Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 5 Marks

Year: May 2015

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

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

   $\dfrac{m_f=\delta_f}{f_m}$

   where

   $\delta_f$ is the frequency deviation and

   $f_m$ is the modulating frequency

3. 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.

4. 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.
5. The more the amount by which carrier frequency is varied from its unmodulated value, the more is the index of modulation.
6. The modulation index influences the amplitudes of the different sideband frequency components in FM which can be computed with the help of Bessel functions.
7. For certain values of modulation index, the carrier can disappear completely and the entire FM wave consists of sideband components for such values.
8. The increase in the modulation index for FM implies increased depth of modulation which requires more bandwidth for transmission.
9. In FM signals, the efficiency and bandwidth both depend on the maximum modulating frequency and the modulation index.