The maximum amplitude of an A.M. wave is found to be 15 V while its minimum amplitude is found to be 3 V. What is the modulation index?
Let the amplitude of the carrier signal be Ac and the amplitude of the message signal be Am.
Maximum amplitude, 
_______(1)
Minimum amplitude, 
_______(2)
Adding both the equations, we get
Subtracting (2) from (1), we get
The modulation index is given by:
Hence, the modulation index is 2/3.
1