Amplitude Modulation and its Applications

Amplitude Modulation is an electronic communication systems technique wherein the baseband signal is superimposed with the amplitude of the carrier wave i.e. the amplitude of the carrier wave is varying with proportion to the base waveform being transmitted. Amplitude Modulation has been in use since the earliest days of radio technology.

Amplitude Modulation

What is Amplitude Modulation (AM)?

When an amplitude modulated signal is created, the amplitude of the created signal represents the original baseband signal to be transmitted. This amplitude forms an envelope over the underlying high-frequency carrier wave. Here, the overall envelope of the carrier is modulated to carry the audio signal. AM is the simplest way of modulating a signal.

Amplitude Modulation Formula

Amplitude Modulation expression is given by:

\(s(t)=[A_{c}+A_{m}cos(2\pi f_{m}t)]cos(2\pi f_{c}t)\)

Where,

  • Am is the amplitude of the modulating signal
  • Ac is the amplitude of the carrier signal
  • fm is the frequency of the modulating signal
  • fc is the frequency of the carrier signal

Demodulation

Demodulation is a process where the signal that is a mixture of the amplitude of the baseband signal and the frequency of the carrier signal, is deconstructed to yield the original signal that is to be transmitted. Amplitude modulation and demodulation are equally simple to perform. The Amplitude modulated signal needs just a simple diode detector circuit to demodulate.

The diode rectifier rectifies the incoming signal, allowing only one half of the signal waveform to pass through. The capacitor is then used to remove the radio frequency parts of the signal, leaving just the original waveform. As you see the equipment for demodulation is very cheap and this enables the cost of the receivers to be kept low.

Amplitude Modulation History

The first recorded instance of amplitude modulation of the baseband wave harks back to 1901 when a Canadian man Reginald Fessenden used a continuous spark transmission to create the first amplitude modulation ever. Into this continuous spark transmission, he puts a carbon microphone in the antenna lead. The sound waves impacting on the microphone varied its resistances and this, in turn, varied the intensity of the transmission.

Though the accuracy and the signal-to-noise ratio in this earlier method of transmission were very low, with the advent of a continuous sine wave generator, the audio quality was greatly improved. This led to Amplitude modulated waves becoming the standard for voice transmission.

 


 

Applications Of Amplitude Modulation

With the improvement of the technology, the uses of Amplitude Modulation waves has become somewhat less prevalent, nevertheless it can still be found playing an important role in;

  1. Broadcast Transmission: AM is still widely used for broadcasting either long or medium or short wave bands. The received signal is simple to break down into the baseband signal and hence the equipment cost to the user is very little and it is easy to manufacture
  2. Air band Radio: The use of AM in the aerospace industry is widespread. The VHF (Very High Frequency) transmissions made by the airborne equipment still use AM. The radio contact between ground to air and also ground to ground use AM signals.
  3. Quadrature Amplitude Modulation: Believe it or not, AM is used in the transmission of data of pretty much everything, from short range transmissions such as Wi-Fi to cellular communications and etc. Quadrature amplitude modulation is formed by mixing two carriers that are out of phase by 90o.

The need for modulation was sky high and the invention of Amplitude modulation has changed the way in which we communicate.

Detection Of Amplitude Modulated Wave (Demodulation)

Demodulation or detection is a process where the signal that is a mixture of the amplitude of the baseband signal and the frequency of the carrier signal, is deconstructed to yield the original signal that is to be transmitted. Simply, it is the recovery of the modulating signal from the modulated wave.Detection of Amplitude Modulated Wave

Amplitude modulation and demodulation are equally simple to perform. The amplitude modulated signal needs just a simple diode detector circuit to demodulate. The diode rectifies the incoming signal, allowing only one-half of the signal waveform to pass through. The capacitor then is used to remove the radio frequency parts of the signal, leaving just the original waveform. As you see, the equipment for demodulation is very cheap, and this enables the cost of the receivers to be kept low.

Thus, amplitude modulated wave can be demodulated in two steps:

  • Rectification of modulated wave.
  • Elimination of the RF component of the modulated wave.

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Practise This Question

Which of the following is not a natural phenomenon?