(1) Since the carrier wave (figure 9) itself does not convey intelligence, information in the
form of a signal wave is super-imposed upon the carrier. This process, which is called modulation,
varies or modifies either the frequency or the amplitude of the carrier waveform. Both amplitude
modulation and frequency modulation methods are used in military radio communications systems.
(2) When antenna frequency signals are superimposed on the RF carrier, additional RF
signals are generated. The additional frequencies are equal to the sum and difference of the audio
frequencies and the radio frequency involved. For example, assume that a 1000 kc carrier is modulated
by a 1 kc audio tone. Two new radio frequencies are developed, one at 1001 kc (the sum of 1000 and 1
kc) and the other at 999 kc (the difference between 1000 and 1 kc). If a complex audio signal is used
instead of a single tone, two new frequencies will be set up for each of the audio frequencies involved.
The new frequencies are called sidebands.
b. Amplitude modulation (AM). Amplitude modulation is defined as the variation of the RF
power of a transmitter at an audio rate. In other words, the RF energy increases and decreases in power
according to the audio (sound) frequencies. In very simple terms, amplitude modulation is the process
of varying the power output of a transmitter (figure 9).
(1) When an RF carrier is modulated by a single audio tone, two additional frequencies are
produced. One is the upper frequency, which equals the sum of the frequency of the RF carrier and the
frequency of the audio note. The other is the lower frequency, which equals the difference between the
frequency of the RF carrier and the frequency of the audio note. The one higher than the carrier
frequency is the upper side frequency; the one lower than the carrier frequency is the lower side
frequency.
(2) When the modulating signal is made up of complex tones, as in speech, each individual
frequency component of the modulating signal produces its own upper and lower side frequencies.
These side frequencies occupy a band of frequencies called sidebands. The sideband that contains the
sum of the carrier and modulating frequencies is called the upper sideband; the sideband that contains
the difference of the carrier and the modulating frequencies is called the lower sideband.
(3) The space that a carrier and its associated sidebands occupy in a frequency spectrum is
called a channel. In amplitude modulation, the width of the channel (bandwidth) is equal to twice the
highest modulating frequency. Consequently, if a 5,000 kc carrier is modulated by a band of
frequencies ranging from 200 to 5,000 cycles (.2 to 5 kc), the upper sideband extends from 5,000.2
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