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ANGLE MODULATION
Angle Modulation
• Introduction
• Types of Angle Modulation – FM & PM
• Definition – FM & PM
• Signal Representation of FM & PM
• Generation of PM using FM
• Generation of FM using PM
Angle Modulation
Consider again the general carrier vc t  = Vc cosωct + φc 
ωc t + φc  represents the angle of the carrier.
There are two ways of varying the angle of the carrier.
a) By varying the frequency, c – Frequency Modulation.
b) By varying the phase, c – Phase Modulation
Frequency Modulation
Phase Modulation
• One of the properties of a sinusoidal wave is its phase, the
offset from a reference time at which the sine wave begins.
• We use the term phase shift to characterize such changes.
• If phase changes after cycle k, the next sinusoidal wave will
start slightly later than the time at which cycle k completes.
Introduction to Angle Modulation
• High degree of noise immunity by bandwidth
expansion.
• They are widely used in high-fidelity music
broadcasting.
• They are of constant envelope, which is beneficial
when amplified by nonlinear amplifiers.
Introduction to Angle Modulation
FM and PM
FREQUENCY
MODULATION
Frequency Modulation (FM)
• Types of FM
• Spectrum of FM
• Transmission Bandwidth of FM
• Deviation Ratio
• Generation of FM
• Detection of FM
• Signal to Noise Ratio of FM
• Pre-Emphasis & De-Emphasis in FM
Types of FM
• Basically 2 types of FM:
– NBFM (Narrow Band Frequency Modulation)
– WBFM (Wide Band Frequency Modulation)
NBFM
• Derivation
• Difference between NBFM & AM DSB FC
• Generation of NBFM
WBFM
• Generating WBFM from NBFM
• Derivation
– Bessel Function
Spectrum of FM
Transmission Bandwidth of FM
Carson’s Rule
Deviation Ratio
Power in FM
Generation of FM
• Mainly there are 2 methods to generate FM Signal.
They are:
1. Direct Method
1. Hartley Oscillator
2. Basic Reactance Modulator
2. Indirect Method
1. Amstrong Modulator (Using NB Phase Modulator)
2. Frequency Multiplier
Generation of FM
• Basically two methods:
1. Direct method
• Build a voltage controlled oscillator (VCO) where the
frequency is varied in response to an applied modulating
voltage by using a voltage-variable capacitor
• The main difficulty is that it is very difficult to maintain the
stability of the carrier frequency of the VCO when used to
generate wide-band FM.
2. Indirect method
• Use a narrow-band FM modulator followed by frequency
multiplier and mixer for up conversion.
• Allows to decouple the problem of carrier frequency stability
from the FM modulation.
Edwin Howard Armstrong
(1890 - †1954)
Edwin Howard Armstrong received his engineering degree in
1913 at the Columbia University.
He was the inventor of the following basic electronic circuits
underlying all modern radio, radar, and television:
 Regenerative Circuit (1912)
 Superheterodyne Circuit (1918)
 Superregenerative Circuit (1922)
 FM System (1933).
Indirect Method – Amstrong
Modulator
Indirect Method
Narrow Band Phase Modulator
(NBPM)
Detection of FM
• Types of FM Detectors:
1.
2.
3.
4.
RL Discriminator
Tuned FM Discriminator
Balanced Slope Detector
Centre Tuned Discriminator / Phase Discriminator /
Foster – Seeley Discriminator
5. Phase Locked Loop (PLL) Demodulator
6. Ratio Detector
RL Discriminator
Signal to Noise Ratio of FM
• Introduction to Signal to Noise Ratio (SNR)
• Noise in FM
– Capture Effect
– FM Threshold Effect
– FM Threshold Reduction
– Pre-Emphasis & De-Emphasis in FM