Mumbai University > Electronics Engineering > Sem4 > Fundamentals of Communication Engineering

Marks: 5M

Year: May2014

• 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:

  $\frac{m_f=δ_f}{f_m}$

  Where,

  $δ_f $ is the frequency deviation and

  $ f_m$ 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.