John C. Lietz

Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)


Electrical Engineering


Rapid transmission of inforn1ation from one point to another is extremely vital to our modern society. Millions of messages are transmitted each day through a variety of communication systems. A large majority of these systems rely upon some type of modulation process. The word "modulate" did not originate in the communications field. Like many other terms it was adopted because it aptly described the process to which it is now commonly associated. Basically the term means: to adjust; to regulate; to change; to alter; to vary; or to inflect as with the voice. For example, speaking and singing are both modulation processes where sound is varied in such a manner as to convey information. In a more technical sense, modulation is impressing an intelligence signal upon a carrier signal that is constant in all respects. The desired result of this over long distances through an available medium. When the transmission medium is the earth's atmosphere, the carrier signal is usually a sinusoidel radio frequency (RF) wave that is constant in both • 1agnitude and frequency. In modulating this carrier one of its characteristics or parameters must be aired in accordance with the intelligence signal more commonly referred to as the modulating signal. The two characteristics of a continuous RF wave that can be varied are the magnitude and the angle. From this it is obvious that there are two basic types of modulation available. The term “amplitude modulation” (AM) is used to describe the process where the magnitude of the RF wave is modulated; and the term "angle modulation Tl is associated with modulating the angle of the RF wave. Angle modulation can be further broken down into phase modulation (PM) and frequency modulation (FM) which are very closely related. Phase modulation results from varying the phase angle of the RF wave; while frequency modulation is produced by varying the instantaneous frequency of the RF wave.

Library of Congress Subject Headings

Modulation theory
Amplifiers, Radio frequency




South Dakota State University