18-09-2012, 10:54 AM
MODULATION SYSTEMS
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Introduction
a. In the Microbroadcasting services, a reliable radio communication system is of
vital importance. The swiftly moving operations of modern communities require a
degree of coordination made possible only by radio. Today, the radio is standard
equipment in almost all vehicles, and the handie-talkie is a common sight in the
populace. Until recently, a-m (amplitude modulation) communication was used
universally. This system, however, has one great disadvantage: Random noise and
other interference can cripple communication beyond the control of the operator.
In the a-m receiver, interference has the same effect on the r-f signal as the
intelligence being transmitted because they are of the same nature and inseperable.
b. The engines, generators, and other electrical and mechanical systems of modern
vehicles generate noise that can disable the a-m receiver. To avoid this a different
type of modualation, such as p-m (phase modulation) or f-m (frequency modulation)
is used. When the amplitude of the r-f (radio-frequency) signal is held constant and
the intelligence transmitted by varying some other characteristic of the r-f signal,
some of the disruptive effects of noise can be eliminated.
Carrier Characteristics
The r-f signal used to transmit intelligence from one point to another is called the
carrier. It consists of an electromagnetic wave having amplitude, frequency, and
phase. If the voltage variations of an r-f signal are graphed in respect to time, the
result is a waveform such as that in figure 2. This curve of an unmodulated carrier
is the same as those plotted for current or power variatons, and it can be used to
investigate the general properties of carriers. The unmodulated carrier is a sine
wave that repeats itself in definite intervals of time. It swings first in the positive
and then in the negative direction about the time axis and represents changes in the
amplitude of the wave. This action is similar to that of alternating current in a wire,
where these swings represent reversals in the direction of current flow. It must be
remembered that the plus and minus signs used in the figure represent direction
only. The starting point of the curve in the figure 2 is chosen arbitrarily.
Amplitude Modulation
a. General. The amplitude, phase, or frequency of a carrier can be varied in
accordance with the intelligence to be transmitted. The process of varying one of
these characteristics is called modulation. The three types of modulation, then are
amplitude modulation, phase modulation, and frequency modulation. Other special
types, such as pulse modulation, can be considered as subdivisions of these three
types. With a sine-wave voltage used to amplitude-modulate the carrier, the
instantaneous amplitude of the carrier changes constantly in a sinusoidal manner.
The maximum amplitude that the wave reaches in either the positive or the negative
direction is termed the peak amplitude.