Amplitude Modulation -power Distribution 3d611k

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

w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w = q m 2 + m 2 + m 2 + mn2 1

2

3

m1 , m2 , m3 , 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