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:
a. Since all intelligence signals occur at approximately
the same frequency, there would be catastrophic
interference problems if these frequencies were used.
b. Audio frequency radio waves do not propagate long
distances very reliably.
c. Efficient transmission and reception of radio waves
are not possible unless extremely large antennas are
used.
d. 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 3.3Vpp. 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