Page 59 - Complete Wireless Design

Modulation

58 Chapter Two

actual frequency deviation of the FM carrier depends on the increase or
decrease in the baseband’s amplitude. Frequency deviation is the amount
the RF carrier deviates from its center frequency in one direction during
modulation. Without any baseband modulation present, however, the fre-
quency of the RF carrier will stay at the transmitter’s predetermined center
frequency, which is the frequency of the master oscillator after any multipli-
cation. Thus, as the baseband modulation occurs, the carrier will increase
and decrease in frequency; as the baseband swings positive in amplitude, the
carrier will increase in frequency, but as the baseband modulation swings
negative in amplitude, the frequency of the carrier will fall below its rest fre-
quency (Fig. 2.11).
The frequency of the baseband signal will change the rate that the frequency-
modulated RF carrier intersects its own rest frequency, and will vary at this
same baseband rate. As an example; if a baseband audio tone is inserted at 2
kHz, the FM carrier will actually swing past its own rest frequency 2000 times
in 1 second.
Unlike AM, the percent of modulation for FM is directed by government
rules and regulations, and not by any natural limitations. For instance, for
narrowband voice communications, 5 kHz is the maximum allowed deviation
for 100 percent frequency modulation, while for wideband FM broadcast, the
maximum allowed deviation is 75 kHz. If the baseband signal’s amplitude
should induce the FM deviation to go above the 100 percent limit, then more
frequency sidebands will be created, broadening the bandwidth and conceiv-
ably causing interference to any adjacent channels.
As shown in Fig. 2.12, when FM is observed on an oscilloscope in the time
domain, the modulated RF carrier will not change in amplitude, but only in

Figure 2.11 A baseband modulating signal’s effect on a carrier after frequency modulation.

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