2-14
10.0. This information was used to develop table 2-1 by listing the modulation indexes in the left column
and the number of significant sidebands in the right. SIGNIFICANT SIDEBANDS (those with
significantly large amplitudes) are shown in both views of figure 2-9 as vertical lines on each side of the
carrier frequency. Actually, an infinite number of sidebands are produced, but only a small portion of
them are of sufficient amplitude to be important. For example, for a modulation index of 0.5 [top
spectrums of both views (A) and (B)], the number of significant sidebands counted is 4. For the next
spectrums down, the modulation index is 1.0 and the number of sidebands is 6, and so forth. This holds
true for any combination of deviating and modulating frequencies that yield identical modulating indexes.
Table 2-1.—Modulation index table
MODULATION
INDEX
SIGNIFICANT
SIDEBANDS
.01
2
.4
2
.5
4
1.0
6
2.0
8
3.0
12
4.0
14
5.0
16
6.0
18
7.0
22
8.0
24
9.0
26
10.0
28
11.0
32
12.0
32
13.0
36
14.0
38
15.0
38
You should be able to see by studying figure 2-9, views (A) and (B), that the modulating frequency
determines the spacing of the sideband frequencies. By using a significant sidebands table (such as table
2-1), you can determine the bandwidth of a given fm signal. Figure 2-10 illustrates the use of this table.
The carrier frequency shown is 500 kilohertz. The modulating frequency is 15 kilohertz and the deviation
frequency is 75 kilohertz.
