Question 1(a) [3 marks]

Define AM, FM and PM.

Answer:

Modulation Type	Definition
AM (Amplitude Modulation)	Process where amplitude of carrier signal varies in accordance with the instantaneous amplitude of the message signal
FM (Frequency Modulation)	Process where frequency of carrier signal varies in accordance with the instantaneous amplitude of the message signal
PM (Phase Modulation)	Process where phase of carrier signal varies in accordance with the instantaneous amplitude of the message signal

Mnemonic: "AFaP" - "Amplitude, Frequency and Phase" are the three parameters changed during modulation.

Question 1(b) [4 marks]

Explain block diagram of communication system.

Answer:

Communication System

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

Components of Communication System:

Information Source: Produces message to be communicated
Transmitter: Converts message to signals suitable for transmission
Channel: Medium through which signals travel
Receiver: Extracts original message from received signal
Destination: Person/device for whom message is intended
Noise Source: Unwanted signals that interfere with transmitted signal

Mnemonic: "I Transmit Communication Reliably Despite Noise"

Question 1(c) [7 marks]

Explain Amplitude modulation with waveform and derive voltage equation for modulated signal also Sketch the frequency spectrum of the DSBFC AM.

Answer:

Amplitude Modulation is the process where the amplitude of a high-frequency carrier wave varies according to the instantaneous value of the modulating signal.

Waveform and Equation:

AM Waveform

graph TD
    subgraph Amplitude Modulation
    A["Message Signal m(t) = Am cos(ωm·t)"]
    B["Carrier Signal c(t) = Ac cos(ωc·t)"]
    C["AM Signal s(t) = Ac[1 + μ·cos(ωm·t)]cos(ωc·t)"]
    end

Derivation of AM equation:

Carrier signal: c(t) = Ac cos(ωc·t)
Modulating signal: m(t) = Am cos(ωm·t)
Modulation Index: μ = Am/Ac
AM signal: s(t) = Ac[1 + μ·cos(ωm·t)]cos(ωc·t)
Expanding: s(t) = Ac·cos(ωc·t) + μ·Ac/2·cos[(ωc+ωm)t] + μ·Ac/2·cos[(ωc-ωm)t]

DSBFC AM Frequency Spectrum:

    |
    |        Carrier
    |          |
    |          |
    |          |
    |    LSB   |   USB
    |     |    |    |
    |_____|____|____|_____
         fc-fm fc  fc+fm

Key Points:

LSB (Lower Sideband): Located at fc-fm
USB (Upper Sideband): Located at fc+fm
Bandwidth: 2fm (twice the highest modulating frequency)

Mnemonic: "CARrying Two SideBands" - DSBFC AM carries both sidebands.

Question 1(c OR) [7 marks]

Derive the equation for total power in AM, calculate percentage of power savings in DSBFC And SSBSC.

Answer:

Total Power in AM:

For AM signal s(t) = Ac[1 + μ·cos(ωm·t)]cos(ωc·t)

graph TD
    subgraph AM Power Distribution
    A[Carrier Power: Pc = Ac²/2]
    B[Total Sideband Power: PUSB + PLSB = Pc·μ²/2]
    C["Total Power: Pt = Pc(1 + μ²/2)"]
    end

Power Calculation:

Carrier Power: Pc = Ac²/2
Power in each sideband: PUSB = PLSB = Pc·μ²/4
Total Sideband Power: PUSB + PLSB = Pc·μ²/2
Total Power: Pt = Pc + PUSB + PLSB = Pc(1 + μ²/2)

Power Savings:

Modulation	Power Distribution	Power Savings
DSBFC AM	Uses carrier + both sidebands	0% (reference)
SSBSC AM	Uses only one sideband, no carrier	(2 - μ²/2)/(1 + μ²/2) × 100%

For μ = 1, SSBSC saves approximately 85% power compared to DSBFC.

Mnemonic: "SSB Saves Power By Cutting Carrier"

Question 2(a) [3 marks]

Compare AM and FM.

Answer:

Parameter	AM	FM
Definition	Amplitude of carrier varies with message signal	Frequency of carrier varies with message signal
Bandwidth	2 × message frequency	2 × (Δf + fm)
Noise Immunity	Poor (noise affects amplitude)	Excellent (noise mainly affects amplitude)
Power Efficiency	Low (carrier contains most power)	High (all transmitted power contains information)
Circuit Complexity	Simple, inexpensive	Complex, expensive

Mnemonic: "AM Needs Power, FM Fights Noise"

Question 2(b) [4 marks]

Draw and explain block diagram for envelope detector.

Answer:

graph LR
    A[AM Signal Input] --> B[Diode]
    B --> C[RC Circuit]
    C --> D[Output Signal]

Components of Envelope Detector:

Diode: Rectifies the AM signal (allows current flow in one direction)
RC Circuit: R and C values chosen such that:
- RC >> 1/fc (to filter carrier frequency)
- RC << 1/fm (to follow the envelope)

Working:

Diode conducts during positive half-cycles of carrier
Capacitor charges to peak value
When input falls, capacitor discharges through resistor
Output follows envelope of AM signal

Mnemonic: "Detect, Rect, and Connect" - Detection through Rectification and RC connection.

Question 2(c) [7 marks]

Draw block diagram of FM radio receiver and explain working of each block.

Answer:

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

Working of Each Block:

Antenna: Receives FM broadcast signals (88-108 MHz)
RF Amplifier: Amplifies weak RF signals, provides selectivity
Mixer & Local Oscillator: Converts RF to fixed IF (10.7 MHz) using heterodyning
IF Amplifier: Provides most of receiver's gain and selectivity
Limiter: Removes amplitude variations from FM signal
FM Detector: Converts frequency variations to audio (uses ratio detector/PLL)
Audio Amplifier: Amplifies recovered audio signal
Speaker: Converts electrical signals to sound

Mnemonic: "Really Mighty Instruments Limit Frequency And Make Sound"

Question 2(a OR) [3 marks]

Define Sensitivity, Selectivity, Fidelity for radio receiver.

Answer:

Parameter	Definition
Sensitivity	Ability of receiver to amplify weak signals (measured in μV)
Selectivity	Ability to separate desired signal from adjacent signals
Fidelity	Ability to reproduce the original signal without distortion

Mnemonic: "SSF" - "Select Signals Faithfully"

Question 2(b OR) [4 marks]

Explain ratio detector for FM.

Answer:

Ratio Detector

graph LR
    A[FM Input] --> B[Secondary Winding]
    B --> C[Diode D1]
    B --> D[Diode D2]
    C --> E[Capacitor C1]
    D --> F[Capacitor C2]
    E --> G[Output]
    F --> G
    E --> H[Stabilizing Capacitor C3]
    F --> H

Working of Ratio Detector:

Uses balanced circuit with two diodes in series
Large stabilizing capacitor keeps sum of voltages constant
Output voltage is proportional to frequency deviation
Inherently insensitive to amplitude variations (no limiter needed)
Less susceptible to impulse noise than discriminator

Mnemonic: "RADS" - "Ratio And Diodes Stabilize"

Question 2(c OR) [7 marks]

Draw block diagram of AM radio receiver and explain working of each block.

Answer:

AM Radio Receiver

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

Working of Each Block:

Antenna: Intercepts AM broadcast signals (535-1605 kHz)
RF Amplifier: Amplifies weak RF signals with good SNR
Mixer & Local Oscillator: Converts RF to fixed IF (455 kHz)
IF Amplifier: Provides most gain and selectivity at 455 kHz
Detector: Extracts audio from AM signal (envelope detector)
AGC (Automatic Gain Control): Maintains constant output level
Audio Amplifier: Boosts detected audio to drive speaker
Speaker: Converts electrical signals to sound waves

Mnemonic: "ARMIDAS" - "Amplify, Mix, IF, Detect, Audio, Speak"

Question 3(a) [3 marks]

Describe the Nyquist criteria.

Answer:

Nyquist Criteria: To accurately reconstruct a signal from its samples, the sampling frequency (fs) must be at least twice the highest frequency (fmax) present in the signal.

Parameter	Formula	Description
Nyquist Rate	fs ≥ 2fmax	Minimum sampling rate required
Nyquist Interval	Ts ≤ 1/2fmax	Maximum time between samples

Consequence if violated: Aliasing occurs - higher frequencies appear as lower frequencies in sampled signal.

Mnemonic: "Sample Double to Dodge Aliasing"

Question 3(b) [4 marks]

Explain Sample and hold Circuit with Waveform.

Answer:

Sample and Hold Circuit

graph LR
    A[Analog Input] --> B[Electronic Switch]
    C[Clock] --> B
    B --> D[Capacitor]
    D --> E[Buffer]
    E --> F[Output]

Sample and Hold Circuit Operation:

Electronic Switch: Closes briefly during sampling
Capacitor: Stores sampled voltage
Buffer Amplifier: Provides high input impedance and low output impedance

Waveform:

Sample and Hold Waveform

Applications:

Analog-to-Digital Conversion
Data Acquisition Systems
Pulse Amplitude Modulation

Mnemonic: "SCAB" - "Switch, Capacitor And Buffer"

Question 3(c) [7 marks]

Define quantization explain uniform and non-uniform quantization in details.

Answer:

Quantization: Process of mapping a large set of input values to a smaller set of discrete output values.

Quantization Process

graph LR
    A[Continuous Amplitude] --> B[Discrete Amplitude]
    B --> C[Digital Code]

Uniform Quantization vs Non-uniform Quantization:

Parameter	Uniform Quantization	Non-uniform Quantization
Step Size	Equal throughout range	Varies (smaller for small signals)
Characteristic	Linear	Non-linear (logarithmic/exponential)
SNR	Poor for small signals	Better for small signals
Implementation	Simple	Complex (companding required)
Applications	Simple signals, images	Speech, audio (μ-law, A-law)

Quantization Error:

Difference between original and quantized signal
Maximum error = ±Q/2 (where Q is quantization step size)
Appears as quantization noise in reconstructed signal

Mnemonic: "UNIQ" - "UNIform has equal steps, non-uniform Quiets noise"

Question 3(a OR) [3 marks]

Explain aliasing error and how to overcome it.

Answer:

Aliasing Error: Distortion that occurs when a signal is sampled at a rate lower than twice its highest frequency component.

graph TD
    A[Aliasing Error]
    A --> B[Original high frequencies appear as false low frequencies]
    A --> C[Causes distortion that cannot be removed after sampling]

How to Overcome Aliasing:

Use anti-aliasing filter (low-pass) before sampling
Increase sampling rate above Nyquist rate (fs > 2fmax)
Bandlimit the input signal before sampling

Mnemonic: "ALIAS" - "Avoid Low sampling by Increasing And Screening"

Question 3(b OR) [4 marks]

Draw following signal in time domain and frequency domain: 1) Sawtooth signal 2) Pulse signal

Answer:

Sawtooth Signal:

Time Domain:

    /|  /|  /|  /|
   / | / | / | / |
  /  |/  |/  |/  |
     T   2T  3T

Frequency Domain:

    |
    |
    |\
    | \
    |  \
    |   \
    |____\____________
    0  f0 2f0 3f0 4f0

Pulse Signal:

Time Domain:

    |‾|     |‾|     |‾|
    | |     | |     | |
____|_|_____|_|_____|_|____
    T       2T      3T

Frequency Domain:

    |
    |    sinc function
    |\       /\
    | \     /  \
    |  \___/    \____
    |
    |___________________
    0   f0    2f0    3f0

Mnemonic: "STPF" - "SawTooth slopes down, Pulse has sinc Function"

Question 3(c OR) [7 marks]

Compare PAM, PWM and PPM with waveform.

Answer:

Parameter	PAM	PWM	PPM
Full Form	Pulse Amplitude Modulation	Pulse Width Modulation	Pulse Position Modulation
Parameter Varied	Amplitude of pulses	Width/duration of pulses	Position/timing of pulses
Noise Immunity	Poor	Good	Excellent
Bandwidth	Lower	Higher	Highest
Power Efficiency	Low	Medium	High
Demodulation	Simple	Moderate	Complex

Waveforms:

Message:    /\/\/\

PAM:        ‖  ‖   ‖ ‖  ‖   ‖
            ‖  ‖   ‖ ‖  ‖   ‖

PWM:        ‖‖‖ ‖‖  ‖ ‖‖‖ ‖‖  ‖
                    
PPM:        ‖  ‖   ‖ ‖  ‖   ‖
            |--|---||-|--|---||

Mnemonic: "APP" - "Amplitude, Pulse-width, Position"

Question 4(a) [3 marks]

Explain Space wave propagation.

Answer:

Space Wave Propagation: Mode where radio waves travel through lower atmosphere (troposphere) directly or via ground reflection.

Space Wave Propagation

graph LR
    A[Transmitter] --> B[Direct Wave]
    A --> C[Ground Reflected Wave]
    B --> D[Receiver]
    C --> D

Characteristics:

Frequency range: VHF, UHF (30 MHz - 3 GHz)
Limited to line-of-sight distance
Range = 4.12(√h₁ + √h₂) km (where h₁, h₂ = heights in meters)
Affected by terrain, buildings, and atmospheric conditions

Mnemonic: "SLOT" - "Straight Line Over Terrain"

Question 4(b) [4 marks]

Explain working of Differential PCM (DPCM) transmitter.

Answer:

DPCM Transmitter

graph LR
    A[Input Signal] --> B[Quantizer]
    B --> C[Encoder]
    C --> D[Output DPCM]
    C --> E[Inverse Quantizer]
    E --> F[Predictor]
    F -- Predicted value --> G{Subtractor}
    A --> G
    G -- Difference --> B

Working of DPCM Transmitter:

Predictor: Estimates current sample based on previous samples
Subtractor: Computes difference between actual and predicted value
Quantizer: Converts difference signal to discrete levels
Encoder: Converts quantized values to binary code
Feedback Loop: Reconstructs signal as receiver would see it

Advantage: Only difference signal is transmitted, which requires fewer bits

Mnemonic: "SPEQIF" - "Subtract, Predict, Encode, Quantize In Feedback"

Question 4(c) [7 marks]

Explain delta modulator in details also explain slop overload noise and granular noise.

Answer:

Delta Modulation (DM): Simplest form of differential PCM where the difference signal is encoded with just 1 bit.

Delta Modulator

graph LR
    A[Input Signal] --> B{Comparator}
    B --> C[1-bit Quantizer]
    C --> D[Output DM] 
    C --> E[Integrator]
    E -- Predicted value --> B

Working Principle:

Compares input signal with integrated version of previous output
If input > integrated value: transmit 1
If input < integrated value: transmit 0
Step size (δ) is fixed

Noise in Delta Modulation:

Type of Noise	Cause	Solution
Slope Overload Noise	Input signal changes faster than δ can track	Increase step size or sampling frequency
Granular Noise	Step size too large for slowly varying signals	Decrease step size

Mnemonic: "DOGS" - "Delta modulation has Overload and Granular noiseS"

Question 4(a OR) [3 marks]

Explain Ground wave propagation.

Answer:

Ground Wave Propagation: Radio wave propagation that follows the curvature of the Earth.

graph LR
    A[Transmitter] --> B[Ground Wave]
    B --> C[Receiver]
    D[Earth Surface] --- B

Characteristics:

Frequency range: LF, MF (30 kHz - 3 MHz)
Propagates along Earth's surface (vertically polarized)
Range depends on transmitter power, ground conductivity, frequency
Signal strength decreases with distance and frequency
Used for AM broadcasting, marine communication

Mnemonic: "GEL" - "Ground waves follow Earth at Low frequencies"

Question 4(b OR) [4 marks]

Explain ADM transmitter.

Answer:

Adaptive Delta Modulation (ADM): Improved version of DM where step size varies according to signal characteristics.

graph LR
    A[Input Signal] --> B{Comparator}
    B --> C[1-bit Quantizer]
    C --> D[Output ADM]
    C --> E[Step Size Controller]
    E --> F[Integrator]
    F -- Predicted value --> B

Working of ADM Transmitter:

Basic Operation: Similar to standard DM
Step Size Control: Analyzes recent output bits
Adaptation Logic:
- If consecutive bits are same: Increase step size
- If consecutive bits alternate: Decrease step size

Advantages over DM:

Reduces both slope overload and granular noise
Better signal tracking
Improved SNR

Mnemonic: "ASIC" - "Adapt Step-size, Improve Coding"

Question 4(c OR) [7 marks]

Explain Block diagram of basic PCM-TDM system.

Answer:

PCM-TDM System: Combines Pulse Code Modulation with Time Division Multiplexing to transmit multiple digital signals over single channel.

graph LR
    subgraph "Transmitter"
    A1[Analog Input 1] --> B1[Sample & Hold]
    A2[Analog Input 2] --> B2[Sample & Hold]
    A3[Analog Input n] --> B3[Sample & Hold]
    B1 --> C[Multiplexer]
    B2 --> C
    B3 --> C
    C --> D[Quantizer]
    D --> E[Encoder]
    E --> F[Frame Formatter]
    end
    
    F --> G[Transmission Channel]
    
    subgraph "Receiver"
    G --> H[Frame Synchronizer]
    H --> I[Decoder]
    I --> J[Demultiplexer]
    J --> K1[LPF 1]
    J --> K2[LPF 2]
    J --> K3[LPF n]
    K1 --> L1[Output 1]
    K2 --> L2[Output 2]
    K3 --> L3[Output n]
    end

Working of PCM-TDM System:

Transmitter:
- Multiple analog signals sampled simultaneously
- Samples time-multiplexed into single stream
- Stream quantized and encoded into PCM format
- Framing bits added for synchronization
Receiver:
- Frame sync detected for alignment
- PCM stream decoded to recover samples
- Demultiplexer separates individual channel samples
- Low-pass filters reconstruct original analog signals

Mnemonic: "SAMPLE-CODE-MUX" - Sampling, Coding, and Multiplexing

Question 5(a) [3 marks]

Define radiation pattern, Directivity and Gain for antenna.

Answer:

Antenna Parameters

Parameter	Definition
Radiation Pattern	Graphical representation of radiation properties (field strength or power) as function of space coordinates
Directivity	Ratio of maximum radiation intensity to average radiation intensity
Gain	Product of directivity and efficiency (practical measure of antenna performance)

Relationship: Gain = Directivity × Efficiency

Mnemonic: "RDG" - "Radiation Directs with Gain"

Question 5(b) [4 marks]

Explain Microstrip Antenna with sketch.

Answer:

Microstrip (Patch) Antenna: Low-profile antenna consisting of a metal patch on a substrate with ground plane.

Microstrip Antenna

graph TD
    subgraph "Microstrip Antenna Structure"
    direction LR
    A[Radiating Patch]
    B[Dielectric Substrate]
    C[Ground Plane]
    D[Feed Point]
    
    A --- B
    B --- C
    D --- A
    end

Key Features:

Patch: Typically rectangular or circular (λ/2 in length)
Substrate: Low-loss dielectric material (εr = 2.2 to 12)
Feeding Methods: Microstrip line, coaxial probe, aperture coupling
Radiation: Primarily from fringing fields at patch edges

Applications: Mobile devices, GPS, RFID, satellite communications

Mnemonic: "PSDG" - "Patch on Substrate with Dielectric over Ground"

Question 5(c) [7 marks]

Explain PCM transmitter and receiver in details.

Answer:

PCM (Pulse Code Modulation) Transmitter:

graph LR
    A[Analog Input] --> B[Anti-aliasing Filter]
    B --> C[Sample & Hold]
    C --> D[Quantizer]
    D --> E[Encoder]
    E --> F[Parallel to Serial]
    F --> G[Line Coder]
    G --> H[PCM Output]

PCM Receiver:

PCM System

graph LR
    A[PCM Input] --> B[Regenerative Repeater]
    B --> C[Line Decoder]
    C --> D[Serial to Parallel]
    D --> E[Decoder]
    E --> F[Reconstruction Filter]
    F --> G[Analog Output]

Working Details:

Block	Function
Anti-aliasing Filter	Limits bandwidth to prevent aliasing
Sample & Hold	Takes samples at regular intervals
Quantizer	Assigns discrete amplitude levels
Encoder	Converts levels to binary codes
Line Coder	Converts digital data to transmission format
Regenerative Repeater	Restores signal quality
Decoder	Converts binary to amplitude levels
Reconstruction Filter	Smoothens staircase output to analog

Mnemonic: "SAFE PCR" - "Sample, Amplify, Filter, Encode, Pulse Code Receiver"

Question 5(a OR) [3 marks]

Explain dipole antenna with sketch.

Answer:

Dipole Antenna: Simplest and most widely used antenna consisting of two conducting elements.

graph LR
    A[Feed Point] --- B[λ/4 Wire]
    A --- C[λ/4 Wire]
    D[Total Length = λ/2] -.-> B
    D -.-> C

Key Characteristics:

Length: Typically λ/2 (half-wavelength dipole)
Radiation Pattern: Figure-8 pattern perpendicular to antenna axis
Impedance: ~73 Ω for half-wave dipole
Polarization: Same as the orientation of the antenna

Applications: Radio broadcasting, TV reception, amateur radio

Mnemonic: "HALF" - "Half-wavelength Antenna Leads Field"

Question 5(b OR) [4 marks]

Explain parabolic reflector antenna With Sketch.

Answer:

Parabolic Reflector Antenna: High-gain antenna using parabolic dish to focus electromagnetic waves.

graph LR
    A[Feed Horn] --> B[Parabolic Reflector]
    B --> C[Focused Beam]
    D[Focal Point] -.-> A

Working Principle:

Feed: Located at focal point of parabola
Reflector: Parabolic surface reflects waves in parallel direction
Reflection Property: All paths from focal point to reflector to parallel line are equal

Applications:

Satellite communications
Radio astronomy
Radar systems
Microwave links

Mnemonic: "PROF" - "Parabola Reflects On Focus"

Question 5(c OR) [7 marks]

Compare PCM, DM, ADM and DPCM.

Answer:

Parameter	PCM	DM	ADM	DPCM
Full Form	Pulse Code Modulation	Delta Modulation	Adaptive Delta Modulation	Differential PCM
Bits per Sample	8-16 bits	1 bit	1 bit	3-4 bits
Step Size	Fixed quantization levels	Fixed step size	Variable step size	Fixed quantization of difference
Bandwidth Requirement	Highest	Lowest	Low	Medium
Signal Quality	Excellent	Poor to moderate	Moderate	Good
Implementation Complexity	Moderate	Very simple	Moderate	Complex
Applications	Digital audio, telephony	Simple telemetry	Voice communication	Video, speech

Key Differences:

PCM: Encodes absolute amplitude values
DM: Encodes only 1-bit difference with fixed step
ADM: Improves DM by adapting step size
DPCM: Encodes multi-bit difference signal

Mnemonic: "PAID" - "PCM, ADM, Integrate in DPCM"