Table 1-1. Radio frequency bands.
3. Modulation. Both amplitude modulation (AM) and frequency modulation (FM) transmitters produce
RF carriers. The carrier is a wave of constant amplitude and frequency, which can be modulated by
changing its amplitude or frequency (Figure 1-7, page 1-10). Thus, the RF carrier "carries" intelligence
by being modulated. Modulation is the process of superimposing intelligence (voice or coded signals)
on the carrier. The following discussion of modulation is based on FM 24-18, Tactical Single-Channel
Radio Communications Techniques.
a. AM is defined as the variation of the RF power output of a transmitter at an audio rate. In
other words, the RF energy increases and decreases in power according to the audio frequencies (AFs)
superimposed on the carrier signal.
(1) When AF signals are superimposed on the RF carrier signal, additional RF signals are
generated. These additional frequencies are equal to the sum and the difference of the AF signals and
the RF used. For example, assume a 500 kHz carrier is modulated by a 1 kHz audio tone. Two new
frequencies are developed. One is at 501 kHz (the sum of 500 kHz and 1 kHz) and the other is at 499
kHz (the difference between 500 kHz and 1 kHz). If a complex audio signal is used instead of a single
tone, two new frequencies will be set up for each of the AF signals involved. The new frequencies that
result from superimposing an AF signal on an RF signal are called sidebands.
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