Frequency Modulation ( FM ) and Amplitude Modulation ( AM )

Amplitude Modulation

• When the amplitude of the carrier wave is changed according to intensity of signal, it is called as amplitude modulation.
• Amplitude of carrier wave changes with modulation
• Carrier frequency remain constant with modulation
• The amplitude of signal changes according to strength of modulating signal.
• The modulation index is always less than unity for AM signal
• Modulation index in AM : it is defined as the ratio of amplitude of change of carrier and amplitude of normal carrier.

        Modulation index = Signal Amplitude / Carrier amplitude

Frequency Modulation

• When the frequency of the carrier wave changed according to intensity of signal, it is called as frequency modulation.
• Carrier frequency changes with modulation
• The carrier frequency changes with respect to strength of modulating signal.
• The value of modulating index is always less than unity.
• Modulation index in FM : It is defined as ratio of maximum frequency deviation and frequency of modulating signal.

      Modulation index = Maximum frequency deviation /                                                                       Modulating  frequency

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