Modulation and Detection MCQ Quiz - Objective Question with Answer for Modulation and Detection - Download Free PDF

The correct answer is Convert analog signals to digital and vice-versa

• The modem is a device that transforms computer-generated digital signals into analog signals to enable their traveling through phone lines. The ‘modulator-demodulator’ or modem may work as a dial-up for LAN or connect to an ISP.
• Modems can be both external, as in the device which connects to the USB or the serial port of a computer machine, or proprietary components for handheld smart devices and other devices, also as internal, in the form of add-on expansion cards for computer systems and PCMCIA cards for laptop computers.

Key Points

• The design of a modem differs for both the external and internal modem. In internal modems, IRQ – Interrupt request is utilized to configure the modem in addition to I/O, which is a memory address. Usually, before the installation of a built-in modem, integrated serial interfaces are disabled, assigning them the COM2 resources at the same time.
• For external types of a modem, the modem assigns and uses the resources itself. This is especially helpful for USB port and laptop users as the non-complex and simpler nature of the process render it far more useful for daily usage.

• At the time of setup, the second step to make sure the proper working of a modem is the installation of drivers. The modem working efficiency and processing is decided by two factors:

• Speed of UART –An acronym for Universal Asynchronous Receiver or Transmitter chip(installed on the motherboard to which the modem connection is established)
• Speed of the modem itself

Explanation:

Amplitude Limiter in FM Receivers

Definition: An amplitude limiter is a circuit found in frequency modulation (FM) receivers that is used to remove amplitude variations in the received signal. These amplitude variations are often caused by noise and interference. The primary purpose of the amplitude limiter is to maintain a consistent amplitude level of the FM signal before it is demodulated, ensuring that the signal's frequency variations, which carry the information, are preserved without being affected by amplitude noise.

Working Principle: In an FM receiver, the amplitude limiter operates by clipping the peaks of the incoming signal. This clipping process effectively removes any amplitude variations that exceed a certain threshold, which is set to be just above the maximum amplitude of the desired signal. By doing so, the amplitude limiter ensures that the output signal has a constant amplitude, which is crucial for the proper demodulation of the FM signal. The frequency variations of the signal, which contain the actual information, remain intact and unaffected by the amplitude limiting process.

Advantages:

• Improves the signal-to-noise ratio (SNR) by eliminating unwanted amplitude variations caused by noise.
• Prevents overloading of subsequent stages in the receiver, such as the demodulator, by maintaining a consistent signal amplitude.
• Enhances the overall performance and reliability of the FM receiver by ensuring that the demodulated signal accurately represents the original transmitted information.

Disadvantages:

• May introduce distortion if the threshold for amplitude limiting is not set correctly.
• Can potentially clip desired signal components if the signal experiences rapid and significant amplitude variations.

Applications: Amplitude limiters are commonly used in various types of FM receivers, including those found in radio broadcasting, two-way communication systems (such as walkie-talkies), and other wireless communication devices that utilize frequency modulation.

Correct Option Analysis:

The correct option is:

Option 1: Remove amplitude variations due to noise.

This option accurately describes the primary function of an amplitude limiter in FM receivers. By removing amplitude variations caused by noise, the amplitude limiter ensures that the signal's amplitude remains constant, allowing for accurate demodulation of the frequency variations that carry the information.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Filtration.

This option is incorrect because filtration refers to the process of removing specific frequency components from a signal, typically using filters. While filtration is an important aspect of signal processing in FM receivers, it is not the primary function of an amplitude limiter.

Option 3: Demodulation.

This option is incorrect because demodulation is the process of extracting the original information signal from the modulated carrier wave. Although demodulation is a critical function in FM receivers, it is not the role of the amplitude limiter. The amplitude limiter prepares the signal for demodulation by removing amplitude variations, but it does not perform the demodulation itself.

Option 4: Amplification.

This option is incorrect because amplification refers to increasing the amplitude of a signal. While amplification is necessary in FM receivers to boost the signal strength, it is not the function of an amplitude limiter. The amplitude limiter, in fact, works to keep the amplitude constant rather than increasing it.

Conclusion:

Understanding the role of an amplitude limiter in FM receivers is essential for appreciating its importance in maintaining signal integrity. By removing amplitude variations caused by noise, the amplitude limiter ensures that the frequency variations, which carry the information, are accurately demodulated. This function significantly enhances the performance and reliability of FM receivers, making amplitude limiters a crucial component in modern communication systems.

Concept:

The efficiency of an AM transmission is defined as the ratio of the useful power (containing information) to the total power used in transmission, i.e.

\(\eta =\frac{P_{useful}}{P_{total}}\)

Analysis:

DSB-SC wave suppresses the carrier to increase efficiency.

The standard expressed for a DSB-SC wave is:

\(s\left( t \right) = \frac{{{A_m}{A_c}}}{2}\cos \left( {{\omega _c} + {\omega _m}} \right)t + \frac{{{A_m}{A_c}}}{2}\cos \left( {{\omega _c} - {\omega _m}} \right)t\)

We observe that the carrier is suppressed in DSB-SC.

Upper sideband power will be:

\({P_u} = {\left[ {\left( {\frac{{{A_m}{A_c}}}{2}} \right) \times \frac{1}{{\sqrt 2 }}} \right]^2}\)

\({P_u} = \frac{{A_m^2A_t^2}}{8}\)

Similarly, lower sideband power will be:

\({P_L} = \frac{{A_m^2A_c^2}}{8}\)

|Now, the total power will be:

P_T = P_u + P_L

\({P_T} = \frac{{A_m^2A_c^2}}{8} + \frac{{A_m^2A_c^2}}{8}\)

\({P_T} = \frac{{A_m^2A_c^2}}{4}\)

The efficiency of DSB-SC wave will be:

\(\eta = \frac{{{P_u}}}{{{P_T}}} = \frac{{{p_L}}}{{{p_T}}}\)

\(\eta = \frac{{A_m^2A_c^2/8}}{{A_m^2/A_c^2/4}} \times 100\)

η % = 50%

Note: The modulation efficiency of a standard AM signal in which the carrier is not suppressed is 33.33 %

In AM modulation, the modulation index 'or' modulation depth (m) is defined as a measure of the extent to which a carrier voltage is varied by the modulating signal.

It is one of the key parameters whose value determines if there is a distortion in the AM signal or not.

It is evaluated mathematically as:

\(μ=\frac{A_m}{A_c}\)

For three different values of modulation index, we have different output waveforms as:

Under Modulation (μ < 1):

Critical Modulation (μ = 1):

Over Modulation (μ > 1):

Observation:

• As the value of the modulation index increases, the carrier experiences a 180° phase reversal, which causes additional sidebands, and hence, the wave gets distorted.
• Such an overmodulated wave causes interference, which cannot be eliminated.

PCM (Pulse Code Modulation):

PCM stands for Pulse Code Modulation, it is a digital modulation scheme in which analog signals are quantized to get digital bit stream which is transmitted using pulses coded by the digital bitstream.

Frequency shift keying:

• Frequency shift keying is true digital modulation i.e. message signal is sent in bits.
• The frequency of the sine carrier is varied according to message signal in digital form i.e. bits

Phase Shift Keying: The modulation system where different data bits are represented by differing phases is phase shift keying.

Phase Modulation:

The Phase of the carrier is varied according to the amplitude of the message signal.

It is an analog modulation technique.

If the lower sideband overlaps the basebands, the distortion is called Aliasing. The main reason for aliasing is undersampling \(i.e.\;{f_s} < 2{f_m}\)

The correct answer is Convert analog signals to digital and vice-versa

• The modem is a device that transforms computer-generated digital signals into analog signals to enable their traveling through phone lines. The ‘modulator-demodulator’ or modem may work as a dial-up for LAN or connect to an ISP.
• Modems can be both external, as in the device which connects to the USB or the serial port of a computer machine, or proprietary components for handheld smart devices and other devices, also as internal, in the form of add-on expansion cards for computer systems and PCMCIA cards for laptop computers.

Key Points

• The design of a modem differs for both the external and internal modem. In internal modems, IRQ – Interrupt request is utilized to configure the modem in addition to I/O, which is a memory address. Usually, before the installation of a built-in modem, integrated serial interfaces are disabled, assigning them the COM2 resources at the same time.
• For external types of a modem, the modem assigns and uses the resources itself. This is especially helpful for USB port and laptop users as the non-complex and simpler nature of the process render it far more useful for daily usage.

• At the time of setup, the second step to make sure the proper working of a modem is the installation of drivers. The modem working efficiency and processing is decided by two factors:

• Speed of UART –An acronym for Universal Asynchronous Receiver or Transmitter chip(installed on the motherboard to which the modem connection is established)
• Speed of the modem itself

• Intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception
• This helps in signal processing as designing amplifiers for microwave applications is difficult
• The intermediate frequency used in commercial Medium Wave Super Heterodyne receiver is 455 kHz

Concept:

FM broadcast: Frequency Modulation is the most widely used modulation system in radio transmission, in FM the audio signals are modulated onto carrier frequencies in bands 88 MHz to 108 MHz.

Standard AM broadcast: It has a range of medium frequency of range 540 kHz to 1600 kHz, used in AM broadcast, navigation of aircraft/marine.

Television: It has a range of very high to ultra-high frequency of range 54 MHz to 890 MHz in a television broadcast.

Cellular Mobile Radio: It has a range of super-high frequency of range 840 MHz to 935 MHz in cellular mobile radio.

Depending on the input voltage VCO produces the different frequencies and it is used in the modulation/generation of frequency.

Explanation:

Amplitude Limiter in FM Receivers

Definition: An amplitude limiter is a circuit found in frequency modulation (FM) receivers that is used to remove amplitude variations in the received signal. These amplitude variations are often caused by noise and interference. The primary purpose of the amplitude limiter is to maintain a consistent amplitude level of the FM signal before it is demodulated, ensuring that the signal's frequency variations, which carry the information, are preserved without being affected by amplitude noise.

Working Principle: In an FM receiver, the amplitude limiter operates by clipping the peaks of the incoming signal. This clipping process effectively removes any amplitude variations that exceed a certain threshold, which is set to be just above the maximum amplitude of the desired signal. By doing so, the amplitude limiter ensures that the output signal has a constant amplitude, which is crucial for the proper demodulation of the FM signal. The frequency variations of the signal, which contain the actual information, remain intact and unaffected by the amplitude limiting process.

Advantages:

• Improves the signal-to-noise ratio (SNR) by eliminating unwanted amplitude variations caused by noise.
• Prevents overloading of subsequent stages in the receiver, such as the demodulator, by maintaining a consistent signal amplitude.
• Enhances the overall performance and reliability of the FM receiver by ensuring that the demodulated signal accurately represents the original transmitted information.

Disadvantages:

• May introduce distortion if the threshold for amplitude limiting is not set correctly.
• Can potentially clip desired signal components if the signal experiences rapid and significant amplitude variations.

Applications: Amplitude limiters are commonly used in various types of FM receivers, including those found in radio broadcasting, two-way communication systems (such as walkie-talkies), and other wireless communication devices that utilize frequency modulation.

Correct Option Analysis:

The correct option is:

Option 1: Remove amplitude variations due to noise.

This option accurately describes the primary function of an amplitude limiter in FM receivers. By removing amplitude variations caused by noise, the amplitude limiter ensures that the signal's amplitude remains constant, allowing for accurate demodulation of the frequency variations that carry the information.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Filtration.

This option is incorrect because filtration refers to the process of removing specific frequency components from a signal, typically using filters. While filtration is an important aspect of signal processing in FM receivers, it is not the primary function of an amplitude limiter.

Option 3: Demodulation.

This option is incorrect because demodulation is the process of extracting the original information signal from the modulated carrier wave. Although demodulation is a critical function in FM receivers, it is not the role of the amplitude limiter. The amplitude limiter prepares the signal for demodulation by removing amplitude variations, but it does not perform the demodulation itself.

Option 4: Amplification.

This option is incorrect because amplification refers to increasing the amplitude of a signal. While amplification is necessary in FM receivers to boost the signal strength, it is not the function of an amplitude limiter. The amplitude limiter, in fact, works to keep the amplitude constant rather than increasing it.

Conclusion:

Understanding the role of an amplitude limiter in FM receivers is essential for appreciating its importance in maintaining signal integrity. By removing amplitude variations caused by noise, the amplitude limiter ensures that the frequency variations, which carry the information, are accurately demodulated. This function significantly enhances the performance and reliability of FM receivers, making amplitude limiters a crucial component in modern communication systems.

• AM detector employs envelope detector which is the low pass filter
• The output of the envelope detector follows the envelope of the message signal

• The envelope detector is used when the modulation index is ≤ 1
• When modulation index is greater than 1, then synchronous detection is used

If the lower sideband overlaps the basebands, the distortion is called Aliasing. The main reason for aliasing is undersampling \(i.e.\;{f_s} < 2{f_m}\)

The correct answer is Convert analog signals to digital and vice-versa

• The modem is a device that transforms computer-generated digital signals into analog signals to enable their traveling through phone lines. The ‘modulator-demodulator’ or modem may work as a dial-up for LAN or connect to an ISP.
• Modems can be both external, as in the device which connects to the USB or the serial port of a computer machine, or proprietary components for handheld smart devices and other devices, also as internal, in the form of add-on expansion cards for computer systems and PCMCIA cards for laptop computers.

Key Points

• The design of a modem differs for both the external and internal modem. In internal modems, IRQ – Interrupt request is utilized to configure the modem in addition to I/O, which is a memory address. Usually, before the installation of a built-in modem, integrated serial interfaces are disabled, assigning them the COM2 resources at the same time.
• For external types of a modem, the modem assigns and uses the resources itself. This is especially helpful for USB port and laptop users as the non-complex and simpler nature of the process render it far more useful for daily usage.

• At the time of setup, the second step to make sure the proper working of a modem is the installation of drivers. The modem working efficiency and processing is decided by two factors:

• Speed of UART –An acronym for Universal Asynchronous Receiver or Transmitter chip(installed on the motherboard to which the modem connection is established)
• Speed of the modem itself

• In TV transmission for bandwidth saving Video Signals are transmitted using Vestigial Modulation
• Vestigial Modulation is a type of Amplitude modulation where a little more than one sideband is transmitted