Page 56 - Complete Wireless Design

Modulation

Modulation 55

Figure 2.8 The amplitude of an AM carrier remains the same whether modulated (a) or not (b).

Figure 2.9 When the amplitude and/or frequency of the baseband modulation changes, only the
sidebands are affected: (a) 5-kHz high-amplitude and (b) 10-kHz low-amplitude modulation.

f CARRIER f AUDIO LSB and f CARRIER f AUDIO USB. As an example, if a 1000-
kHz carrier is modulated with a 10-kHz baseband audio tone, then the side-
band frequencies can be located at 1000 kHz 10 kHz 1010 kHz, and at
1000 kHz 10 kHz 990 kHz. The bandwidth of this signal will thus be
twice the baseband frequency, or the upper sideband minus the lower side-
band. In the above example, the bandwidth is 2 10 kHz, or 20 kHz.
Only the total power changes in an AM signal, or P P P P . This
T C LSB USB
is because, as discussed above, the carrier power remains unchanged no mat-
ter what baseband modulation amplitude we use, and only the sideband
amplitudes will vary. Thus, the total power of an AM signal will equal the sum
of the carrier and sideband powers. In fact, when the carrier is amplitude

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