Frenzel Chapter 4: Frequency Modulation
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Questions and Answers
- 1.
The amount of frequency deviation from the carrier center frequency in an FM transmitter is proportional to what characteristic of the modulating signal?
- A.
Amplitude
- B.
Frequency
- C.
Phase
- D.
Shape
Correct Answer
A. AmplitudeExplanation
In an FM transmitter, the amount of frequency deviation from the carrier center frequency is proportional to the amplitude of the modulating signal. This means that as the amplitude of the modulating signal increases, the frequency deviation also increases. The modulation index, which represents the ratio of the amplitude of the modulating signal to the frequency deviation, determines the extent of frequency modulation. Therefore, the amplitude of the modulating signal directly affects the frequency deviation in an FM transmitter.Rate this question:
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- 2.
Both FM and PM are types of what kind of modulation?
- A.
Amplitude
- B.
Phase
- C.
Angle
- D.
Duty Cycle
Correct Answer
C. AngleExplanation
FM (Frequency Modulation) and PM (Phase Modulation) are both types of angle modulation. Angle modulation refers to the modulation techniques where the angle of the carrier wave is varied in accordance with the modulating signal. In FM, the frequency of the carrier wave is varied, while in PM, the phase of the carrier wave is varied. Therefore, both FM and PM fall under the category of angle modulation.Rate this question:
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- 3.
If the amplitude of the modulating signal decreases, the carrier deviation
- A.
Increases
- B.
Decreases
- C.
Remains constant
- D.
Goes to zero
Correct Answer
B. DecreasesExplanation
When the amplitude of the modulating signal decreases, it means that the signal is not varying as much in intensity. This results in a smaller range of variations in the carrier signal's frequency. As a result, the carrier deviation, which is the difference between the carrier signal's frequency and its unmodulated frequency, decreases. Therefore, the correct answer is "Decreases".Rate this question:
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- 4.
On an FM signal, maximum deviation occurs at what point on the modulating signal?
- A.
Zero crossing points
- B.
Peak positive amplitude
- C.
Peak negative amplitude
- D.
Both B and C
Correct Answer
D. Both B and CExplanation
On an FM signal, maximum deviation occurs at both the peak positive and peak negative amplitudes of the modulating signal. This is because frequency modulation involves varying the frequency of the carrier signal based on the amplitude of the modulating signal. The maximum deviation in frequency occurs when the modulating signal reaches its peak positive or peak negative amplitude, resulting in the maximum change in the carrier signal's frequency.Rate this question:
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- 5.
In PM, a frequency shift occurs while what characteristic of the modulating signal is changing?
- A.
Shape
- B.
Phase
- C.
Frequency
- D.
Amplitude
Correct Answer
D. AmplitudeExplanation
In PM (Phase Modulation), the frequency of the carrier signal remains constant. However, the phase of the carrier signal is shifted in proportion to the amplitude of the modulating signal. Therefore, when the amplitude of the modulating signal changes, it causes a frequency shift in the PM signal.Rate this question:
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- 6.
Maximum frequency deviation of a PM signal occurs at
- A.
Zero crossing points
- B.
Peak positive amplitude
- C.
Peak negative amplitude
- D.
Peak positive or negative amplitude
Correct Answer
A. Zero crossing pointsExplanation
The maximum frequency deviation of a PM (Phase Modulation) signal occurs at the zero crossing points. This is because at these points, the phase of the signal is changing rapidly, resulting in a larger frequency deviation. At the peak positive or negative amplitude, the phase is not changing as quickly, leading to a smaller frequency deviation. Therefore, the zero crossing points are where the maximum frequency deviation occurs in a PM signal.Rate this question:
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- 7.
In PM, carrier frequency deviation is not proportional to
- A.
Modulating signal amplitude
- B.
Carrier amplitude and frequency
- C.
Modulating signal frequency
- D.
Modulator phase shift
Correct Answer
B. Carrier amplitude and frequencyExplanation
In PM (Phase Modulation), the carrier frequency deviation is not proportional to the carrier amplitude and frequency. In PM, the phase of the carrier signal is varied in proportion to the modulating signal. The amplitude and frequency of the carrier signal remain constant. Therefore, any changes in the carrier amplitude and frequency do not affect the carrier frequency deviation in PM.Rate this question:
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- 8.
To compensate for increases in carrier frequency deviation with an increase in modulating signal frequency, what circuit is used between the modulating signal and the phase modulator?
- A.
Low pass filter
- B.
High pass filter
- C.
Phase shifter
- D.
Bandpass filter
Correct Answer
A. Low pass filterExplanation
A low pass filter is used between the modulating signal and the phase modulator to compensate for increases in carrier frequency deviation with an increase in modulating signal frequency. A low pass filter allows low-frequency signals to pass through while attenuating high-frequency signals. By using a low pass filter, the high-frequency components of the modulating signal that can cause carrier frequency deviation are filtered out, ensuring that the modulation remains within the desired range.Rate this question:
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- 9.
The FM produced by PM is called
- A.
FM
- B.
PM
- C.
Indirect FM
- D.
Indirect PM
Correct Answer
C. Indirect FMExplanation
When frequency modulation (FM) is produced by phase modulation (PM), it is referred to as indirect FM. In indirect FM, the frequency of the carrier signal is varied based on the instantaneous phase of the modulating signal. This modulation technique is commonly used in telecommunications and broadcasting systems to transmit audio signals.Rate this question:
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- 10.
If the amplitude of the modulating signal applied to a phase modulator is constant, the output signal will be
- A.
Zero
- B.
The carrier frequency
- C.
Above the carrier frequency
- D.
Below the carrier frequency
Correct Answer
B. The carrier frequencyExplanation
If the amplitude of the modulating signal applied to a phase modulator is constant, the output signal will be the carrier frequency. In phase modulation, the phase of the carrier signal is varied in proportion to the instantaneous amplitude of the modulating signal. When the amplitude of the modulating signal is constant, the phase modulation will not change, resulting in the output signal being the carrier frequency.Rate this question:
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- 11.
A 100 MHz carrier is deviated 50 KHz by a 4KHz signal. The modulation index is
- A.
5
- B.
8
- C.
12.5
- D.
20
Correct Answer
C. 12.5Explanation
The modulation index is calculated by dividing the frequency deviation of the carrier signal by the frequency of the modulating signal. In this case, the carrier signal is deviated by 50 KHz and the modulating signal has a frequency of 4 KHz. Therefore, the modulation index is 50 KHz / 4 KHz = 12.5.Rate this question:
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- 12.
The maximum deviation of an FM carrier is 2KHz by a maximum modulating signal of 400Hz. The deviation ratio is
- A.
0.2
- B.
5
- C.
8
- D.
40
Correct Answer
B. 5Explanation
The deviation ratio in frequency modulation (FM) is the ratio of the maximum frequency deviation to the maximum modulating frequency. In this case, the maximum frequency deviation is given as 2KHz and the maximum modulating frequency is given as 400Hz. Therefore, the deviation ratio can be calculated as 2KHz / 400Hz = 5.Rate this question:
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- 13.
A 70 KHz carrier has a frequency deviation of 4KHz with a 1000Hz signal. How many significant sideband pairs are produced?
- A.
4
- B.
5
- C.
6
- D.
7
Correct Answer
D. 7Explanation
When a carrier signal is modulated with a signal of 1000Hz, the frequency deviation is given as 4KHz. According to Carson's rule, the bandwidth required for frequency modulation is equal to the sum of the carrier frequency and the maximum frequency deviation. In this case, the bandwidth required would be 70KHz + 4KHz = 74KHz.
In frequency modulation, sidebands are produced above and below the carrier frequency. The number of significant sideband pairs can be determined by dividing the total bandwidth by the bandwidth of each sideband. In this case, each sideband would have a bandwidth of 1000Hz.
Therefore, the number of significant sideband pairs would be 74KHz / 1000Hz = 74. Since each pair consists of an upper and lower sideband, the total number of sidebands would be 74 x 2 = 148. However, only the significant sidebands are considered, which are those that are above the carrier frequency.
In this case, there are 74 significant sidebands above the carrier frequency, resulting in 74 / 2 = 37 significant sideband pairs. Since each pair consists of an upper and lower sideband, the total number of significant sidebands is 37 x 2 = 74.
Therefore, the correct answer is 7.Rate this question:
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- 14.
What is the relative amplitude of the third pair of sidebands of an FM signal with m=6?
- A.
0.11
- B.
0.17
- C.
0.24
- D.
0.36
Correct Answer
A. 0.11Explanation
The relative amplitude of the third pair of sidebands of an FM signal with m=6 is 0.11. This means that the amplitude of the third pair of sidebands is 0.11 times the amplitude of the carrier signal. In FM modulation, the sidebands are created by varying the frequency of the carrier signal, and the amplitude of the sidebands decreases as the modulation index (m) increases. With a modulation index of 6, the relative amplitude of the third pair of sidebands is 0.11.Rate this question:
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- 15.
A 200KHz carrier is modulated by a 2.5KHz signal. The fourth pair of sidebands are spaced from the carrier by
- A.
2.5KHz
- B.
5KHz
- C.
10KHz
- D.
15KHz
Correct Answer
C. 10KHzExplanation
The fourth pair of sidebands in a modulated signal are spaced from the carrier by a frequency equal to the modulation frequency multiplied by the number of sidebands. In this case, the modulation frequency is 2.5KHz and the number of sidebands is 4. Therefore, the fourth pair of sidebands will be spaced from the carrier by 10KHz.Rate this question:
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- 16.
An FM transmitter has a maximum deviation of 12 KHz and a maxium modulating frequency of 12 KHz. The bandwidth by Carson's rule is
- A.
24KHz
- B.
33.6KHz
- C.
36.8KHz
- D.
48KHz
Correct Answer
D. 48KHzExplanation
Carson's rule states that the bandwidth of an FM signal is equal to twice the sum of the maximum deviation and the maximum modulating frequency. In this case, the maximum deviation is 12 KHz and the maximum modulating frequency is also 12 KHz. Therefore, the bandwidth is equal to 2 * (12 KHz + 12 KHz) = 48 KHz.Rate this question:
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- 17.
The maximum allowed deviation of the FM sound signal in TV is 25KHz. If the actual deviation is 18KHz, the percent modulation is
- A.
43 percent
- B.
72 percent
- C.
96 percent
- D.
139 percent
Correct Answer
B. 72 percentExplanation
The percent modulation is calculated by dividing the actual deviation by the maximum allowed deviation and multiplying by 100. In this case, the actual deviation is 18KHz and the maximum allowed deviation is 25KHz. Therefore, the percent modulation is (18/25) * 100 = 72 percent.Rate this question:
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- 18.
Which of the following is not a major benefit of FM over AM?
- A.
Greater efficiency
- B.
Noise immunity
- C.
Capture effect
- D.
Lower complexity and cost
Correct Answer
D. Lower complexity and costExplanation
FM (Frequency Modulation) offers several benefits over AM (Amplitude Modulation), such as greater efficiency, noise immunity, and the capture effect. However, lower complexity and cost are not major advantages of FM over AM. This means that FM does not necessarily require less complexity or cost compared to AM.Rate this question:
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- 19.
The primary disadvantage of FM is its
- A.
Higher cost and complexity
- B.
Excessive use of spectrum space
- C.
Noise susceptibility
- D.
Lower efficiency
Correct Answer
B. Excessive use of spectrum spaceExplanation
FM (Frequency Modulation) is a method of transmitting information through varying the frequency of the carrier wave. The primary disadvantage of FM is its excessive use of spectrum space. This means that FM requires a wider bandwidth compared to other modulation techniques, such as AM (Amplitude Modulation). As a result, FM signals can occupy more space in the frequency spectrum, limiting the number of available channels and potentially causing interference with other signals. This excessive use of spectrum space is a drawback of FM, making it less efficient in terms of spectrum utilization compared to other modulation methods.Rate this question:
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- 20.
Noise is primarily
- A.
High frequency spikes
- B.
Low frequency variations
- C.
Random level shifts
- D.
Random frequency variations
Correct Answer
A. High frequency spikesExplanation
High frequency spikes refer to sudden and rapid changes in the signal's amplitude, occurring at a fast rate. This type of noise is characterized by short-duration disturbances that can occur randomly or periodically. It is considered as the primary noise because it affects the signal across a wide frequency range, potentially causing interference and distortion.Rate this question:
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- 21.
The receiver circuit that rids FM of noise is the
- A.
Modulator
- B.
Demodulator
- C.
Limiter
- D.
Low pass filter
Correct Answer
C. LimiterExplanation
A limiter is a circuit that helps in removing noise from an FM signal. It sets a maximum limit on the amplitude of the signal, preventing any sudden and drastic changes in the signal strength. By doing so, it effectively reduces any noise or interference that may be present in the FM signal. Hence, the limiter circuit is responsible for improving the quality of the received FM signal by eliminating unwanted noise.Rate this question:
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- 22.
The phenomenon of a strong FM signal dominating a weaker signal on a common frequency is referred to as the
- A.
Capture effect
- B.
Blot out
- C.
Quieting factor
- D.
Domination syndrome
Correct Answer
A. Capture effectExplanation
The phenomenon of a strong FM signal dominating a weaker signal on a common frequency is referred to as the capture effect. This occurs when a receiver locks onto the stronger signal and ignores or "captures" the weaker signal, resulting in the weaker signal being suppressed or not heard at all.Rate this question:
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- 23.
The AM signals generated at a low level may only be amplified by what type of amplifier?
- A.
Op Amp
- B.
Linear
- C.
Class C
- D.
Push-pull
Correct Answer
B. LinearExplanation
A linear amplifier is the correct answer because it amplifies the AM signals generated at a low level without distorting the original waveform. Unlike other types of amplifiers, such as class C or push-pull, which are more commonly used for specific applications, a linear amplifier is designed to provide a faithful reproduction of the input signal without introducing any significant distortion or non-linearities. Therefore, it is the most suitable type of amplifier for amplifying low-level AM signals.Rate this question:
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- 24.
Frequency modulation transmitters are more efficient because their power is increased by what type of amplifier?
- A.
Class A
- B.
Class B
- C.
Class C
- D.
All of the above
Correct Answer
C. Class CExplanation
Class C amplifiers are more efficient for frequency modulation transmitters because they operate in a highly nonlinear region, allowing for high power amplification with low power dissipation. Class A amplifiers have low efficiency as they operate in a linear region, while Class B amplifiers have better efficiency but suffer from distortion. Class C amplifiers, on the other hand, have high efficiency and minimal distortion, making them the preferred choice for FM transmitters.Rate this question:
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- 25.
Noise interfere mainly with modulating signals that are
- A.
Sinusoidal
- B.
Nonsinusoidal
- C.
Low frequencies
- D.
High frequencies
Correct Answer
D. High frequenciesExplanation
Noise interference primarily affects high-frequency modulating signals because high-frequency signals have a shorter wavelength and are more susceptible to distortion and interference from external factors. Low-frequency signals, on the other hand, have longer wavelengths and are less affected by noise. Sinusoidal and nonsinusoidal signals refer to the shape of the waveform and do not directly determine the susceptibility to noise interference. Therefore, the correct answer is high frequencies.Rate this question:
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- 26.
Pre emphasis circuits boost what modulating frequencies before modulation?
- A.
High frequencies
- B.
Mid range frequencies
- C.
Low frequencies
- D.
All of the above
Correct Answer
A. High frequenciesExplanation
Pre emphasis circuits are used in communication systems to improve the signal-to-noise ratio. They work by boosting the higher frequencies of the audio signal before it is modulated. This is done because high frequencies are more susceptible to noise and distortion during transmission. By emphasizing the high frequencies, the overall quality of the signal can be improved, resulting in better clarity and intelligibility.Rate this question:
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- 27.
A pre emphasis circuit is a
- A.
Low pass filter
- B.
High pass filter
- C.
Phase shifter
- D.
Bandpass filter
Correct Answer
B. High pass filterExplanation
A pre emphasis circuit is a high pass filter. This type of filter is used in audio systems to boost the higher frequencies and attenuate the lower frequencies. It is commonly used in audio recording and broadcasting to improve the signal quality and reduce noise. By emphasizing the higher frequencies, the circuit helps to enhance the clarity and intelligibility of the audio signal.Rate this question:
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- 28.
Pre emphasis is compensated for at the receiver by a
- A.
Phase inverter
- B.
Bandpass filter
- C.
High pass filter
- D.
Low pass filter
Correct Answer
D. Low pass filterExplanation
Pre emphasis is a technique used in audio processing to boost the higher frequencies before transmission to improve the signal-to-noise ratio. At the receiver, this pre-emphasized signal needs to be compensated for to restore the original frequency balance. A low pass filter is used for this purpose as it allows the lower frequencies to pass through while attenuating the higher frequencies. This effectively reverses the pre-emphasis process and restores the original frequency response of the audio signal.Rate this question:
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- 29.
The cutoff frequency of pre-emphasis and de-emphasis circuits is
- A.
1KHz
- B.
2122KHz
- C.
5KHz
- D.
75KHz
Correct Answer
B. 2122KHzExplanation
The cutoff frequency of pre-emphasis and de-emphasis circuits is 2122KHz. This means that frequencies below 2122KHz will be attenuated or reduced, while frequencies above 2122KHz will pass through with little or no attenuation. Pre-emphasis and de-emphasis circuits are commonly used in audio systems to improve the signal-to-noise ratio and compensate for the frequency response of the system. By emphasizing higher frequencies during recording and de-emphasizing them during playback, the circuit helps to reduce noise and improve the overall audio quality.Rate this question:
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