Communication Systems - Class 12 Physics | Bihar Board

Communication Systems

Class 12 Physics | Bihar Board | Chapter 10

Lecture Navigation

Lecture 1: Introduction & Elements
Lecture 2: Signals & Media
Lecture 3: Wave Propagation
Lecture 4: Amplitude Modulation
Lecture 5: Detection & Components

Key Concepts

Elements of Communication System
Analog vs Digital Signals
Propagation of EM Waves
Amplitude Modulation (AM)
Demodulation Process
Antenna Fundamentals

Exam Weightage

Chapter Weight: 10 Marks
Expected Questions: 2 Short + 1 Long
Important Topics: Modulation, Wave Propagation, AM

Key Formulas

Modulation Index: \( m = \frac{A_{\text{max}} - A_{\text{min}}}{A_{\text{max}} + A_{\text{min}}} \)
AM Power: \( P_t = P_c\left(1 + \frac{m^2}{2}\right) \)
Coverage Distance: \( d = \sqrt{2Rh_t} + \sqrt{2Rh_r} \)
Antenna Height: \( h = \frac{\lambda}{4} \)

Introduction to Communication Systems & Elements

Introduction to Communication Systems

Definition: A communication system is technology for exchanging information between two points using electrical/electromagnetic signals.

Purpose:

Overcome geographical barriers
Enable real-time information exchange
Facilitate mass communication

Communication System = Transmitter + Channel + Receiver

Key Concepts:

Signal: Information-carrying quantity (voltage/current)
Information: Message (voice, image, data)
Noise: Unwanted disturbances degrading signal quality

Elements of a Communication System

Every communication system has three essential elements:

Communication System Block Diagram

Source → Transmitter → Channel → Receiver → Destination

Component	Description	Example
Transmitter	Converts message to transmittable form	Radio transmitter circuit
Transmission Channel	Physical medium carrying signal	Optical fiber, atmosphere
Receiver	Extracts original message from received signal	TV receiver circuitry

Critical Devices:

Transducer: Converts energy forms (Microphone, Speaker)
Amplifier: Boosts signal strength
Modulator/Demodulator: Alters signal for transmission

Signals & Transmission Media

Types of Communication

Mode	Characteristics	Applications
Point-to-Point	Dedicated channel between two users	Telephone, FAX
Broadcast	One-to-many transmission	TV, Radio, Satellite

Signals & Their Properties

Analog Signals:

Continuous time-varying signals
Representation: \( x(t) = A \sin(\omega t + \phi) \)
Disadvantages: Noise accumulation, difficult processing

Digital Signals:

Discrete values (0/1 or Low/High)
Advantages: Noise immunity, easy processing and storage

Signal-to-Noise Ratio: \( \text{SNR}_{\text{dB}} = 10 \log_{10}\left(\frac{P_{\text{signal}}}{P_{\text{noise}}}\right) \)

Transmission Media

Guided Media:

Medium	Bandwidth	Application
Twisted Pair	< 1 MHz	Telephone lines
Coaxial Cable	100-500 MHz	Cable TV
Optical Fiber	10¹⁴ Hz	Internet backbone

Unguided Media: Electromagnetic waves propagation

MF (300 kHz-3 MHz): AM radio
VHF (30-300 MHz): FM radio
UHF (300 MHz-3 GHz): TV, mobile

Wave Propagation & Modulation

Propagation of EM Waves

Ground Wave

Follows earth's curvature

Frequency: < 2 MHz

Application: AM radio (local)

Sky Wave

Ionospheric reflection

Frequency: 3-30 MHz

Application: Shortwave radio

Space Wave

Line-of-sight

Frequency: > 30 MHz

Application: TV, FM, Mobile

Space Wave Range: \( d = \sqrt{2Rh_t} + \sqrt{2Rh_r} \)

Where R = Earth radius (6400 km), h_t, h_r = antenna heights

Satellite Communication

Geostationary orbit: 35,786 km altitude
Uplink frequency: ~6 GHz
Downlink frequency: ~4 GHz
Applications: TV broadcasting, weather forecasting, GPS

Need for Modulation

Why Modulate?

Antenna Size Reduction: Antenna length ∝ λ/4 → Higher frequency → Smaller antenna
Multiplexing: Multiple signals on single channel (FDM, TDM)
Noise Reduction: Higher frequency signals less affected
Range Increase: Low-frequency signals attenuate quickly

Message Signal

Carrier Wave

Modulated Signal

Amplitude Modulation (AM)

Principles of Amplitude Modulation

Definition: Amplitude Modulation varies the amplitude of a high-frequency carrier wave in proportion to the instantaneous amplitude of the modulating signal.

\( s(t) = A_c[1 + k_a m(t)]\cos(\omega_c t) \)

Where:

\( A_c \) = Carrier amplitude
\( m(t) \) = Message signal
\( k_a \) = Sensitivity constant
\( \omega_c \) = Angular frequency of carrier

AM Waveform

[Graph showing carrier wave with varying amplitude]

Modulation Index

The modulation index (m) determines the depth of modulation and is given by:

\( m = \frac{A_{\text{max}} - A_{\text{min}}}{A_{\text{max}} + A_{\text{min}}} \)

Characteristics:

Range: 0 ≤ m ≤ 1 (m > 1 causes distortion)
Ideal value: 0.6-0.8
Percentage modulation = m × 100%

Frequency Spectrum of AM

Carrier frequency: \( f_c \)
Upper Sideband (USB): \( f_c + f_m \)
Lower Sideband (LSB): \( f_c - f_m \)
Bandwidth required: \( BW = 2f_m \)

Total Power: \( P_t = P_c\left(1 + \frac{m^2}{2}\right) \)

Where \( P_c \) is the carrier power

Detection & System Components

Detection and Demodulation

Definition: The process of extracting the original information-bearing signal from the modulated carrier wave.

AM Detection Process:

Rectification: Using diode to allow only positive half of AM wave
Envelope Detection: RC circuit to follow the envelope of rectified wave
Filtering: Remove RF components using capacitor

Diode Detector Circuit

[Diagram of diode detector circuit with AM input and output]

Working Principle: The diode removes the negative half of the AM wave. The capacitor charges to the peak value during each cycle and discharges through the resistor during the intervals, thereby reconstructing the original modulating signal.

Antennas & System Components

Antenna Fundamentals:

Transducer: Converts electrical energy to EM waves and vice versa
Radiation pattern: Directional properties of antenna
Gain: Measure of directionality

Minimum Antenna Height: \( h = \frac{\lambda}{4} \)

Types of Antennas:

Type	Characteristics	Application
Dipole	λ/2 length, omnidirectional	Radio receivers
Yagi-Uda	Directional, high gain	TV reception
Parabolic Reflector	Highly directional	Satellite communication

TV Tower Height Calculation:

\( h = \frac{d^2}{2R} \)

Where d = coverage distance, R = Earth radius

Bihar Board Exam Focus

10 Marks Weightage: Expect 2 short + 1 long question
Important Questions:
- AM modulation index calculation (numerical)
- Differentiate between sky wave and space wave propagation
- Explain working principle of optical fiber with diagram
- Draw block diagram of satellite communication system
Diagrams to Practice:
- AM waveform showing modulation index
- Optical fiber structure with core and cladding
- Satellite communication setup
- Block diagram of transmitter and receiver
Formulas to Remember:
- Modulation index: \( m = \frac{V_m}{V_c} \)
- Coverage distance: \( d = \sqrt{2Rh} \)
- AM power distribution: \( P_t = P_c(1 + m^2/2) \)
- Critical frequency: \( f_c = 9\sqrt{N_{\text{max}}} \)