Amplitude Modulation or AM, is a modulation technology mainly used for radio carrier wave-based message transmission which modifies the carrier wave’s amplitude (signal intensity) in accordance with the message signal, such as an audio signal, i.e., a modulating signal.

In this article, we will learn what amplitude modulation is, its definition, historical overview, equations to explain the process , its applications in telecommunication, and some related methodologies. We will also solve some numerical problems based on amplitude modulation and attempt to answer FAQs regarding this topic

What is Amplitude Modulation?

Amplitude modulation, or AM, is a modulation technology mainly used for radio carrier wave-based message transmission. Amplitude modulation modifies the carrier wave’s amplitude (signal intensity) in accordance with the message signal, such as an audio signal, i.e., a modulating signal. The mechanism of AM can be:

• A complex interaction between modulating waves and carrier signals is the basis of AM.
• By means of a rigorous modulation procedure, information is encoded for transmission by the carrier wave’s amplitude, which experiences minute variations in response to the modulating signal.
• Demodulation techniques use complex circuitry to accurately detect and amplify the modulating waveform; these techniques are crucial for recovering the original signal from AM transmissions.
• In simple terms, AM works by altering the height/magnitude of the carrier wave to match the information we want to send, then changing it back at the other end to hear or see the message.

Historical Overview

Amplitude modulation, known as AM, emerged as one of the initial modulation techniques employed for transmitting data via radio waves. It was developed during the 20th century as a result of the radiotelephone experiments conducted by Landell de Moura and Reginald Fessenden in the 1900s. Following multiple successful trials, this modulation method was further refined and implemented in electronic communication systems and has been a great boon to the field of telecommunications ever since. The history of AM, from when it started to when it became a normal part of electronic communication, shows big improvements and new ideas in technology.

Why Do We Need Modulation?

In the current world it is not necessary to have wired connection to every communication device. Within minutes, signals, messages, and information can be sent across the globe. The modulation process plays a major role in achieving this.

Modulation enables efficient and reliable communication over long distances, even in the presence of noise and interference, making it an important component of modern telecommunications. Here are some of the ways in which modulation plays a crucial role in modern telecommunications:

High Range Transmission

Modulation is essential for high-range transmission because it allows signals to be sent over long distances without degradation. In the absence of modulation, signals would quickly weaken over distance and long-distance communication would become impossible.

The height of the transmitting and receiving antennas should be similar to a quarter of the wavelength of the frequency being used for efficient communication. Therefore, the minimum antenna length required for the effective transmission of λ wavelength signals is λ/4.

h_min = λ/4 where λ is the wavelength of the modulating signal

Quality of Transmission

Through the improvement of the signal-to-noise ratio, modulation techniques increase the quality of transmission. During transmission, noise and interference may reduce the quality of signals, particularly over crowded frequency bands or noisy channels.

By distributing the signal energy across a larger frequency range, modulation reduces the signal’s sensitivity to noise and interference and enhances the transmission quality overall.

λ = Wavelength of the transmitting signal

• For small λ or high frequency ⇒ Transmission power is high and quality of transmission is high
• For high λ or small frequency ⇒ Transmission power is small and quality of transmission is low

Avoiding the Overlapping of Signals

There is a high probability of two different signals transmitted from different transmitting stations to have the same frequency which in turn causes them to get mixed up or overlap with one another. This problem requires modulation as a solution to prevent them from getting mixed up.

We can avoid signal interference by modulating each signal onto a different carrier frequency or with a different modulation method. This increases the overall efficiency and capacity of the system by allowing several users to share the same communication channel without interfering with each other’s messages.

Terms Related to Amplitude Modulation

Following terms are related to amplitude modulation:

Carrier Wave

A carrier wave is a pure wave of constant frequency analogous to a sine wave of electronic signal. It doesn’t carry much information itself that we can relate to (such as speech or data).

Modulating Signal

To include speech information or data information to the carrier wave which can be interpreted, another wave needs to be imposed, called an input signal, on top of the carrier wave. This input signal is known as the modulating signal.

Modulation

Modulation is defined as the process of superimposing a low-frequency signal which is coined as the input signal or modulating signal on a high-frequency carrier signal whose data or speech needs to be interpreted.

Amplitude Phase

Amplitude is the maximum distance from the center line to the peak; phase, on the other hand, tells us where any particle in a periodic waveform is located and frequency is the number of waves passing through a given point in a second.

• Amplitude modulation alters the carrier wave’s magnitude to convey information, which is then restored at the receiver to interpret the message.
• In contrast, Frequency modulation adjusts the carrier wave’s frequency, allowing data and speech information to be transmitted via frequency changes.
• Similarly, phase modulation varies the carrier wave’s phase to convey data and speech information. Each modulation technique offers unique methods for encoding and transmitting information

Amplitude Modulated Waveform

The combination of the modulating signal and the carrier wave produces the amplitude modulated signal, which is the final waveform as shown in the figure given below.

Expression for Amplitude Modulated Wave

Carrier wave will be generally a high frequency wave and similar to sine or cosine wave and can be represented as :

C(t)=A_csinω_ct ……..(1)

or

C(t)= A_csin( 2πf_ct)

where,

• A_c is the amplitude of carrier wave,
• sinω_ct is phase of carrier wave,
• f_c is frequency of carrier wave
• C(t) is our carrier wave,

Modulating signal is also analogous to a sine or cosine wave and represented as:

m(t)= A_msinω_mt ….. 2

or

m(t)= A_msin( 2πf_mt)

where

• A_m is the amplitude of modulating wave,
• sinω_mt is phase of modulating wave,
• f_m is frequency of modulating signal
• m(t) is our modulating or input signal.

We are now superimposing modulating signal into a carrier wave to interpret the data and speech information and thus also varying the amplitude of the carrier wave in accordance with the amplitude of the modulating signal, and hence the amplitude-modulated wave C_m(t) will be:

C_m(t) = (A_c + A_m sin ω_mt) sin ω_ct ………..(3)

This is the general form of an amplitude-modulated wave.

The degree of modulation is given by index of modulation.

From equation (3), we have,

C_m(t) = (A_c + A_m sin ω_mt) sin ω_ct

Taking Ac common in (3), we get

C_m(t)=A_c(1+(A_m/A_c)sin(ω_mt))sin(ω_ct ) …….. 4

where

A_m/A_c= μ, which is known as modulation index also known as modulation factor, modulation coefficient or degree of modulation.

Thus modulation index is defined as the ratio of the amplitude of the modulating signal to the amplitude of the carrier wave.

Modulation Index

Modulation index (μ) is defined as ratio of the amplitude of the modulating signal to the amplitude of the carrier wave. Its mathematical expression and detailed explanation is discussed later in the article.

Communication Systems and Modulation

Communication systems are essential for the transmission of information over long distances, enabling people to exchange messages, data, and signals reliably. Information is encoded, transmitted, received, and decoded in these systems using a variety of media, including wires, cables, optical fibers, and wireless channels in the form of electronic signals. They are mainly carried out in two different ways:

• Analog signal transmission
• Digital signal transmission

An analogue electronic signal can be represented in to form of either cosine or a sine wave:

m(t) = A_msin(ω_mt+θ)

or

m(t) = A_mcos(ω_mt+θ)

where

• m(t) is the modulating signal
• A_m is amplitude of the modulating signal,
• (ω_mt+θ) is the phase of the signal

Types Of Amplitude Modulation

Amplitude Modulation can be categorized into three main domains:

Double sideband-suppressed carrier modulation (DSB-SC): In this, the frequency spectrum of the modulating signal is symmetrically below and above that of the carrier signal. Sidebands are represented by the incoming information signal’s lower and upper frequencies. Higher frequency components are found in upper sidebands compared to lower sidebands and lower frequency components in carrier frequencies.

Modulation of a single sideband ( SSB): Amplification of single sideband refers to the transmission of only one sideband through an antenna. It has a sideband on either the top or lower half.

Modulation of the Vestigial Sideband (VSB): It is a solution to the problem of distracting noises created as a result of bandpass filter not having the capacity to block off frequencies outside of the cut off zone. In this instance, one of the sidebands ( upper or lower) is transmitted but a part of the other is not.

ITU type designations

In the year of 1982, the International telecommunication Union (ITU) designated the types of amplitude modulation as given in the table below: