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Electrical Lab Experiments

To study the different modulation and demodulation techniques



Aim

To study the different modulation and demodulation techniques

Theory:

Modulation and demodulation: Communication is the basic attraction of mankind as it gives the knowledge of what is going on around us. In our daily life, we communicate with many people and use the entertainment media like television, radio, internet and newspaper to get ourselves involved. These entertainment media act as a source of communication. Electronic communication comprises TV, radio, internet, etc. When we want to transmit a signal from one location to another, we have to strengthen the signal. After undergoing strengthening process the signal travels to a long distance. This is called as modulation, and this article gives an overview of the modulation and types of modulation techniques. Communication is nothing but, the process of exchanging (two way communications) or passing (one way communication) information from one person to another. The basic electronic communication system consists of these components: transmitter, receiver and communication channel.

Types of Modulation

Communication is the basic attraction of mankind as it gives the knowledge of what is going on around us. In our daily life, we communicate with many people and use the entertainment media like television, radio, internet and newspaper to get ourselves involved. These entertainment media act as a source of communication. Electronic communication comprises TV, radio, internet, etc. When we want to transmit a signal from one location to another, we have to strengthen the signal. After undergoing strengthening process the signal travels to a long distance. This is called as modulation, and this article gives an overview of the modulation and types of modulation techniques. Communication is nothing but, the process of exchanging (two way communications) or passing (one way communication) information from one person to another. The basic electronic communication system consists of these components: transmitter, receiver and communication channel.

Figure 1. Communication System

A transmitter is a group of electronic circuits designed to convert the information into a signal for transmission over a given communication medium. A receiver is a group of electronic circuits designed to convert the signal back to the original information. The communication channel is the medium which is designed to transmit the electronic signal from one place to another. Modulation is a way of sending signals of low frequency over long distances without a huge loss of energy by the use of another wave of very high frequency called a carrier wave. Modulation is nothing but, a carrier signal that varies in accordance with the message signal. Modulation technique is used to change the signal characteristics. Basically, the modulation is of following two types:

  • Analog Modulation
  • Digital Modulation
Figure 1. Types of Modulation
Figure 2. Modulation Techniques
Figure 3. Types of Digital Modulation Techniques

High frequency signals are more directional and because high frequency waves have a small wavelength there is less diffraction. Also smaller aerials are needed because the size of the aerial has to be of the same sort of size as the wavelength of the signal to be transmitted. Therefore a sound with a frequency of 256 Hz (middle C) received by a microphone, converted to an electrical signal and then transmitted would have a wavelength of 300000000/256 = 1 170 000 m, over 1000 km! However a wave of frequency 100 MHz has a wavelength of only 3 m. We can compare amplitude modulation with a long tube of soft clay on a conveyor belt. The clay moves between the hands of two people, one at each end of the belt. Modulation - the person at one end moulds the clay by pressing on it as it moves between their hands. By squeezing and relaxing they make a tube of clay with a changing diameter. At the other end of the belt there is a person with their eyes shut and their hands on either side of the clay at the other end. As the clay moves past their hands are forced in and out by the changing diameter of the clay cylinder. This is called detection In reality we start with a carrier wave of very high frequency and add to it the audio signal (of relatively low frequency). This addition of the audio signal is called modulation. This can be done either by changing the amplitude of the carrier wave (amplitude modulation) or by changing its frequency (frequency modulation).

The audio signal is produced (Figure 1) and converted to an electrical signal by the microphone.

Figure 1.

The high frequency carrier wave (with a frequency of say 100 MHz) (Figure 2) is produced by the transmitter

Figure 2.

These two signals are added together (modulation) (Figure 3).

Figure 3.

The modulated signal is transmitted (Figure 4).

Figure 4.

The modulated signal is received by the aerial.

A diode is used to separate the high frequency carrier wave from the low frequency audio signal (demodulation) by removing half the signal. This leaves the outline of the audio signal (Figure 5).

Figure 5.

This signal is now amplified and fed to a loudspeaker.

The receiver is tuned to the carrier wave frequency. The following two diagrams (Figures 6 and 7) show two carrier waves of different frequency both modulated by the same frequency audio signal.

Figure 6 and 7

Result

Post Lab Questions

  1. What are the different types of modulation?
  2. Which type of modulation is used in television?
  3. What is PPM modulation?
  4. What are NTSC and PAL?











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