Modulation Index Power Relations and Frequency Spectrum of

Modulation Index, Power Relations and Frequency Spectrum of AM

• Modulation Index( ): The ratio between the amplitude of message wave and the amplitude of carrier wave is known as modulation index • It is also known as modulation factor, modulation Co-efficient, degree of modulation or depth of modulation. Modulation index specifies the amount of energy of carrier wave used in the modulation.

AM Wave

• Frequency Spectrum representation of an AM signal Vc Vm/2 fc-fm Vm/2 f. C fc+fm 2 fm • Bandwidth = (fc+fm)-(fc-fm) = (fc+fm-fc+fm) = 2 fm

• The frequency spectrum of AM waveform contains three parts: • A component at the carrier frequency fc ( having highest amplitude) • An upper side band (USB), whose highest frequency component is at fc+fm • A lower side band (LSB), whose highest frequency component is at fc-fm • The bandwidth of the modulated waveform is twice the information signal bandwidth. • Because of the two side bands in the frequency spectrum its often called Double Sideband with Large Carrier. (DSB-LC) or full AM.

• Power Relations in the AM wave All 3 values are values.

• Total power in Side Bands

• Total power in Lower Side Bands & Upper side bands

• Transmission Efficiency of the AM wave is defined as the ratio of the transmitted power which contains information (sum of lower side bands and upper side band power to the total power. Percentage of Transmission Efficiency is given as

Effect of Modulation on AM wave 1. Linear modulation (Under modulation) 2. Over modulation. Linear: - If or if the modulation is less than 100% then type of modulation is linear amplitude modulation. Over modulation: - If the modulation is greater than 1(m>1) then type of modulation is called over modulation.

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