prirciples of communication engineering

7/29/2019 prirciples of communication engineering

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ANGLEMODULATION


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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


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Angle Modulation

Consider again the general carrier cccc+tV=tv cos

cc +t represents the angle of the carrier.

There are two ways of varying the angle of the carrier.

a) By varying the frequency, cFrequency Modulation.

b) By varying the phase, cPhase Modulation


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Frequency Modulation


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Phase Modulation

One of the properties of a sinusoidal wave is its phase, theoffset 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 willstart slightly later than the time at which cyclekcompletes.


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Introduction to Angle Modulation

High degree of noise immunity by bandwidth

expansion.

They are widely used in high-fidelity music

broadcasting.

They are ofconstant envelope, which is beneficial

when amplified by nonlinear amplifiers.


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Introduction to Angle Modulation


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FM and PM


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FREQUENCYMODULATION


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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


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Types of FM

Basically 2 types of FM:

NBFM (Narrow Band Frequency Modulation)

WBFM (Wide Band Frequency Modulation)


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NBFM

Derivation

Difference between NBFM & AM DSB FC

Generation of NBFM


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WBFM

Generating WBFM from NBFM

Derivation

Bessel Function


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Spectrum of FM


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Transmission Bandwidth of FM


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Carsons Rule


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Deviation Ratio


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Power in FM


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Generation of FM

Mainly there are 2 methods to generate FM Signal.

They are:

1. Direct Method1. Hartley Oscillator

2. Basic Reactance Modulator

2. Indirect Method1. Amstrong Modulator (Using NB Phase Modulator)

2. Frequency Multiplier


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Generation of FM

Basically two methods:1. Direct method

Build a voltage controlled oscillator (VCO) where thefrequency is varied in response to an applied modulatingvoltage by using a voltage-variable capacitor

The main difficulty is that it is very difficult to maintain thestability of the carrier frequency of the VCO when used togenerate 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.


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Edwin Howard Armstrong

( 1890 - 1954)

Edwin Howard Armstrong received his engineering degree in1913 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).


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Indirect Method Amstrong

Modulator


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Indirect Method


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Narrow Band Phase Modulator

(NBPM)


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Detection of FM

Types of FM Detectors:

1. RL Discriminator

2. Tuned FM Discriminator

3. Balanced Slope Detector

4. Centre Tuned Discriminator / Phase Discriminator /

Foster Seeley Discriminator

5. Phase Locked Loop (PLL) Demodulator

6. Ratio Detector


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RL Discriminator


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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

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