5-16
each sideband would be 6 dB less than the carrier, or one-fourth of the power of the carrier. Since the
carrier component is not changed with AM transmission, the total power in the 100-percent-modulated
wave is 50% higher than in the unmodulated carrier. The primary advantage of the log display that is
provided by the spectrum analyzer over the linear display provided by the oscilloscopes for percentage of
modulation measurements is that the high dynamic range of the spectrum analyzer (up to 70 dB) allows
accurate measurements of values as low as 0.06%. It also allows the measurements of low-level distortion
of AM signals. Both capabilities are illustrated in figure 5-15, view A, view B, and view C. The chart in
figure 5-16 provides an easy conversion of dB down from carrier into percentage of modulation.
Figure 5-14.—Spectrum analyzer display of an AM signal.
Figure 5-15A.—Spectrum analyzer displays of AM signals.
