Principles of Electronic Communication (4331104) - Winter 2022 Solution

Question 1(a) [3 marks]

#

What is modulation? What is the need of it?

Answer: Modulation is the process of varying one or more properties (amplitude, frequency, or phase) of a high-frequency carrier signal with a modulating signal containing information.

Need for modulation:

• Antenna size reduction: Makes practical antenna size possible (λ = c/f)
• Multiplexing: Allows multiple signals to share the medium
• Noise reduction: Improves SNR by shifting to higher frequency bands
• Range extension: Increases transmission distance

Mnemonic: “AMEN” - Antenna size, Multiplexing, Eliminate noise, New range

Question 1(b) [4 marks]

#

Derive voltage equation for Amplitude modulation.

Answer: For AM, the carrier signal is modulated by the message signal.

Mathematical derivation:

• Carrier signal: $e_c(t) = A_c \cos(2\pi f_c t)$
• Message signal: $e_m(t) = A_m \cos(2\pi f_m t)$
• Instantaneous amplitude: $A_i = A_c + e_m(t)$
• AM signal: $e_{AM}(t) = A_i \cos(2\pi f_c t)$
• Substituting: $e_{AM}(t) = [A_c + A_m \cos(2\pi f_m t)] \cos(2\pi f_c t)$
• Expanding: $e_{AM}(t) = A_c\cos(2\pi f_c t) + A_m\cos(2\pi f_m t)\cos(2\pi f_c t)$
• Final equation: $e_{AM}(t) = A_c\cos(2\pi f_c t) + \frac{A_m}{2}\cos(2\pi(f_c+f_m)t) + \frac{A_m}{2}\cos(2\pi(f_c-f_m)t)$

Mnemonic: “CAT” - Carrier, Addition, Three components (carrier + 2 sidebands)

Question 1(c) [7 marks]

#

Classify Noise signal and explain flicker noise, shot noise and thermal noise.

Answer:

Noise classification:

Types of Internal Noise:

• Flicker Noise:

  • Occurs at low frequencies (below 1 kHz)
  • Inversely proportional to frequency (1/f noise)
  • Common in semiconductor devices and carbon resistors

• Shot Noise:

  • Caused by random fluctuations of current carriers
  • White noise with constant power density
  • Occurs in active devices like diodes and transistors

• Thermal Noise:

  • Due to random motion of electrons in a conductor
  • Directly proportional to temperature and bandwidth
  • Present in all passive components
  • Also called Johnson noise or white noise

Mnemonic: “FAST” - Flicker (low frequency), Active (shot), Semiconductor (flicker), Temperature (thermal)

Question 1(c) OR [7 marks]

#

Write application of different band of EM wave spectrum.

Answer:

EM Spectrum Applications:

Mnemonic: “Every Very Lovely Monkey Has Visited Uncle Sam’s House Easily In Visible Upper Xtra Gamma” (first letter of each band)

Question 2(a) [3 marks]

#

State advantages of SSB over DSB.

Answer:

Advantages of SSB over DSB:

Mnemonic: “BPFDS” - Bandwidth, Power, Fading, Distortion, Simple

Question 2(b) [4 marks]

#

Explain generation of FM using Phase lock loop technique.

Answer:

FM Generation using PLL:

A Phase-Locked Loop (PLL) generates FM signals by applying the modulating signal to the VCO control input.

PLL FM Modulator:

graph LR
    A[Modulating Signal] --> B[Summing Circuit]
    E[Reference Oscillator] --> F[Phase Detector]
    F --> G[Low Pass Filter]
    G --> B
    B --> H[VCO]
    H --> I[FM Output]
    H --> J[Feedback]
    J --> F

Operation:

• Reference Oscillator: Provides stable reference frequency
• Phase Detector: Compares reference and feedback signals
• Low Pass Filter: Removes high-frequency components
• VCO: Generates output frequency that varies with control voltage
• Modulating Signal: Added to control voltage to produce FM output

Mnemonic: “PROVE” - Phase detector, Reference oscillator, Output VCO, Voltage controlled

Question 2(c) [7 marks]

#

Derive the equation for total power in AM, calculate percentage of power savings in DSB and SSB.

Answer:

Power in AM:

The AM wave equation: $e_{AM}(t) = A_c[1 + m\cos(2\pi f_m t)]\cos(2\pi f_c t)$

Power derivation:

• Total power: $P_T = P_c\left(1 + \frac{m^2}{2}\right)$
• Where $P_c = \frac{A_c^2}{2R}$ (carrier power) and $m$ is modulation index

Power distribution:

• Carrier power: $P_c = \frac{A_c^2}{2R}$
• Total sideband power: $P_{SB} = \frac{m^2 P_c}{2}$
• Each sideband: $P_{LSB} = P_{USB} = \frac{m^2 P_c}{4}$

Power savings:

• In DSB-SC: No carrier power, so savings = $\frac{P_c}{P_T} \times 100% = \frac{1}{1+\frac{m^2}{2}} \times 100%$
  • For m=1, savings = 66.67%
• In SSB: No carrier and one sideband, so savings = $\frac{P_c + P_{SB}/2}{P_T} \times 100%$
  • For m=1, savings = 83.33%

Mnemonic: “CEPTS” - Carrier Eliminated Provides Tremendous Savings

Question 2(a) OR [3 marks]

#

Draw and explain Time domain and Frequency domain display of AM wave.

Answer:

Time and Frequency Domain of AM:

Diagram:

Time Domain:

• Shows amplitude variation of carrier with time
• Envelope follows modulating signal
• Upper and lower envelopes = carrier peak × (1±m)

Frequency Domain:

• Shows frequency components and their amplitudes
• Carrier at frequency fc with amplitude Ac
• Two sidebands at fc±fm with amplitude mAc/2
• Bandwidth = 2fm (twice the modulating frequency)

Mnemonic: “EBS” - Envelope in time, Bandwidth in frequency, Sidebands symmetric

Question 2(b) OR [4 marks]

#

Explain pre-emphasis & de-emphasis circuit.

Answer:

Pre-emphasis and De-emphasis:

Circuit Diagrams:

Purpose:

• Pre-emphasis: Boosts high-frequency components at transmitter
• De-emphasis: Attenuates high-frequency components at receiver

Operation:

• Pre-emphasis: High-pass RC circuit (R series, C parallel)
• De-emphasis: Low-pass RC circuit (R parallel, C series)
• Time constants are identical: τ = RC = 75μs (standard)

Benefits:

• Improves SNR for higher frequencies in FM
• Compensates for higher noise power at high frequencies
• Restores original frequency response at receiver

Mnemonic: “BETH” - Boost (pre-emphasis), Emphasizes Treble, Helps SNR

Question 2(c) OR [7 marks]

#

Compare AM, FM and PM.

Answer:

Comparison of AM, FM and PM:

Mnemonic: “BANCP-MAP” - Bandwidth, Amplitude, Noise, Complexity, Power, Modulation, Applications, Parameters

Question 3(a) [3 marks]

#

Define any FOUR characteristics of radio receiver.

Answer:

Radio Receiver Characteristics:

Mnemonic: “SFIS-SS” - Sensitivity, Fidelity, Image rejection, Selectivity, SNR, Stability

Question 3(b) [4 marks]

#

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

Answer:

FM Receiver Block Diagram:

graph LR
    A[Antenna] --> B[RF Amplifier]
    B --> C[Mixer]
    D[Local Oscillator] --> C
    C --> E[IF Amplifier]
    E --> F[Limiter]
    F --> G[FM Detector]
    G --> H[Audio Amplifier]
    H --> I[Speaker]

Use of Limiter in FM Receiver:

• Primary function: Removes amplitude variations/noise
• Operation: Clips the signal to provide constant amplitude
• Benefits:
  • Eliminates AM interference
  • Improves SNR
  • Ensures proper FM detection
  • Prevents false frequency demodulation
• Location: Placed between IF amplifier and FM detector

Mnemonic: “CARE” - Clips Amplitude, Removes noise, Ensures constant signal

Question 3(c) [7 marks]

#

Draw and explain block diagram of super heterodyne receiver.

Answer:

Super Heterodyne Receiver:

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

Function of each block:

• Antenna: Captures RF signals from electromagnetic waves
• RF Amplifier: Amplifies weak signals, provides selectivity
• Local Oscillator: Generates signal to mix with incoming RF
• Mixer: Produces IF by heterodyning RF with local oscillator
• IF Amplifier: Main amplification and selectivity at fixed frequency
• Detector: Extracts audio from modulated IF signal
• Audio Amplifier: Amplifies audio signal to drive speaker
• AGC (Automatic Gain Control): Maintains constant output level
• Speaker: Converts electrical signal to sound

Super Heterodyne Principle:

• Converts high-frequency RF to fixed IF for better amplification
• IF = |RF ± LO| (typically 455 kHz for AM, 10.7 MHz for FM)

Mnemonic: “ARLMIDAS” - Antenna Receives, Local Mixes, IF Delivers, Audio Sounds

Question 3(a) OR [3 marks]

#

Draw and explain block diagram for envelope detector.

Answer:

Envelope Detector:

Circuit Diagram:

Component Functions:

• Diode (D): Rectifies AM signal (allows only positive half-cycles)
• Capacitor (C): Charges to peak of input, filters carrier frequency
• Resistor (R): Discharges capacitor, follows modulating signal envelope

Operation:

1. Diode conducts during positive half-cycles
2. Capacitor charges to peak voltage
3. During negative half-cycles, diode blocks
4. Capacitor discharges through resistor
5. RC time constant follows envelope variations

RC Selection Criteria: $\frac{1}{f_c} « RC « \frac{1}{f_m}$

Mnemonic: “DRIVER” - Diode Rectifies, RC Values Extract Envelope, Restores audio

Question 3(b) OR [4 marks]

#

What is IF? Explain its importance in brief.

Answer:

Intermediate Frequency (IF):

Definition: IF is a fixed frequency to which incoming RF signals are converted in superheterodyne receivers.

Importance of IF:

Typical IF Values:

• AM receivers: 455 kHz
• FM receivers: 10.7 MHz
• Television: 45 MHz

Mnemonic: “FIGS-ST” - Fixed frequency, Improved selectivity, Gain stability, Simplified tuning

Question 3(c) OR [7 marks]

#

Explain phase discriminator circuit for FM detection.

Answer:

Phase Discriminator for FM Detection:

Circuit Diagram:

Operation:

1. Center-tapped transformer (T2) creates 180° phase difference
2. Primary transformer (T1) sets reference phase
3. Diode D1 and D2 form phase comparators
4. When carrier at center frequency:
   • Equal currents through both diodes
   • Equal voltages across C1 and C2
   • Net output is zero
5. When frequency deviates:
   • Phase changes
   • Unequal diode currents
   • Output voltage proportional to frequency deviation

Advantages:

• Good linearity
• Reduced distortion
• Better noise performance than slope detector

Mnemonic: “PERFECT” - Phase Ensures Rectification For Extracting Carrier Transitions

Question 4(a) [3 marks]

#

Explain quantization process and its necessity.

Answer:

Quantization Process:

Definition: Quantization is the process of mapping continuous analog values to discrete digital levels.

Process:

1. Sampling converts continuous-time signal to discrete-time
2. Range of amplitudes divided into finite number of levels
3. Each sample assigned to nearest quantization level
4. Difference between original and quantized value is quantization error

Necessity of Quantization:

Mnemonic: “DENSE” - Digital conversion, Error control, Noise immunity, Storage, Efficient transmission

Question 4(b) [4 marks]

#

Give difference between DM and ADM.

Answer:

Difference between DM and ADM:

Diagram:

Mnemonic: “SAVAGES” - Step size, Adaptable, Variable tracking, Avoids overload, Granular noise reduction, Error performance, Signal fidelity

Question 4(c) [7 marks]

#

Draw & explain block diagram of PCM system.

Answer:

PCM System Block Diagram:

graph LR
    subgraph "PCM Transmitter"
    A[Input Signal] --> B[Anti-aliasing Filter]
    B --> C[Sample & Hold]
    C --> D[Quantizer]
    D --> E[Encoder]
    E --> F[Parallel to Serial]
    end

    F --> G[Transmission Channel]
    
    subgraph "PCM Receiver"
    G --> H[Serial to Parallel]
    H --> I[Decoder]
    I --> J[Reconstruction Filter]
    J --> K[Output Signal]
    end

PCM Transmitter:

• Anti-aliasing Filter: Limits input signal bandwidth to satisfy Nyquist criterion
• Sample & Hold: Converts continuous signal to discrete-time samples
• Quantizer: Approximates sample amplitudes to nearest discrete levels
• Encoder: Converts quantized levels to binary code
• Parallel-to-Serial: Converts parallel bits to serial for transmission

PCM Receiver:

• Serial-to-Parallel: Converts serial data back to parallel form
• Decoder: Converts binary code back to amplitude levels
• Reconstruction Filter: Smooths stepped output to recover analog signal

PCM Parameters:

• Sampling rate: fs > 2fm (Nyquist rate)
• Quantization levels: L = 2^n (n = number of bits)
• Resolution: Smallest distinguishable change = Vmax/L
• Bit rate: R = n × fs bits/second

Mnemonic: “SAFE-PETS” - Sample, Amplify, Filter, Encode, Pulse train, Extract, Transform, Smooth

Question 4(a) OR [3 marks]

#

Define quantization. Explain non uniform quantization in brief.

Answer:

Quantization Definition: Quantization is the process of converting continuous amplitude values to a finite set of discrete levels in analog-to-digital conversion.

Non-uniform Quantization:

Diagram:

Characteristics:

• Unequal step sizes throughout the amplitude range
• Smaller steps for low amplitudes, larger for high amplitudes
• Better matches human perception (logarithmic response)
• Improves SNR for small signals without increasing bit rate

Implementation Methods:

• Companding: Compressing at transmitter, expanding at receiver
• Logarithmic coding: μ-law (North America) and A-law (Europe)
• Adaptive quantization: Adjusts levels based on signal statistics

Mnemonic: “CLASP” - Compressed Levels, Adaptive Steps, Small steps for small signals, Perceptual matching

Question 4(b) OR [4 marks]

#

Explain Adaptive delta modulation with its application.

Answer:

Adaptive Delta Modulation (ADM):

Diagram:

graph LR
    A[Input Signal] --> B[Comparator]
    B --> C[1-bit Quantizer]
    C --> D[Transmission Channel]
    D --> E[Step Size Control]
    E --> F[Integrator]
    F --Feedback--> B
    F --> G[Output Signal]
    C --Controls--> E

Operation:

• Adapts step size based on input signal slope
• Increases step size for rapid changes (prevents slope overload)
• Decreases step size for slow changes (reduces granular noise)
• Uses previous bits pattern to determine slope changes

Advantages:

• Better signal tracking than DM
• Lower bit rate for same quality
• Reduced slope overload and granular noise
• Wider dynamic range

Applications:

• Speech and audio compression
• Voice-grade communication channels
• Digital telephony systems
• Video signal encoding
• Telemetry systems

Mnemonic: “ADAPT” - Automatically Decides Appropriate Pulse Transitions

Question 4(c) OR [7 marks]

#

What is sampling? Explain types of sampling in brief.

Answer:

Sampling Definition: Sampling is the process of converting a continuous-time signal to a discrete-time signal by taking measurements at regular intervals.

Types of Sampling:

Diagrams:

Sampling Parameters:

• Sampling period (Ts): Time between consecutive samples
• Sampling frequency (fs): Number of samples per second (fs = 1/Ts)
• Nyquist rate: Minimum sampling rate (fs > 2fm) to avoid aliasing

Mnemonic: “INFS” - Ideal (impulses), Natural (follows signal), Flat-top (constant), Sufficient rate

Question 5(a) [3 marks]

#

Define bit rate and baud rate.

Answer:

Bit Rate and Baud Rate:

Relationship:

• For binary signaling: Bit Rate = Baud Rate
• For M-ary signaling: Bit Rate = Baud Rate × log₂M
  • Where M = number of different signal elements

Example:

• 4-QAM (M=4): Each symbol carries log₂4 = 2 bits
• If Baud Rate = 1000 symbols/s, then Bit Rate = 2000 bits/s

Mnemonic: “BBSM” - Bits per second, Baud for Symbols, Modulation determines relationship

Question 5(b) [4 marks]

#

Explain working of DPCM.

Answer:

Differential Pulse Code Modulation (DPCM):

Block Diagram:

graph LR
    subgraph "Transmitter"
    A[Input] --> B[Difference]
    B --> C[Quantizer]
    C --> D[Encoder]
    D --> E[Output]
    F[Predictor] --> B
    C --> F
    end

    subgraph "Receiver"
    G[Input] --> H[Decoder]
    H --> I[Output]
    H --> J[Predictor]
    J --> I
    end

Working Principle:

• Encodes difference between current sample and predicted sample
• Prediction based on previous samples (correlation)
• Smaller dynamic range of differences allows fewer bits per sample

Advantages:

• Higher compression ratio than PCM
• Reduced bit rate for same quality
• Exploits signal correlation
• Improved SNR performance

Mnemonic: “DEEP” - Difference Encoded, Efficient Prediction, Exploits correlation, Preserves quality

Question 5(c) [7 marks]

#

The binary data 1011001 is to be transmitted using following line coding techniques: (i) Unipolar RZ and NRZ (ii) Polar RZ and NRZ (iii) AMI (iv) Manchester. Draw all the waveforms.

Answer:

Line Coding Waveforms for 1011001:

Description of Line Coding Techniques:

Mnemonic: “UPAM” - Unipolar, Polar, AMI, Manchester encoding options

Question 5(a) OR [3 marks]

#

Compare RZ and NRZ coding with example.

Answer:

Comparison of RZ and NRZ Coding:

Example for binary data 101:

Mnemonic: “BPSIDC” - Bandwidth, Power, Synchronization, Implementation, DC component

Question 5(b) OR [4 marks]

#

Explain delta modulation in brief.

Answer:

Delta Modulation (DM):

Block Diagram:

graph LR
    A[Input Signal] --> B[(Comparator)]
    B --> C[1-bit Quantizer]
    C --> D[Transmission]
    C --> E[Integrator]
    E --Feedback--> B
    D --> F[Integrator]
    F --> G[Output Signal]

Working Principle:

• Encodes only the difference between samples using 1 bit
• Comparator checks if input is higher/lower than predicted value
• Integrator accumulates the bits to approximate original signal
• Output is series of 1s and 0s representing up/down steps

Limitations:

• Slope Overload: Cannot track rapidly changing signals
• Granular Noise: Small variations around steady signal

Advantages:

• Simplest form of differential encoding
• Low bit rate (1 bit per sample)
• Simple implementation
• Hardware efficiency

Mnemonic: “SIDE” - Single-bit, Integrates Differences, Encodes changes

Question 5(c) OR [7 marks]

#

Explain PCM-TDM system.

Answer:

PCM-TDM System:

Block Diagram:

graph LR
    subgraph "Transmitter"
    A1[Channel 1] --> B1[LPF]
    A2[Channel 2] --> B2[LPF]
    A3[Channel 3] --> B3[LPF]
    A4[Channel n] --> B4[LPF]
    B1 --> C[Multiplexer]
    B2 --> C
    B3 --> C
    B4 --> C
    C --> D[Sample & Hold]
    D --> E[Quantizer]
    E --> F[Encoder]
    F --> G[Frame Generator]
    G --> H[Line Coder]
    H --> I[Transmission Medium]
    end

    subgraph "Receiver"
    I --> J[Line Decoder]
    J --> K[Frame Sync]
    K --> L[Decoder]
    L --> M[Demultiplexer]
    M --> N1[LPF]
    M --> N2[LPF]
    M --> N3[LPF]
    M --> N4[LPF]
    N1 --> O1[Channel 1]
    N2 --> O2[Channel 2]
    N3 --> O3[Channel 3]
    N4 --> O4[Channel n]
    end

PCM-TDM Operation:

System Parameters:

• Channel Capacity: N channels
• Sampling Rate: fs per channel
• Quantization: n bits per sample
• Frame Structure: 1 sample from each channel + sync
• Total Bit Rate: N × n × fs + overhead

Mnemonic: “MOST-FDR” - Multiplex, Quantize, Sample, Transmit, Frame, Demultiplex, Reconstruct