem 39 Determine the frequency of the AM carrier shown in Figure 2-6. a. 1 MHz b. 10 kHz c. 10 MHz d. none of the above

Problem 40 The typical output impedance for an RF transmitter is a. 75 Ω b. 50 Ω c. 8 Ω d. 16 Ω e. none of the above