Amplitude modulation encodes an audio signal onto a radio carrier wave by varying the amplitude of the carrier wave in accordance with the audio signal. The modulating audio signal and carrier radio frequency signal are combined in a modulator, which varies the amplitude of the carrier wave while keeping its frequency and phase constant. The mathematical representation of AM shows the instantaneous amplitude of the modulated signal is equal to the sum of the amplitudes of the modulating signal and carrier signal.

1. Amplitude Modulation

2. Amplitude Modulation
• Modulating signal- Audio signal
• Carrier signal –Radio frequency
signal
• Electronic device used for
modulation- Modulator
• Frequency and phase of the
carrier is not changed.

3. Amplitude Modulation

4. Mathematical representation of Amplitude Modulation
 AM is defined as system of modulation in which the
instantaneous value of the carrier changes in
accordance with amplitude of the modulating
signal.
 instantaneous value of the modulating signal
em =Em sinωmt
 em – instantaneous amplitude of the modulating signal
 Em – maximum amplitude of the modulating signal
 ωm = 2πfm – angular frequency of the modulating signal

5. Contd.
 instantaneous value of the carrier signal
ec =Ec sinωct
 ec – instantaneous amplitude of the carrier signal
 Ec– maximum amplitude of the carrier signal
 ωc = 2πfc – angular frequency of the carrier signal

6. Contd.
 instantaneous value of the amplitude modulated signal
eAM =EAM sinωct
 eAM – instantaneous amplitude of the modulated signal
 EAM– maximum amplitude of the modulated signal
 ωc = 2πfc – angular frequency of the carrier signal
EAM

7. Contd.

8. Contd.

9. Example
 A carrier wave of frequency 10MHz and peak value 10Volts is amplitude
modulated by a 5KHz sine wave of amplitude 6Volts. What is the modulation
index?

10. Frequency Spectrum of AM Wave

11. Total Power of amplitude modulated wave