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

Explanation:

Digital Modulation Technique

Definition: Digital modulation refers to the process of modifying a carrier signal to encode digital information. This is achieved by altering the carrier's properties such as amplitude, frequency, or phase. Digital modulation techniques are fundamental in modern communication systems, as they enable efficient and reliable transmission of digital data over various communication channels.

Phase Shift Keying (PSK):

Definition: Phase Shift Keying (PSK) is a digital modulation technique where the phase of the carrier signal is varied to represent digital data. Each unique phase state corresponds to a specific binary or multi-bit symbol, allowing the transmission of information.

Working Principle: In PSK, the carrier signal's phase is shifted to encode digital information. For example:

• BPSK (Binary Phase Shift Keying): Two distinct phase states (e.g., 0° and 180°) are used to represent binary data (0 and 1).
• QPSK (Quadrature Phase Shift Keying): Four phase states (e.g., 0°, 90°, 180°, and 270°) are used, with each phase state representing two bits of data.
• Higher-Order PSK: More phase states (e.g., 8-PSK, 16-PSK) are used to represent more bits per symbol, increasing the data transmission rate.

Advantages of PSK:

• High spectral efficiency, allowing more data to be transmitted within a given bandwidth.
• Resilience to noise and interference, especially in low signal-to-noise ratio (SNR) environments.
• Widely used in modern digital communication systems, including wireless networks, satellite communications, and optical fiber systems.

Disadvantages of PSK:

• Complexity increases with higher-order PSK, requiring sophisticated demodulation techniques.
• Performance can degrade in the presence of severe phase noise or synchronization issues.

Applications: PSK is extensively used in various digital communication systems, including Wi-Fi, cellular networks, and satellite communications, due to its efficiency and robustness.

Correct Option Analysis:

The correct option is:

Option 3: Phase Shift Keying (PSK)

This option correctly identifies PSK as an example of a digital modulation technique. PSK is a widely used method for encoding digital information by varying the phase of the carrier signal, making it a fundamental concept in digital communication systems.

Important Information

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

Option 1: Single Sideband Modulation (SSB)

SSB is a type of analog modulation technique used in amplitude modulation (AM) systems. It involves transmitting only one sideband (either the upper or lower sideband) of the modulated signal while suppressing the carrier and the other sideband. SSB is primarily used in analog communication systems, such as amateur radio and long-distance voice communication, and is not an example of a digital modulation technique.

Option 2: Narrow Band FM

Narrow Band FM is another analog modulation technique where the frequency of the carrier signal is varied in proportion to the modulating signal. It is used in applications such as two-way radio communication and public safety communication. Similar to SSB, it is not a digital modulation technique.

Option 4: Double Sideband Amplitude Modulation

This is a classical analog modulation method where both the upper and lower sidebands are transmitted along with the carrier signal. It is commonly used in AM radio broadcasting but is not related to digital modulation techniques.

Conclusion:

Understanding the distinction between analog and digital modulation techniques is crucial for identifying the correct option. Among the options provided, only Phase Shift Keying (PSK) is a digital modulation technique, as it involves encoding digital information by varying the phase of the carrier signal. The other options, including Single Sideband Modulation (SSB), Narrow Band FM, and Double Sideband Amplitude Modulation, are examples of analog modulation techniques, which are fundamentally different from digital modulation.

Concept:

The quantization noise power P_q for a uniform quantizer is given by:

P_q = 

where is the quantization step size.

Given:

Full-scale input voltage =

Number of quantization levels = 2^N

Calculation:

Quantization step size:

Quantization noise power:

Hence, the correct answer is option 1) 

The correct answer is 2f.

Key Points

• The bandwidth requirement for simulating an analog signal is determined by the Nyquist criterion.
  • According to the Nyquist theorem, the sampling rate must be at least twice the highest frequency of the analog signal to accurately reconstruct the signal without aliasing.
  • If the frequency of the analog signal is f" id="MathJax-Element-147-Frame" role="presentation" style="position: relative;" tabindex="0">ff $f$ , the minimum sampling rate required is 2f" id="MathJax-Element-148-Frame" role="presentation" style="position: relative;" tabindex="0">2f2f $2f$ .
  • This ensures that the signal can be accurately digitized and reconstructed.

Additional Information

• Aliasing occurs when the sampling rate is less than twice the signal frequency, leading to distortion in the reconstructed signal.
• To avoid aliasing, a low-pass filter is often used before sampling to limit the bandwidth of the analog signal to f" id="MathJax-Element-149-Frame" role="presentation" style="position: relative;" tabindex="0">ff $f$ .
• In practical systems, the sampling rate is often slightly higher than 2f" id="MathJax-Element-150-Frame" role="presentation" style="position: relative;" tabindex="0">2f2f $2f$ to provide a margin of safety.
• This principle is widely used in digital audio, communications, and other fields involving analog-to-digital conversion.

The correct answer is Option 2: TDM requires the transmitter and receiver to be synchronized periodically.

Key Points

• Time Division Multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches.
• TDM divides the available time on a channel into time slots and allocates these slots to different data streams.
• Synchronization between the transmitter and receiver is crucial in TDM to ensure that the correct data is received in the correct time slot.
• If the transmitter and receiver are not synchronized, data meant for one slot could be received in another, causing data loss or corruption.
• Effective synchronization involves periodic checks and adjustments to maintain alignment between the transmitter and receiver.

Additional Information

• TDM is used in various communication systems such as telephone networks, where multiple calls are transmitted over a single line.
• There are different types of TDM, including Synchronous TDM and Statistical TDM.
• Synchronous TDM assigns fixed time slots to each data stream, whether or not there is data to send, leading to potential inefficiency.
• Statistical TDM, on the other hand, dynamically allocates time slots based on demand, improving efficiency.
• TDM systems often include mechanisms for error detection and correction to ensure data integrity.

Calculation:

The total modulation index is the square root of the sum of the squares of the individual modulation indices. If the modulation indices for the two waves are 0.3 and 0.4, then the total modulation index is:

m_total = √(0.3² + 0.4²)

m_total = √(0.09 + 0.16) = √0.25 = 0.5

Therefore, the total modulation index is 0.5.

In TV transmission:

• A video is Vestigial sideband modulated, which is a type of amplitude modulated waveform
• The Video signals are thus encoded in amplitude variations of the carriers
• The Audio signal is encoded in FM waveform
• Thus, the audio signals are encoded as frequency variations of the carrier

Noise is the signal that affects amplitude majorly. Thus, the video signal is distorted from amplitude variations.

Quantization: 

• It is the process through which a range of continuous analog values are quantized or rounded off to a single value, thereby forming samples of a discrete digital signal.
• Quantization Error occurs when there is a difference between an input value and it’s quantized value. 
• Quantization occurs when an analog signal is converted into it’s digital form, thus it occurs in Pulse Code modulation (PCM).

PCM:

• PCM stands for Pulse Code Modulation.
• It is a technique by which an analog signal gets converted into digital form to have signal transmission through a digital network.

• The major steps involved in PCM are sampling, quantizing, and encoding.

• With PCM, the amplitude of the analog signal is sampled at regular intervals and translated into a binary number.
• The difference between the original signal and the translated digital signal is called the quantizing error.

Some Advantages associated with PCM are:

• Immunity to transmission noise and interference.
• It is possible to regenerate the coded signal along the transmission path.
• The Quantization Noise depends on the number of quantization levels and not on the number of samples produced per second.
• The storage of Coded signals is easy.

The disadvantages of PCM includes:

• Requires larger Bandwidth.
• Need synchronization
• Not compatible with analog systems.

PCM:

• PCM stands for Pulse Code Modulation.
• With PCM, the amplitude of the analog signal is sampled at regular intervals and translated into a binary number.
• The difference between the original signal and the translated digital signal is called the quantizing error.

Some Advantages associated with PCM are:

• Immunity to transmission noise and interference.
• It is possible to regenerate the coded signal along the transmission path.
• The Quantization Noise depends on the number of quantization levels and not on the number of samples produced per second.
• The storage of Coded signals is easy.

The disadvantages of PCM includes:

• Requires larger Bandwidth.
• Need synchronization
• Not compatible with analog systems.

Advantages of FM over AM are:

• Improved signal to noise ratio.
• Smaller geographical interference between neighbouring stations.
• Less radiated power.
• Well defined service areas for given transmitter power.

Disadvantages of FM:

• Much more Bandwidth
• More complicated receiver and transmitter.

Concept:

The number of levels for an n-bit PCM system is given by:

L = 2n

We can also state that the number of bits for a given quantization level will be:

n = log2 L

Calculation:

The number of levels given = 4096, i.e. L = 4096

The number of bits used will be:

n = log2 (4096)

= log2 (2¹²)

= 12 log2 (2)

n = 1

The bandwidth of PCM is given by:

n = number of bits to encode

fs = sampling frequency

Concept:

The number of levels for an n-bit PCM system is given by:

L = 2n

We can also state that the number of bits for a given quantization level will be:

n = log₂ L

Also, the bandwidth of PCM is given by:

n = number of bits to encode

fs = sampling frequency

Calculation:

For L = 4 quantization levels, the number of bits n = log2 4 = 2 bits. The bandwidth is, therefore:

B.W. = 2 fs

Similarly, For L = 64 quantization levels, the number of bits n = log2 64 = 6 bits. The bandwidth is, therefore:

B.W. = 6 fs

Clearly, the Bandwidth is increased by 3 times.

The AM, FM and PM output waveforms are as shown:

In Frequency modulation, the frequency of the carrier is varied according to the amplitude of the message signal and the amplitude of the carrier remains constant.

Concept:

Two types of distortions (Errors) occurs in Delta modulation system i.e.

1) Slope overload error

2) Granular Error

This is explained with the help of the given figure:

To avoid the slope overload error, the optimum or desired condition is:

i.e. the step rise of the quantized output must follow the input

Slope overload occurs when:

To prevent/avoid slope overload error, the condition that shall be satisfied is:

m(t) is a sinusoidal waveform given as:

m(t) = A_m sin ω_m t

The condition to avoid slope overload, therefore, becomes:

A_m cos (2π (mt)).2πf_m ≤ Δ⋅f_s

For maximum Amplitude, the above condition becomes:

• PCM (Pulse Code Modulation) is a digital scheme for transmitting analog data.
• The amplitude of an analog signal can take any value over a continuous range, i.e. it can take on infinite values.
• But, digital signal amplitude can take on finite values.
• Analog signals can be converted into digital by sampling and quantizing.

 

Advantages of PCM:

• Encoding is possible in PCM.
• Very high noise immunity, i.e. better performance in the presence of noise.
• Convenient for long-distance communication.
• Good signal to noise ratio.

 

Disadvantage of PCM:

• The circuitry is complex.
• It requires a large bandwidth.
• Synchronization is required between transmitter and receiver.

Concept:

Slope overload distortion: When the maximum slope of the message is more than Δ f_s then there is slope overload distortion in the delta modulator.

condition to prevent slope overload distortion 

 ,

where m(t)→ message signal, f_s→ sampling frequency

Calculation:

Given f_m = 10 kHz

Input is 1 V_pp so the maximum  input voltage is 0.5 volt

let applied input is sinusoidal than

m(t) = A_msin(2πf_mt)

where 

f_s = 10× (Nyquist rate)

f_s =10×(2f_m)= 20(10k)=200kHZ

 By the condition to prevent slope overload distortion 

2πA_mf_m ≤ Δ (200k)

2π(0.5)(10k) ≤ Δ (200k)

Δ ≥ 0.157

Δ _minimum = 0.157 V