Electronic Communication, Electronics

Modulation Index, | Depth of Modulation | and Percentage Modulation

It is one of the important factors in AM Modulation (Amplitude Modulation) if the Modulation index is applied less than the overdamping will occur or in simple words, the envelope will be overdamped and the Information signal will not be same at the receiver side. The same case is true when the Modulation index is made greater than the normal limit.

In order to make AM Modulation perfect, we need to study Modulation Index which is also known Modulation Depth.

In this article, we will learn about Depth of Modulation or Modulation Index Step by step. We will also cover Percentage Modulation in this article.

What is Modulation Index?

For AM Modulation (Amplitude Modulation): “Modulation Index is defined as the relationship between the amplitude of the Information signal and the amplitude of the carrier signal“.

Modulation Index is the ratio.

m=Vm/Vc

Where:

  • Vm is the amplitude voltage of modulating (Information) signal.
  • Vc is the amplitude voltage of carrier signal.

Multiplying the ratio of modulation index by 100 gives the percentage modulation.

m=Vm/Vc*100

For example, if the carrier voltage is 8 V and the modulating (information) signal voltage is 6.5 V, then the modulation index will be 0.8125

Using the above equation it can be seen that a Modulation Index of 0.90 means that the signal will increase by a factor of 0.90 and also decrease to 0.10 of its original level.

Importance of Modulation Index in AM Modulation:

The modulation index is a ratio and hence the modulation index should be a number between 0 and 1.

If the amplitude of the information signal voltage is higher than the carrier voltage, m will be greater than 1, causing distortion in AM wave.

Vm>Vc (Distortion will occur)

If the distortion is great enough, the information signal becomes not usable. Distortion of voice transmissions produces unnatural sounds or noise in the speaker. Distortion of video signals produces an inaccurate picture on a TV screen.

Effect of Modulation Index on AM Modulation:

To understand Modulation Index in the better way let’s have few examples of AM Modulated waveforms with different levels of Modulation Index.

First Case: When m=0

When there is no information signal there is no presence of AM modulation, from given picture you can see there is only carrier signal hence this condition is not useful because it does not provide us any information.

modulation index at m=0
modulation index at m=0

Second Case: When m=0.5

In this case, you can see the modulation index is 0.5, this is a condition in which AM modulation is not desirable because the carrier signal will not fall to zero references hence some amount of information is also lost in this condition.

In this condition, carrier voltage is much greater than the information signal voltage. And also this condition is not suitable for AM modulation because we do not get exact information, and in this condition, 50% modulation is achieved.

modulation index at m=0.5
modulation index at m=0.5

Third Case: When m=1

Here you can observe that carrier voltage and information signal voltages are equal hence this condition is achieved.

In this condition we get perfect AM modulation, you can see from given figure that the carrier signal falls on zero references and in this condition 100% modulation is achieved.

modulation index at m=1
modulation index at m=1

Fourth Case: When m>1

This is the worst case when the information signal voltage is much greater than the carrier signal this case happens and in this case greater than 100% modulation is achieved which is not useful because it goes in overmodulation.

In this condition we do not get exact envelope hence we do not get exact information of information signal as shown below.

modulation index at m>1
modulation index at m>1

Summary of All four Cases of Modulation Index:

modulation index
Summary of modulation index

AM modulation is widely used in AM broadcasting in order to ensure that the transmission never becomes distorted or overmodulated. They use Automatic circuits called compression circuits, which solve this problem by amplifying the lower-level signals and suppressing or compressing the higher-level signals. These circuit results in higher average power without overmodulation.

Also, read about Communication:

  1. Introduction to Electronic Communication.
  2. Types of Electronic Communication (Simplex, Duplex, and Half Duplex).
  3. Applications of Simplex, and Duplex Communication.
  4. What is Amplitude Modulation?

Image courtesy to: Learn CBSE Forum

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