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.
hmin = 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)=Acsinωct ……..(1)
or
C(t)= Acsin( 2πfct)
where,
- Ac is the amplitude of carrier wave,
- sinωct is phase of carrier wave,
- fc 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)= Amsinωmt ….. 2
or
m(t)= Amsin( 2πfmt)
where
- Am is the amplitude of modulating wave,
- sinωmt is phase of modulating wave,
- fm 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 Cm(t) will be:
Cm(t) = (Ac + Am sin ωmt) sin ωct ………..(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,
Cm(t) = (Ac + Am sin ωmt) sin ωct
Taking Ac common in (3), we get
Cm(t)=Ac(1+(Am/Ac)sin(ωmt))sin(ωct ) …….. 4
where
Am/Ac= 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) = Amsin(ωmt+θ)
or
m(t) = Amcos(ωmt+θ)
where
- m(t) is the modulating signal
- Am is amplitude of the modulating signal,
- (ωmt+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:
|
Designation |
Description |
|---|---|
|
A3E |
DOUBLE-SIDEBAND A FULL CARRIER |
|
R3E |
SINGLE-SIDEBAND REDUCED-CARRIER |
|
H3E |
SINGLE-SIDEBAND FULL-CARRIER |
|
J3E |
SINGLE-SIDEBAND SUPPRESSED-CARRIER |
|
B8E |
INDEPENDENT-SIDEBAND EMISSION |
|
C3F |
VESTIGIAL-SIDEBAND |
|
LINCOMPEX |
LINKED COMPRESSOR AND EXPANDER |
Linear Modulation and Over Modulation
There are two categories of amplitude modulated wave determined from the value of modulation index μ.
Linear Modulation: Linear modulation is observed when the modulation index μ is less than 1.
Over Modulation: Over modulation is observed when the modulation index μ is greater than 1.
Linearly modulated waves are preferred over overly modulated ones. This is due to the fact that the overmodulated wave’s amplitude eventually drops to zero. It indicates that the amplitude is not changed in accordance with the message signal at that point. hence, the signal will be distorted.
Transmission Efficiency
The transmission efficiency (η) of AM wave is defined as the percentage of total power contributed by side bands of the AM signal. In other words, it is the ratio of the power used to send the message signal to the total power used to send or transmit data over the AM wave system.
As soon as the broadcast signal is received, it is necessary to separate the message signal from the carrier wave. Therefore, there is a difference between the total power consumption and the power needed to convey the message signal.
It is related to the modulation index as given by the equation:
η = μ2/(2+μ2) × 100 % ….. 5
Frequencies of Amplitude Modulated Wave
From equation (4), we have :
Cm(t) = Ac(1+(Am/Ac)sin(ωmt))sin(ωct ) = Acsin(ωct) + Acμsin(ωmt)sin(ωct)
The above equation can be re-written as :
Cm(t) = Acsin(ωct) + (Acμ/2) × [cos(ωc-ωm) – cos(ωc+ωm)] ( ∵ sinA.sinB = (1/2) × [cos(A-B)-cos(A+B)]
Cm(t) = Acsin(ωct) + (Acμ/2) × cos(ωc-ωm)- (Acμ/2) × cos(ωc+ωm) ……………….. 6
From the above equation, we can see that amplitude modulated wave is the sum of three sine or cosine waves.
Thus, there are three frequencies in amplitude modulated waves- f1 ,f2 and f3 corresponding to ωc, ωc + ωm and ωc – ωm respectively.
ω1 = ωc ⇒ it is corresponding f1 = fc
ω2 = ωc + ωm ⇒ it is corresponding f2 = fc+fm
ω3 = ωc – ωm ⇒ it is corresponding f3 = fc-fm
where
- fc is Carrier wave frequency
- fc + fm is Upper side band frequency
- fc – fm is Lower side band frequency
- fm is Modulating signal frequency
In general fc >> fm
Bandwidth
Bandwidth (BW) is defined as the difference between the highest and lowest frequencies of the signal.
BW = Upper sideband frequency (fc + fm) – Lower sideband frequency (fc – fm)
or
BW= fmax – fmin
BW= (fc + fm) – (fc – fm)= fc + fm – fc + fm= 2fm
BW= 2fm = Twice the frequency of the modulating signal
Advantages and Disadvantages of Amplitude Modulation
The advantages and disadvantages of amplitude modulation is tabulated below:
|
Advantages |
Disadvantages |
|---|---|
|
Most straightforward mode of modulation and easier to implement. |
It is not so efficient when comes to power usage. |
|
Demodulation of the transmitted wave is easily carried out using a few components and a circuit. |
There is requirement of a very high bandwidth that is equivalent to the highest audio frequency. |
|
It is a cheaper and cost-effective technique of modulation. |
There is high signal attenuation due to noise interference which is highly noticeable. |
Applications of Amplitude Modulation
Amplitude Modulation ( AM) is a modulation technique employed in diverse applications across different fields. Here are some key applications of amplitude modulation :
1. Radio Broadcasting: AM is frequently utilized to send audio signals over great distances in radio broadcasting. The amplitude of a high-frequency carrier wave in AM radio is modulated by the audio signal (music, voice, etc.), which enables the wave to transport audio data.
2. Shortwave Broadcasting: AM modulation is used in shortwave radio broadcasting to send signals over great distances. Large geographic areas can be covered via shortwave broadcasts, which are frequently utilized for distant locations, emergency communication, and worldwide broadcasting.
3. Wireless Communication: AM modulation is used in a number of wireless communication systems, including audio transmission systems, wireless intercom systems, and wireless microphones. It offers an easy-to-use and efficient way to wirelessly transmit audio signals.
Demodulation Techniques
Demodulation methods in amplitude modulation (AM) are techniques employed to extract the original modulating signal (such as audio and speech) from the modulated carrier wave. Several demodulation methods are commonly employed in AM systems:
Envelope Detection
To recover the original modulating signal, it involves detecting the modulated signal to create a unipolar waveform, and then smoothing the resulting signal. Although envelope detection is simple and cheap, it performs poorly in noisy environments and introduces distortion.
Diode Demodulation
The process of diode demodulation involves rectifying the modulated signal using diodes. The diode produces a waveform that depicts the modulating signal’s envelope by allowing just half of the modulated waveform to pass through.
Difference between AM and FM
The differences between Amplitude Modulation and Frequency Modulation can easily learnt form the table added below:
|
Amplitude Modulation |
Frequency Modulation |
|---|---|
|
AM has a bad sound quality as compared to FM |
FM has a good sound quality as compared to AM |
|
It is cheaper technology |
It is expensive |
|
AM can be sent over a large distance |
FM can be blocked by things like walls and can’t sent over long distance |
|
Less power efficient |
More power efficient |
|
Shorter broadcast range |
Long Broadcast Range |
|
Modulates amplitude of the carrier signal |
Modulates frequency of carrier signal |
Also, Check
Amplitude Modulation Solved Examples
Example 1: A carrier wave of frequency 3 MHz with a pack voltage of 30V is used to modulate a signal of frequency 2kHz with a pack voltage of 25V. Find the following:
i) Modulation index
ii) Frequencies of the modulated wave
iii) Bandwidth
Solution:
Given, Am = Amplitude of modulating wave = 25 V
Ac = Amplitude of carrier wave = 30 V
i) Modulation index, μ = Am/Ac = 25/30 = 0.83
ii) Frequencies of modulated wave is given by fc, fc + fm, fc-fm
fc = 3 MHz = 3 × 106 Hz, fm= 2 kHz = 2 × 103 Hz
fc + fm = 3 × 106 + 2 × 103 = 3002 × 103 Hz = 3002 kHz (Upper sideband frequency)
fc – fm = 3 × 106 – 2 x 103 = 2998 x 103 Hz = 2998 kHz (Lower sideband frequency)
iii) Bandwidth (BW)
BW = Upper side band frequency – Lower side band frequency
BW = (fc + fm) – ( fc – fm )
= 2fm = 2 × 2 kHz = 4kHz
Example 2: A modulating signal m(t) = 10cos(2π × 103t) is amplitude modulated with a carrier signal c(t) = 50cos(2π × 105t). Find the modulation index and band width.
Solution:
Given, the equation of modulating signal is :
m(t) = 10cos (2π × 103t)
It is known to us that the standard equation of modulating signal is given as :
m(t) = Am cos(2πfmt)
By comparing the above two equations, we will obtain
Amplitude of modulating signal, Am = 10 V and frequency of modulating signal, fm = 103 Hz =1kHz
Given, the equation of carrier signal is:
C(t) = 50cos(2π × 105t)
It is known to us that the standard equation of carrier signal is:
C(t) = Accos(2πfct)
By comparing these two equations, we will obtain
Amplitude of the carrier signal as Ac = 50 V and frequency of the carrier signal as fc = 105 Hz = 100KHz
So modulation index can be determined from the equation , μ = Am/Ac
Therefore, μ = 10/50 = 0.2
The value of modulation index is thereby 0.2 and percentage of modulation is 20% ( μ × 100 %)
Band width is given by , BW = 2fm = 2 × 103 Hz = 2 kHz
Amplitude Modulation FAQs
What is meaning of amplitude modulation?
Amplitude modulation is a modulation technique employed in electronic communication and modifies the carrier wave’s amplitude (signal intensity) in accordance to the message signal, such as an audio signal i.e. a modulating signal.
What is a modulation index?
Modulation index (μ) is defined as ratio of the amplitude of the modulating signal to the amplitude of the carrier wave. It is given as μ = Am/Ac where Am is amplitude of modulating signal and Ac is amplitude of carrier signal.
Who developed the amplitude modulation technique?
Amplitude modulation was developed by Landell de Moura and Reginald Fessenden in 1900s.
Give the expression for amplitude modulated wave.
Amplitude modulated wave is given by: Cm(t) = (Ac + Am sin ωmt) sin ωct
How do you differentiate between amplitude modulation and frequency modulation?
The difference lies in the modulation or alteration of the carrier wave. Amplitude modulation works by altering the amplitude 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.
It differs from frequency modulation or FM where the frequency of the carrier wave is altered by imparting the input signal and thus the users are able to use changes of frequency to carry data and speech information.
Which one is preferred out of frequency modulation and amplitude modulation?
There is lot of noise interference in amplitude modulation causing the attenuation of the signal. Owing to the increased bandwidth in case of frequency modulation, frequency modulation has better sound quality and is the preferable one.
What is the band width for amplitude modulated (AM) wave?
Band width (BW) is defined as the difference between the extreme frequencies.
BW = Upper sideband frequency (fc + fm) – Lower sideband frequency (fc – fm).
What are the frequency components in an AM wave?
There are three frequencies in amplitude modulated waves f1 ,f2 and f3 corresponding to ωc, ωc + ωm and ωc – ωm respectively.
ω1 = ωc is corresponding f1 = fc which is the carrier wave frequency
ω2 = ωc + ωm is corresponding f2 = fc+fm which is upper sideband frequency
ω3 = ωc – ωm is corresponding f3 = fc-fm which is lower sideband frequency.
What is transmission efficiency in AM wave?
The transmission efficiency (η) of AM wave is defined as the percentage of total power contributed by side bands of the AM signal.
What is meant by demodulation?
Demodulation is defined as the process of extracting a modulating/input signal from the modulated signal. In simpler terms, demodulation or detection is the process by which the message is recovered from the modulated signal at receiver.