Figure 1-38.Radio-frequency spectrum.
Radiating energy at audio frequencies (discussed earlier in this chapter) is not practical. The
heterodyning principle, however, makes possible the conversion of an af signal (intelligence) to an rf
signal (with af intelligence) which can be radiated or transmitted through space.
Look again at figure 1-38. The sum and difference frequencies are located very near the rf signal
(100 kilohertz), while the audio signal (5 kilohertz) is spaced a considerable distance away. Because of
this frequency separation, the audio frequency can be easily removed by filter circuits, leaving just three
radio frequencies of 95, 100, and 105 kilohertz. These three radio frequencies are radiated through space
to the receiving station. At the receiver, the process is reversed. The frequency of 95 kilohertz, for
example, is heterodyned with the frequency of 100 kilohertz and the sum and difference frequencies are
again produced. (A similar process occurs between the frequencies of 100 and 105 kilohertz.) Of the
resultant frequencies (95, 100, 105, and 5 kilohertz), all are filtered out except the 5 kilohertz difference
frequency. This frequency, which is identical to the original 5 kilohertz audio applied at the transmitter, is
retained and amplified. Thus, the 5 kilohertz audio tone appears to have been radiated through space
from the transmitter to the receiver.
In the process just described, the 100 kilohertz frequency is referred to as the CARRIER
FREQUENCY, and the sum and difference frequencies are referred to as SIDE FREQUENCIES. Since
the sum frequency appears above the carrier frequency, it is referred to as the UPPER SIDE
FREQUENCY. The difference frequency appears below the carrier and is referred to as the LOWER
When a carrier is modulated by voice or music signals, a large number of sum and difference
frequencies are produced. All of the sum frequencies above the carrier are spoken of collectively as the
UPPER SIDEBAND. All the difference frequencies below the carrier, also considered as a group, are
called the LOWER SIDEBAND.
If the carrier and the modulating signal are constant in amplitude, the sum and difference frequencies
will also be constant in amplitude. However, when the carrier and sidebands are combined in a single
impedance and viewed simultaneously with an oscilloscope, the resultant waveform appears as shown in
figure 1-39. This resultant wave is called the MODULATION ENVELOPE. The modulation envelope
has the same frequency as the carrier. However, it rises and falls in amplitude with the continual phase
shift between the carrier and sidebands. This causes these signals to first aid and then oppose one another.
These cyclic variations in the amplitude of the envelope have the same frequency as the audio-modulating