Principles of Electronic Communication (4331104) - Summer 2024 Solution

Question 1(a) [3 marks]

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Draw and explain block diagram of communication system.

Answer:

flowchart LR
    A[Information Source] --> B[Transmitter]
    B --> C[Channel/Medium]
    C --> D[Receiver]
    D --> E[Destination]
    F[Noise Source] --> C

• Information Source: Generates message signal (voice, video, data)
• Transmitter: Converts message to suitable form for transmission
• Channel: Medium through which signal travels (wires, fiber, air)
• Receiver: Extracts original message from received signal
• Destination: End-user who receives the information

Mnemonic: “Information Travels Carefully Reaching Destination”

Question 1(b) [4 marks]

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Explain applications of EM wave spectrum.

Answer:

Mnemonic: “Radio Makes Invisible Very eXtreme Gamma signals”

Question 1(c) [7 marks]

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State and explain external and internal noise.

Answer:

Diagram:

graph TD
    A[Noise in Communication] --> B[External Noise]
    A --> C[Internal Noise]
    B --> D[Atmospheric Noise]
    B --> E[Space Noise]
    B --> F[Industrial Noise]
    B --> G[Man-made Noise]
    C --> H[Thermal Noise]
    C --> I[Shot Noise]
    C --> J[Transit-time Noise]
    C --> K[Flicker Noise]

Mnemonic: “External Environmental Sources Invade; Internal Components Generate Noise”

Question 1(c) OR [7 marks]

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Draw and explain the block diagram of a Superheterodyne AM receiver.

Answer:

flowchart LR
    A[Antenna] --> B[RF Amplifier]
    B --> C[Mixer]
    D[Local Oscillator] --> C
    C --> E[IF Amplifier]
    E --> F[Detector]
    F --> G[AF Amplifier]
    G --> H[Speaker]
    I[AGC] --> B
    I --> E
    F --> I

Mnemonic: “Radio Loves Making Interesting Detected Audio Sounds”

Question 2(a) [3 marks]

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Define modulation. State types of modulation.

Answer:

Modulation: Process of varying one or more properties of a high-frequency carrier signal with a modulating signal containing information.

Types of Modulation:

graph TD
    A[Modulation] --> B[Analog Modulation]
    A --> C[Digital Modulation]
    A --> D[Pulse Modulation]
    B --> E[AM]
    B --> F[FM]
    B --> G[PM]
    C --> H[ASK]
    C --> I[FSK]
    C --> J[PSK]
    D --> K[PAM]
    D --> L[PWM]
    D --> M[PPM]
    D --> N[PCM]

Mnemonic: “All Modulations Alter Properties: Frequency, Amplitude, Phase”

Question 2(b) [4 marks]

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Define: Signal to noise ratio and Noise figure.

Answer:

Mnemonic: “SNR Shows Necessary Reception; Noise Figure Finds Fault”

Question 2(c) [7 marks]

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Compare PAM, PWM and PPM techniques.

Answer:

Diagram:

Mnemonic: “Amplitude varies height, Width varies length, Position varies timing”

Question 2(a) OR [3 marks]

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Differentiate between bit, symbol and Baud rate.

Answer:

Mnemonic: “Bits Build Symbols, Bauds Show Speed”

Question 2(b) OR [4 marks]

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State advantages and disadvantage of SSB over DSB.

Answer:

Mnemonic: “SSB Saves Bandwidth Power but Costs Complex Hardware”

Question 2(c) OR [7 marks]

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Compare Amplitude Modulation (AM) and Frequency Modulation (FM).

Answer:

Diagram:

Mnemonic: “AM Alters strength, FM Fluctuates timing”

Question 3(a) [3 marks]

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Compare AM receiver with FM receiver.

Answer:

Mnemonic: “AM Accepts Minimal bandwidth; FM Features More circuits”

Question 3(b) [4 marks]

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Define sampling? Explain types of sampling in brief.

Answer:

Sampling: Process of converting continuous-time signal into discrete-time signal by taking samples at regular intervals.

Diagram:

Mnemonic: “Ideal takes Instants, Natural follows Nicely, Flat stays Fixed”

Question 3(c) [7 marks]

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Draw and explain the block diagram of FM receiver. What is the use of Limiter in FM receiver?

Answer:

flowchart LR
    A[Antenna] --> B[RF Amplifier]
    B --> C[Mixer]
    D[Local Oscillator] --> C
    C --> E[IF Amplifier]
    E --> F[Limiter]
    F --> G[Discriminator]
    G --> H[De-emphasis]
    H --> I[AF Amplifier]
    I --> J[Speaker]

Limiter Function: Removes amplitude variations from the FM signal before demodulation to ensure noise immunity, as information in FM is contained in frequency variations, not amplitude.

Mnemonic: “Radio Mixers Increase Frequency; Limiters Discriminate Audio Sound”

Question 3(a) OR [3 marks]

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Describe the concept of single side band (SSB) transmission.

Answer:

Single Sideband (SSB) Transmission: Technique where only one sideband (upper or lower) is transmitted while suppressing the carrier and other sideband.

graph LR
    A[AM Signal] --> B[DSBFC]
    A --> C[DSBSC]
    A --> D[SSB]
    D --> E[USB]
    D --> F[LSB]

• Bandwidth: Requires only half the bandwidth (fc ± fm)
• Power Efficiency: More efficient as power concentrated in one sideband
• Types: USB (Upper Sideband) and LSB (Lower Sideband)

Mnemonic: “SSB Saves Spectrum Bandwidth”

Question 3(b) OR [4 marks]

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Explain pre-emphasis & de-emphasis circuit.

Answer:

Circuit Diagram:

Mnemonic: “Pre Pushes highs, De Drops them”

Question 3(c) OR [7 marks]

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Illustrate generation of FM signal using Phase lock loop technique.

Answer:

flowchart LR
    A[Modulating Signal] --> B[Loop Filter]
    B --> C[VCO]
    C --> D[FM Output]
    C --> E[Phase Detector]
    F[Reference Oscillator] --> E
    E --> B

Working Process:

1. Modulating signal is applied to loop filter
2. VCO frequency shifts proportional to modulating signal
3. Phase detector generates error signal
4. Loop maintains lock while allowing frequency modulation
5. Output of VCO is the FM signal

Mnemonic: “Phase Locks, Voltage Controls, Frequency Modulates”

Question 4(a) [3 marks]

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Explain quantization process and its importance.

Answer:

Quantization: Process of mapping continuous amplitude values to a finite set of discrete levels in analog-to-digital conversion.

Importance:

• Enables digital representation of analog signals
• Determines resolution and accuracy of digital signal
• Affects signal-to-noise ratio in digital systems

Mnemonic: “Quantization Creates Digital from Analog”

Question 4(b) [4 marks]

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Explain different characteristics of Radio receiver.

Answer:

Diagram:

graph TD
    A[Radio Receiver Characteristics] --> B[Sensitivity]
    A --> C[Selectivity]
    A --> D[Fidelity]
    A --> E[Image Rejection]
    B --> F[Measured in μV]
    C --> G[Bandwidth and Q factor]
    D --> H[Frequency response]
    E --> I[Image ratio]

Mnemonic: “Sensitive Selection Faithfully Images”

Question 4(c) [7 marks]

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Draw and explain the block diagram of PCM transmitter and receiver.

Answer:

PCM Transmitter:

flowchart LR
    A[Input Signal] --> B[Anti-aliasing Filter]
    B --> C[Sample & Hold]
    C --> D[Quantizer]
    D --> E[Encoder]
    E --> F[Line Coder]
    F --> G[Transmission Channel]

PCM Receiver:

flowchart LR
    A[Received Signal] --> B[Line Decoder]
    B --> C[Regenerative Repeater]
    C --> D[Decoder]
    D --> E[Reconstruction Filter]
    E --> F[Output Signal]

Mnemonic: “Sample, Quantize, Encode, Transmit; Decode, Reconstruct, Output”

Question 4(a) OR [3 marks]

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Compare Natural and Flat top sampling.

Answer:

Diagram:

Mnemonic: “Natural Follows, Flat Freezes”

Question 4(b) OR [4 marks]

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Explain Diode Detector circuit.

Answer:

Diode Detector Circuit: Used for demodulation of AM signals by extracting the envelope of the modulated wave.

Working:

1. Diode rectifies AM signal
2. Capacitor charges to peak value
3. RC time constant allows capacitor to follow envelope
4. Output follows the original modulating signal

Mnemonic: “Diode Detects, Capacitor Captures”

Question 4(c) OR [7 marks]

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Draw and explain the block diagram of Delta Modulation.

Answer:

Delta Modulation Transmitter:

flowchart LR
    A[Input Signal] --> B[Comparator]
    B --> C[1-bit Quantizer]
    C --> D[Transmission Channel]
    C --> E[Integrator]
    E --> B
    D --> F[To Receiver]

Delta Modulation Receiver:

flowchart LR
    A[Received Signal] --> B[Integrator]
    B --> C[Low-pass Filter]
    C --> D[Output Signal]

Limitations:

• Slope Overload: Occurs when signal changes faster than step size can track
• Granular Noise: Occurs during idle or constant parts of signal

Mnemonic: “Delta Detects Differences, Integrator Increments”

Question 5(a) [3 marks]

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Illustrate working of DPCM.

Answer:

DPCM (Differential Pulse Code Modulation): Encodes the difference between current sample and predicted value.

flowchart LR
    A[Input] --> B[Sampler]
    B --> C[Difference Generator]
    D[Predictor] --> C
    C --> E[Quantizer]
    E --> F[Encoder]
    F --> G[Transmission]
    E --> H[Inverse Quantizer]
    H --> D

• Predictor: Estimates current sample based on previous samples
• Difference: Only difference between actual and predicted is encoded
• Advantage: Reduces bit rate compared to PCM by exploiting signal correlation

Mnemonic: “Differences Predicted Create Minimized bits”

Question 5(b) [4 marks]

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Illustrate Adaptive Delta Modulation.

Answer:

Adaptive Delta Modulation (ADM): Improved version of DM that varies step size based on signal characteristics.

flowchart LR
    A[Input] --> B[Comparator]
    B --> C[Pulse Generator]
    C --> D[Step Size Adapter]
    D --> E[Integrator]
    E --> B
    C --> F[Transmission]

Operation:

1. If multiple 1’s detected: increase step size to avoid slope overload
2. If multiple 0’s detected: increase step size to track falling signal
3. If alternating 1’s and 0’s: decrease step size to reduce granular noise

Mnemonic: “Adapting Delta Makes Slopes Trackable”

Question 5(c) [7 marks]

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Illustrate TDM frame.

Answer:

TDM (Time Division Multiplexing) Frame: Structure used to combine multiple signals by assigning time slots.

Frame Structure:

TDM Hierarchy:

graph LR
    A[Primary Multiplexing 2.048 Mbps] --> B[Secondary Multiplexing 8.448 Mbps]
    B --> C[Tertiary Multiplexing 34.368 Mbps]
    C --> D[Quaternary Multiplexing 139.264 Mbps]

Mnemonic: “Frames Synchronize Time Slots During Multiplexing”

Question 5(a) OR [3 marks]

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State difference between DM and ADM.

Answer:

Mnemonic: “DM’s Fixed Steps; ADM Adapts”

Question 5(b) OR [4 marks]

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Explain the need of line coding. Explain AMI technique.

Answer:

Need for Line Coding:

• DC Component: To eliminate DC component for AC-coupled systems
• Synchronization: To provide timing information for clock recovery
• Error Detection: To enable detection of transmission errors
• Spectral Efficiency: To shape signal spectrum for efficient bandwidth use
• Noise Immunity: To provide resistance against channel noise

AMI (Alternate Mark Inversion) Technique:

Diagram:

Mnemonic: “Alternating Marks Invert Polarity”

Question 5(c) OR [7 marks]

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Draw and explain block diagram of basic PCM-TDM system.

Answer:

flowchart TD
    subgraph "PCM-TDM Transmitter"
    A1[Channel 1] --> B1[Low-pass Filter]
    A2[Channel 2] --> B2[Low-pass Filter]
    A3[Channel 3] --> B3[Low-pass Filter]
    B1 --> C1[Sample & Hold]
    B2 --> C2[Sample & Hold]
    B3 --> C3[Sample & Hold]
    C1 --> D[Multiplexer]
    C2 --> D
    C3 --> D
    D --> E[Quantizer]
    E --> F[Encoder]
    F --> G[Line Coder]
    end
    
    G --> H[Transmission Channel]
    
    subgraph "PCM-TDM Receiver"
    H --> I[Line Decoder]
    I --> J[Regenerator]
    J --> K[Decoder]
    K --> L[Demultiplexer]
    L --> M1[Hold Circuit]
    L --> M2[Hold Circuit]
    L --> M3[Hold Circuit]
    M1 --> N1[Low-pass Filter]
    M2 --> N2[Low-pass Filter]
    M3 --> N3[Low-pass Filter]
    N1 --> O1[Channel 1]
    N2 --> O2[Channel 2]
    N3 --> O3[Channel 3]
    end

Mnemonic: “Multiple Channels Sample, Quantize, Encode; Decode, Demultiplex, Filter”