In this lecture, we use an Analog Arts (http://analogarts.com/) SL987 oscilloscope to review the basics of waves, antennas, modulation concept, AM, and FM.
Waves transfer energy without moving matter, similar to ocean waves or sun light.
In general in air, electromagnetic waves such as radio signals travel longer than mechanical waves such as sound.
To transmit information over very long distances, electromagnetic waves are used. Electromagnetic waves or EM waves spread without the need for a medium, travel at the speed of light, do not lose energy in vacuum, and can travel forever.
Using a signal frequency of about 850 kHz, in December 1901, Guglielmo Marconi received the first transatlantic radio signals, the Morse code for the letter "S".
An antenna converts an electrical signal to EM waves to make it transmittable. The length of the antenna inversely depends on the frequency of the signal. For low frequency signals such as electrical representation of sound out of a microphone, the size of the antenna grows to hundreds of kilometers, making the transmission impractical. Modulation overcomes this problem by mixing the low frequency information with a high frequency sinusoidal.
In the modulation process, a low frequency information signal is put on a higher frequency sine wave, to produce a new waveform, which is suitable for transmission. The information is referred to as the modulating signal, the sine wave as the carrier signal, and the mix as the modulated carrier wave. Modulation process can be viewed as a ride carrying cargo from one place to another, where the cargo is the message and the ride is the carrier signal.
At the destination, the detected signal is mixed with the carrier, and the resulting signal is filtered to produce the message. This process is called demodulation.
AM and FM are two of the most popular modulation techniques.
In AM the amplitude of the modulated signal represents the message.
In FM however, the frequency of the modulated signal characterizes the message.
Modulation concept is the process of varying one or more properties of a high frequency periodic waveform, referred to as the carrier signal, with an information signal, to transmit the information. It is like throwing a paper note across a distance by wrapping it around a stone. Since a sine wave is completely defined by its amplitude, frequency, and phase, in a modulation process, one or a combination of these properties is altered by the modulation signal.
Once a low frequency wave is modulated, it travels longer and requires smaller transmission antennas. Modulation also allows multiple transmissions, each with its own unique carrier frequency. Each modulation technique offers its own benefits and is intended for a particular application.
Both AM and FM techniques are widely used. Since lower frequencies are reflected better by the electrically charged atoms in the ionosphere, AM waves are spread in a wider area, particularly at night when the ionosphere is not affected by sun. An important advantage of FM is that it is not disturbed by outside interferences. Therefore, FM results in a higher quality transmission.
The advent of digital signal processing, gave birth to digital modulation. Conceptually it is like analog modulation. However, it uses a discrete signal to modulate the carrier signal.
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