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Modulation and demodulation

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.

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 300 000 000/256 = 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.



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


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



The modulated signal is transmitted (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).


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.



 
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© Keith Gibbs 2020