Chapter 4-Frequency Modulation and Demodulation
Page 1: Introduction to Frequency Modulation
Course: Communication Engineering Principles EE003-3-2-CEP
Institution: Asia Pacific University of Technology & Innovation
Topic: Frequency Modulation and Demodulation
Page 2: Lesson Structure
Topics Covered:
Angle Modulation
Spectrum of an FM Signal
FM Signal Generation
FM Demodulation
Page 3: Learning Outcomes
Objectives:
Analyze modulation and demodulation techniques for analogue and digital communication systems.
Perform modulation and demodulation for analogue communication systems.
Page 4: Key Terms
Relevant Terminology:
Instantaneous frequency
Frequency deviation
Carrier frequency
Instantaneous phase
Phase deviation
Carrier swing
Modulation index
Page 5: Definition of Modulation
Process: Converting low frequency signals into high frequency for transmission.
Purpose: Enables radiated signals to be recovered remotely, usually for audio, video, or data.
Amplitude Modulation (AM): Varies total power of the transmitted wave while RF carrier power remains constant.
Page 6: Advantages of Frequency Modulation (FM)
Noise Resistance: FM is less susceptible to interference compared to AM because natural noise affects amplitude rather than frequency.
Efficiency: No wasted power in the carrier; all power is useful in FM.
Band Operation: FM operates in high frequency bands (88 MHz to 108 MHz), reducing noise effects.
Audio Quality: Provides higher fidelity and less interference due to guard bands separating channels.
Applications: Used in radio broadcasting, two-way radios, satellite communications, and modems.
Page 7: Angle Modulation
Definition: The angle of the carrier wave varies with the baseband signals.
Forms of Angle Modulation:
Phase modulation (PM)
Frequency modulation (FM)
Noise Performance: Angle modulation offers better discrimination against noise than amplitude modulation.
Page 8: Performance Trade-offs
Bandwidth vs. Noise: Increased transmission bandwidth results in better noise performance.
Complexity: Increased system complexity in both transmitter and receiver compared to amplitude modulation.
Page 9: Types of Angle Modulation
Basic Types:
Phase Modulation (PM)
Frequency Modulation (FM)
Page 10: Signal Equations
FM Signal:
General form: ( x(t) = A_c ext{cos}(2 ext{π}f_c t + k_f m(t)) )
PM Signal:
General form: ( x(t) = A_c ext{cos}(2 ext{π}f_ct + heta(t)) )
Page 11: Frequency Modulation Waveforms
Characteristics:
Carrier wave: Cosine wave representation.
Modulation wave and resultant FM waveform illustrate frequency changes due to modulation.
Page 12: Phase Modulation Waveforms
Characteristics:
Similar to FM but focused on phase changes rather than frequency.
Visual representations of the carrier and modulation waveforms.
Page 13: General Angle Modulated Signal
Defining Characteristics:
Amplitude of the modulated carrier remains constant while the phase varies with the message signal ( m(t) ).
Instantaneous phase and phase deviation are critical quantities in this modulation technique.
Page 14: Instantaneous Frequency
Definitions:
Angular frequency deviation: The rate of change in instantaneous phase.
Instantaneous frequency: Related to the modulation and varies with the message signal.
Frequency sensitivity ( k_f ): Important in defining how much deviation is produced per voltage change in the modulating signal.
Page 15: Instantaneous Frequency in FM
Variability: Instantaneous frequency varies linearly with the modulating signal, defined by the relationship incorporating the carrier frequency ( f_c ) and modulation signal ( m(t) ).
Page 16: Phase Deviation in FM
Phase Deviation: Defined by the modulation index, which gives the ratio of frequency deviation to modulation frequency.
( eta ): Represents maximum departure of phase from the carrier's angle.
Page 17: Frequency Deviation in FM
Definitions:
Instantaneous frequency derived from the modulating signal impacts frequency deviation.
Frequency deviation is crucial for determining bandwidth.
Page 18: FM Signal Characteristics
Components: Visual representation of sinusoidal modulating signals, their instantaneous frequencies, and corresponding FM signals.
Page 19: Carrier Swing
Definition: Maximum change in instantaneous frequency from the lowest to highest is termed carrier swing, computed as twice the frequency deviation.
Page 20: Frequency Relations
Carrier Frequencies:
Higher frequency and lower frequency definitions, as well as the carrier frequency midpoint.
Regulation impact on maximum frequency deviation set by federal communications commission.
Page 21: Example Calculations
Example 1: Calculation of maximum frequency deviation, modulation index, and modifications in modulation voltage influence on these values.
Page 22: More Example Calculations
Example 2: Based on percent modulation, frequency deviation and sensitivity calculations are explored.
Page 23: Narrow-band vs Wide-band FM
Modulation Index Differences:
Distinction defined by narrow-band FM (( eta << 0.3 )) and wide-band FM (( eta >> 0.3 )).
Page 24: Narrow-band FM Characteristics
Relation to FM Signals: Mathematical representation illustrates how narrow-band FM behaves similarly to AM.
Page 25: Continued Narrow-band FM Characteristics
Mathematical Representations: Further details of signal relationships under narrow-band assumptions.
Page 26: Comparison to AM
Bandwidth: Narrow-band FM requires similar bandwidth as AM, but with critical differences in frequency modulation behavior.
Page 27: Bessel Functions Introduction
Bandwidth Determination: FM signal bandwidth relates to modulation index utilizing Bessel functions for calculations.
Page 28: Bessel Function Series
Series Representation: Expresses the FM signal in relation to Bessel function coefficients.
Page 29: FM Spectrum via Bessel Functions
Graphical Interpretation: Bessel functions plotted for standard analytical interpretation of FM signals.
Page 30: FM Signal Spectrum Visualization
Illustration: Theoretical amplitude spectrum plot representing FM signal components utilizing Bessel functions.
Page 31: Bessel Functions Details
Spectrum Analysis: Overview of Bessel function amplitudes and their role in defining the FM spectrum and bandwidth characteristics.
Page 32: Amplitude Spectrum Changes
Amplitude Impact: Increases in modulation index lead to reduced carrier amplitude and increased sideband amplitudes.
Page 33: Transmission Bandwidth of FM
Infinite Bandwidth: Discusses the theoretical and practical aspects of FM signal bandwidth linked to modulation index and spectral power concerns.
Page 34: Bandwidth Calculation Example
Example 3: Evaluating bandwidth for given modulation indexes and highest modulating frequencies using Bessel functions.
Page 35: Carson's Rule for Bandwidth
Rule Definition: Offers a practical method to determine FM bandwidth, emphasizing significant sideband energy and defining limits.
Page 36: Application of Carson's Rule
Comparison Exercises: Example calculations to assess bandwidth via Carson's rule versus Bessel function methodology.
Page 37: FM Modulator Analysis
Investigative Component: Structured exploration of sidebands and their amplitudes in the context of an FM modulator.
Page 38: FM Power Distribution
Power Characteristics: Relates Bessel functions and power distribution in FM across carrier and sideband contributions.
Page 39: Average Power Calculations
Total and Unmodulated Power: Comparison of power quantifications in FM with breakdowns of carrier contributions.
Page 40: Additional Power Calculations
Continued Example Work: Power evaluations relating to earlier examples and load resistance implications.
Page 41: Advantages of FM
Comparison to AM:
Reduced radiated power
Lower distortion and improved noise immunity
Minimization of geographical interference with fewer neighboring station conflicts.
Page 42: FM Signal Generation Methods
Core Methods:
Direct Method
Indirect Method
Page 43: Direct FM Signal Generation
Overview: Describes the use of VCO to produce NBFM directly through modulation techniques.
Page 44: Oscillator Behavior
Capacitor Influence: Oscillator frequency changes based on capacitor configurations directly modulated.
Page 45: Pros and Cons of Direct Generation
Benefits and Limitations: Evaluates large frequency deviations versus stability and calibration challenges inherent to VCO systems.
Page 46: Indirect FM Signal Generation
Definition: Describes how phase modulation can lead to frequency modulation outcomes.
Page 47: PM and FM Interrelation
Relationship Dynamics: Explains the interconnectedness of PM and FM modulation types.
Page 48: Wideband FM Generation
Signal Modulation: Illustrates how narrowband FM is manipulated into wideband FM through techniques like frequency multiplication.
Page 49: Multiplication Methodology
Technique Description: Involves the manipulation of narrowband signals to achieve desired wideband characteristics utilizing precise frequency multiplication.
Page 50: Quick Review Questions
Discussion Points: Examines the necessity of modulation and the specific advantages and drawbacks of FM, as well as practical distinctions between narrow and wide-band scenarios.
Page 51: Main Teaching Points Summary
Key Focus Areas: Bandwidth determination, Bessel functions, spectrum plotting, and power calculations.
Page 52: Q & A Session Overview
Engagement: invites student questions and clarifications regarding content covered.
Page 53: Upcoming Topics
Next Subject: Introduction to noise factors in communication systems.