Page 60 - Complete Wireless Design

Modulation

Modulation 59

Figure 2.12 A time-domain view of frequency
modulation.

frequency (exaggerated here for clarity). These rapid FM frequency fluctua-
tions are evidenced by the shortening and lengthening of the carrier’s wave-
length on the scope’s screen, creating a blurring of the signal. And since
wavelength equals the speed of light divided by the frequency, we can readily
see that any shift in wavelength corresponds to a change in frequency.
The total FM transmitter power will always stay constant during baseband
modulation, so the combined power or voltage in an FM signal will not vary
whether it is modulated or unmodulated. However, any sidebands formed by
the modulation must gain their power from the carrier itself. This carrier must
then sacrifice some of its own power in the creation of the FM sidebands. For
instance, let us assume that an FM transmitter is sending out an unmodu-
lated carrier at 100 watts. When the RF carrier is modulated by the baseband
signal it must give some—or even all—of its power to these sidebands. Thus,
the carrier and its significant sidebands must all total up to the original 100
watts that was present in the unmodulated carrier. Indeed, at certain modu-
lation indexes (see below), the carrier itself will actually vanish, while the side-
bands will now contain all of the power.
An infinite number of sidebands will be created during the modulation
process, since the carrier is sent through an infinite number of frequency or
phase values by the continuously changing baseband frequencies. This action
produces an infinite amount of sideband frequencies; even a single test-tone,
changing in a sinusoidal manner, has an infinite number of discrete ampli-
tudes within a single cycle.
Because of the difficulties inherent in “infinite,” the concept of the signifi-
cant sideband was created. Significant sidebands are any sidebands with an
amplitude that is 1 percent or more of the amplitude of the unmodulated car-
rier. When a sideband is below this level it can be ignored, while the higher the
amplitude of the baseband modulation, the higher the number of these signif-
icant sideband frequencies that will be produced.

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