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 AM power distribution

• Understand modulation by a complex waave

AM POWER DISTRIBUTION

• The average power dissipated in a load by

unmodulated carrier is given by

Pc = (0.707 Ec )2/R = (Ec )2/2R

• The upper and lower sideband powers are expressed

mathematically as

Pusb = Plsb = (m Ec/2)2/2R

= m2 Ec2 /8R

= m2 Pc /4

AM POWER DISTRIBUTION

• Total power in an AM DSBFC envelope is

Pt = Pc+ Pusb+ Plsb

= Pc+ (m2 Pc /4) + (m2 Pc /4)

= Pc+ (m2 Pc /2)

AM POWER DISTRIBUTION

• Factoring Pc gives

Pt = Pc[1+ (m2 /2)]

AM POWER DISTRIBUTION

• From the AM power analysis the following

characteristics can be noted

1. Carrier power is unaffected by the modulation

process

2. m increases, Pt increases

AM POWER DISTRIBUTION

• For m = 0, Pt is equal to Pc

• For m = 1, total sideband power is one half of carrier

power – most of the power is wasted in carrier since

information is contained in the sideband –

disadvantage of AM DSBFC

AM POWER DISTRIBUTION

AM CURRENT CALCULATION

• The relationship between carrier current and the

current of the modulated wave is

MODULATION BY COMPLEX

INFORMATION SIGNAL

• If carrier is amplitude modulated by complex wave,

the combined coefficient of modulation is given by,

mt = m12+ m2

2+ m3

2+ mn

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m1, m

2, m

3, mn @coefficient of modulation for input signals 1,2,3, and n

MODULATION BY COMPLEX

INFORMATION SIGNAL

• The upper and lower sideband and total

transmitted power is given as,

PROBLEMS

2. For an AM DSBFC wave with a peak unmodulated

carrier voltage Pc =10 Vp, a load resistance RL = 10Ω,

and a modulation coefficient m = 1, determine

(a) powers of the carrier and upper and lower

sidebands

(b) total sideband power

(c) total power of the modulated carrier

(d) draw the power spectrum