As the potential for the uses of electricity were explored, scientists in the United States and England
worked to develop the telegraph. The first practical system was established in London, England, in 1838.
Just 20 years later, the final link to connect the major countries with electrical communications was
completed when a transatlantic submarine cable was connected. Commercial telegraphy was practically
worldwide by 1890. The telegraph key, wire lines, and Morse code made possible almost instantaneous
communications between points at great distances. Submarine cables solved the problems of transoceanic
communications, but communications with ships at sea and mobile forces were still poor.
In 1897 Marconi demonstrated the first practical wireless transmitter. He sent and received messages
over a distance of 8 miles. By 1898 he had demonstrated the usefulness of wireless telegraph
communications at sea. In 1899 he established a wireless telegraphic link across the English Channel. His
company also established general usage of the wireless telegraph between coastal light ships (floating
lighthouses) and land. The first successful transatlantic transmissions were achieved in 1902. From that
time to the present, radio communication has grown at an extraordinary rate. Early systems transmitted a
few words per minute with doubtful reliability. Today, communications systems reliably transmit
information across millions of miles.
The desire to communicate directly by voice, at a higher rate of speed than possible through basic
telegraphy, led to further research. That research led to the development of MODULATION. Modulation
is the ability to impress intelligence upon a TRANSMISSION MEDIUM, such as radio waves. A
transmission medium can be described as light, smoke, sound, wire lines, or radio-frequency waves. In
this module, you will study the basic principles of modulation and DEMODULATION (removing
intelligence from the medium).
In your studies, you will learn about modulation as it applies to radio-frequency communications. To
modulate is to impress the characteristics (intelligence) of one waveform onto a second waveform by
varying the amplitude, frequency, phase, or other characteristics of the second waveform. First, however,
you will review the characteristics and generation of a sine wave. This review will help you to better
understand the principles of modulation. Then, an important principle called HETERODYNING (mixing
two frequencies across a nonlinear impedance) will be studied and applied to modulation. Nonlinear
impedance will be discussed in the heterodyning section. You will also study several methods of
modulating a radio-frequency carrier. You will come to a better understanding of the demodulation
principle by studying the various circuits used to demodulate a modulated carrier.
Q-1. What is modulation?
Q-2. What is a transmission medium?
Q-3. What is heterodyning?
Q-4. What is demodulation?
SINE WAVE CHARACTERISTICS
The basic alternating waveform for all complex waveforms is the sine wave. Therefore, an
understanding of sine wave characteristics and how they can be acted upon is essential for you to
understand modulation. You may want to review sine waves in chapter 1 of NEETS, Module 2,
Introduction to Alternating Current and Transformers at this point.