Subject: Principles of Communication Engineering

Difficulty : Medium

Marks : 05

Amplitude modulation is defined as the process in which amplitude of the carrier is varied in accordance with instantaneous amplitude of the modulating signal. Here the amplitude of AM is variable but frequency and phase of the signal is constant.

Modulation index $m_a$ is defined as the ratio of amplitude of modulating signal to the amplitude of carrier signal.

$m_a = \frac{v_m}{v_c}$

If modulation index $m_a \leq1$ or if percentage of modulation is less than 100 % than type of modulation is linear.

If modulation index is greater than 1 i.e. if $m_a \gt 1$i.e. if percent modulation is greater than 100% type of modulation is over modulation.

When $m_a \gt 1$ ($m_a \gt 100%$) envelope of AM does not have exactly same shape and no one to one correspondence with the message signal is present. Over modulation introduces envelope distortion. Hence it should be avoided.

Shape of Am waveform is not saame as modulating signal if $m_a \gt1$.This will incrase the complexity of the demodulation circuit to a great extent.As a result modulation index of AM should be less than one.