Amplitude Modulation -Parameters

ANALOG AND DIGITAL COMMUNICATION

Presentation by:

KARTHIKA S.

AP,ECE

SSN College of Engineering

UNIT I

FUNDAMENTALS OF ANALOG

COMMUNICATION

Learning Objectives

On completion of this session you will be able to:

• understand analog communication systems

• Define Amplitude modulation

• Know about generation, frequency spectrum, bandwidth and

modulation index for amplitude modulation

INTRODUCTION

• Information signal (low freq signals) are seldom in a

form that is suitable for transmission – affected by

noise and interference – needs to be converted to a

form more suitable for transmission

• MODULATION – impressing low freq information

signals onto high freq carrier signal

• DEMODULATION – reverse process of modulation

– received signals are transformed back to original

information signal

• Amplitude Modulation is the process of changing the

amplitude of a relatively high frequency carrier signal

in proportion with the instantaneous value of the

modulating signal (information)

• AM is used in commercial broadcasting of both audio

and video signals.

• It is also used for two way mobile radio

communications such as citizens band (CB) radio.

AMPLITUDE MODULATION

AMPLITUDE MODULATION

AM ENVELOPE

• The shape of the modulated wave is called AM

envelope

• Repetition rate of the envelope is equal to modulating

signal freq

• Shape of the envelope is identical to the shape of the

modulating signal

AM GENERATION

• Linear addition will not suffice. With Linear addition we are just Shifting the carrier wave form its original position by the instantaneous value of the information signal.

• The method utilized to produce a useable AM signal to continue the carrier and intelligent signal through a non linear device

• AM modulator is a nonlinear device – nonlinear mixing occurs – output envelope is made up of

• A dc voltage

• Carrier frequency

• Components at the sum and difference frequencies of the two original frequencies (cross products)

AM GENERATION

FREQUENCY SPECTRUM OF AM

FREQUENCY SPECTRUM OF AM

• The three components that form the AM Waveform are listed below:

1. The lower-side Frequency (fc – fm(max))

2. The Carrier Frequency (fc )

3. The Upper-side Frequency (fc + fm(max))

• The band of frequencies above the carrier is term the Upper Sideband (USB).

• The band of frequencies below the carrier is term the Lower Sideband (LSB).

AM BANDWIDTH

• Equal to twice the highest modulating signal

frequency

• B = (fc + fm(max) ) – (fc – fm(max))

= 2fm(max)

PROBLEM

• For an AM DSBFC modulator with a carrier

frequency fc = 100kHz and a maximum modulating

signal frequency fm(max) = 5kHz, determine

(a) frequency limits of the upper and lower sidebands

(b) bandwidth

(c) draw the output frequency spectrum

COEFFICIENT OF MODULATION

AND PERCENT MODULATION

• Describes the amount of amplitude change percent in an AM waveform

• Also known as modulation index or modulation factor and it is symbolized by m

m = Em / Ec

0≤m ≤1

0→ no modulation

1 → 100% modulation

COEFFICIENT OF MODULATION

AND PERCENT MODULATION

• Percent modulation gives the percentage change in

the amplitude of output wave, denoted by M

M = Em / Ec * 100%

= m * 100 %

Modulation coefficient, Em and Ec

Modulation coefficient, Em and Ec

• Referring to above figure, percent modulation can be

expressed as,

M = [(Vmax – Vmin) / (Vmax + Vmin)] * 100

• Peak amplitude of upper and lower side frequencies is

given by,

Eusf = Elsf = (Vmax – Vmin) / 4

since, Em = Eusf+ Elsf

50% AND 100% MODULATED WAVE

PROBLEM

• For an AM waveform with Vmax = 18v, Vmin = 2v,

Em= 8v and Ec = 10v, determine,

(a) peak amp of the upper and lower side freq

(b) peak amp of unmodulated carrier

(c) peak change in the amp of the envelope

(d) coefficient of modulation

(e) percent modulation

Questions

• Define modulation?

• What are the types of analog modulation?

• Define amplitude modulation?

• Define coefficient of modulation and percent modulation?

• Define modulation index for AM?

Summary

On completion of this lesson you have learned:

• About analog communication systems

• To define Amplitude modulation – generation

• frequency spectrum, bandwidth and

modulation index for AM