Frenzel Chapter 4: Frequency Modulation

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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".

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.