Subject : Principle of Communication Engineering

Topic : Angle Modulation and Demodulation

Difficulty : High

• Modulation index is defined as the ratio of the maximum frequency deviation to the modulating frequency.

• Modulation index for FM is always more than one. It is given as:

  mf=δf/fm

  Where,

  δf is the frequency deviation and

  fm is the modulating frequency.

• Modulation index for FM is dimensionless quantity and is used to describe the depth of modulation achieved for a modulating signal with a given peak amplitude and radian frequency.

• As the modulating frequency decreases and the modulating voltage amplitude remains constant, the modulation index increases. This helps in differentiating frequency modulation from phase modulation.

• The more the amount by which carrier frequency is varied from its unmodulated value, the more is the index of modulation.

• The modulation index influences the amplitudes of the different sideband frequency components in FM which can be computed with the help of Bessel functions.

• For certain values of modulation index, the carrier can disappear completely and the entire FM wave consists of sideband components for such values.

• The increase in the modulation index for FM implies increased depth of modulation which requires more bandwidth for transmission.

• In FM signals, the efficiency and bandwidth both depend on the maximum modulating frequency and the modulation index.