Understanding Amplitude Modulation

The amplitude modulation(AM) technique is used in electronic communication, most commonly for transmitting messages with a radio wave. Amplitude modulation is a form of modulation used for radio transmissions for broadcasting and two-way radio communication applications.

Amplitude modulation is a type of modulation where the height of the carrier signal is changed in accordance with the height of the message signal. One of the key reasons for the use of amplitude modulation was its ease of use. The system simply required the carrier amplitude to be modulated, but more usefully the detector required in the receiver could be a simple diode-based circuit.

In order that a radio signal can carry audio or other information for broadcasting or for two-way radio communication, it must be modulated or changed in some way. Although there are a number of ways in which a radio signal may be modulated, one of the easiest is to change its amplitude in line with variations of the sound. In this way, the amplitude of the radio frequency signal varies in line with the instantaneous value of the intensity of the modulation.

Types of Amplitude Modulation

There are three main types of amplitude modulation

Double Sideband-suppressed Carrier Modulation (DSB-SC)

Single Sideband Modulation (SSB)

Vestigial Sideband Modulation (VSB)

Mathematical Expression

Consider a sinusoidal modulating signal or message signal  (a_m) of frequency (ω_m) and amplitude (A_m) given by:

                     a_m = A_m sin ω_mt   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  (1)

and carrier wave (a_c) of frequency (ω_c) and amplitude (A_c) given by:

                     a_c = A_c sin ω_ct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2)

Where,

• a_m is the modulating signal or message signal
• a_c is the carrier signal
• A_m is the maximum amplitude of the message signal
• A_c is the maximum amplitude of the carrier signal
• ω_m is the frequency of the message signal
• ω_c is the frequency of the carrier signal

Using the above mathematical expressions, we can create a new mathematical expression for the complete modulated wave.

The amplitude-modulated wave (A) is given as:

                   A = a_c + a_m. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  (3)

Put a_m value from equation (1) into equation (3), then we get

                   A = a_c + a_m sin ω_mt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (4)

The instantaneous value of the amplitude-modulated wave (a) can be given as:

                 a = A sin θ

                  a = A sin ω_ct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (5)

Put A value from equation (4) into equation (5), then we get

                 a = (A_c + A_m sin ω_mt) sin ω_ct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  (6)

This is an equation of amplitude modulated (AM) wave.

Advantages of Amplitude Modulation

• Low cost
• Few components needed
• It is simple to implement
• Long-distance communication
• it can be demodulated using a circuit consisting of very few components

Disadvantages of Amplitude Modulation

• It requires high bandwidth
• Amplitude modulation is prone to high levels of noise
• It is inefficient in terms of its power usage
• Amplitude modulation cannot be used for the transmission of sensitive information
• It is a type of transmission that can be easily affected by external radiation.
• Amplitude modulation (AM) cannot be used for transmitting music Frequency modulation (FM).

Use of  Amplitude modulation | Amplitude modulation applications

• Airband radio
• Single sideband
• Broadcast transmissions
• Quadrature amplitude modulation