Analog Communications Dr. M. Venu Gopala Rao A.M.I.E.T.E, M.Tech, Ph.D(Engg) Cert. in R.S.T ( City & Guild’s London Institute, London) F.I.E.T.E, L.M.I.S.T.E, I.S.O.I., S.S.I., M.I.A.E. Professor, Dept. of ECE, K L University mvgr03@kluniversity.in

AM: The amplitude of the carrier signal is varied according to the instantaneous value of m(t)

FM: The frequency of the carrier signal is varied according to the instantaneous value of m(t)

Lectrure - 1 AMPLITUDE MODULATION

AM-FC (or simply AM) Introduction Single Tone, Multi tone and Baseband signals Time domain description Frequency domain description Power and Current Relations 2. Modulators Switching Modulator Square Law Modulator 3. Demodulators Square Law Demodulator Detector Demodulator Envelope Detector 4. Virtues and Limitations of AM

Types of Amplitude Modulation Non Linear AM AM DSB FC Amplitude Modulation AM DSB SC Linear AM SSB VSB

Amplitude Modulation (AM) The Amplitude Modulation (A.M.) is defined as a system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating signal. DSB C: Double Side Band with Carrier DSB FC: Double Side Band with Full Carrier DSB LC: Double Side Band with Large Carrier DSB TC: Double Side Band with Transmitted Carrier

Block Schematic Diagram of AM .

Time Domain Description: The standard form of an A.M wave as a function of time is represented by where is constant called the amplitude sensitivity of the modulator. The term is called envelope of AM wave

Case1: Single Tone Modulation Let a message signal be Time domain Representation of AM where Modulation index

Freq. domain Representation

Single Tone Modulation

AM Signal Modulating Signal Carrier Signal

Transmission Bandwidth The AM transmission bandwidth is defined as the frequency range occupied between upper sideband and lower sideband. The AM transmission Band Width = USB – LSB = which is the twice of the message signal frequency (or bandwidth). LSB USB

AM for various Modulation Indices

Power Relations in AM where the total power, carrier power, lower sideband power higher sideband power Carrier Signal

Power Relations in AM If R = 1 ohm

Transmission Efficiency of AM

Carrier Power Vs Sideband Power

Ex2: In an AM transmission the carrier signal is modulated with a message signal (a) What is the modulation index? (b) What are the frequency components are available in AM signal? (c) Determine the Carrier power, sideband power and total power. (d) What is the efficiency of the AM system?

(c) Determine the Carrier power, sideband power and total power (c) Determine the Carrier power, sideband power and total power. (d) What is the efficiency of the AM system?

Ex3: A 400W carrier is modulated to a depth of 75% Ex3: A 400W carrier is modulated to a depth of 75%. Calculate the total power in the modulated wave. Also determine the total sideband power

Ex4: A broadcast radio transmitter radiates 10 KW, when the modulation percentage is 60. Determine the carrier, lower sideband, upper sideband and total sideband power.

Current Calculations Let be un-modulated current and be the total or modulated current of an AM transmitter both being rms values. If R is the resistance,

, Ex5: The antenna current of an AM transmitter is 8 A, when only the carrier is sent, but increases to 8.93 A, when the carrier is modulated by a single sine wave. Find the percentage of modulation. Determine the antenna current when the modulation index is 0.8. Ans: (a) Given that the carrier current and total currents are and The total current and carrier currents are related by or (b) For

Given an Amplitude Modulated waveform as shown. (a) Find the modulation index (b) Find the expression of the modulated wave. (c) Determine the total average power of the modulated signal, the carrier power, the USB power and the LSB power (assume unit load). (d) Determine the modulation efficiency.

Develop Matlab codes for the following problem

Case2: Multitone Modulation Let a message signal consists of three sinusoidal signals of different frequencies.

Case2: Multitone Modulation The frequency domain representation Multitone modulation is expressed as

Case2: Multitone Modulation

Modulation Index for Multitone modulating signal The total magnitude of several sinusoidal modulating signals may be represented as By dividing by on both sides of the equation, we get For three sinusoidal signals and

Ex6: A certain transmitter radiates 9 KW power with the carrier un-modulated and 10.125 KW, when the carrier is simultaneously modulated. Calculate the modulation index. If another sine wave corresponding to 40% modulation is transmitted simultaneously, determine the resultant modulation index and total radiated power.

Ex7: The antenna current of an AM broadcast transmitter modulated to a depth of 40% by an audio sine wave is 11 A. It increases to 12 A as a result of simultaneous modulation by another audio sine wave. What is the modulation index due to the second wave?

Case3: Baseband Signal Modulation Let a message signal is band limited to ‘W’ Hz Then AM signal is represented in time domain by Frequency domain representation of AM wave is given by

AM (Baseband Signal)

Transmission Bandwidth = USB-LSB = 2W

Conditions for AM The amplitude of is always less than unity. That is for all t. If envelope distortion is introduced. 2. The carrier frequency is much higher than that of message frequency W , Otherwise envelope cannot be visualized satisfactorily at the receiver.

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