5-18
Figure 5-16.—Modulation percentage versus sideband levels.
NOTE: Anything greater than -6 dB exceeds 100% modulation and produces distortion, as shown in
figure 5-16.
In modern, long-range hf communications, the most important form of amplitude modulation is ssb
(single-sideband). In ssb either the upper or lower sideband is transmitted, and the carrier is suppressed.
Ssb requires only one-sixth of the output power required by AM to transmit an equal amount of
intelligence power and less than half the bandwidth. Figure 5-17 shows the effects of balancing the carrier
of an AM signal. The most common distortion experienced in ssb is intermodulation distortion, which is
caused by nonlinear mixing of intelligence signals. The two-tone test is used to determine if any
intermodulation distortion exists. Figure 5-18 illustrates the spectrum analyzer display of the two-tone test
with the modulation applied to the upper sideband input.
Figure 5-17.—Double sideband carrier suppressed.
