13-07-2012, 11:23 AM
Frequency Modulation
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Introduction
This chapter will introduce the student to the other forms of analog
modulation—frequency modulation (FM) and phase modulation (PM)—
both of which are commonly known as angle modulation. Both FM and
PM are used extensively in communications systems. FM is used in radio
broadcasting, for the transmission of the sound signal in standard (NTSC)
TV, for private land-mobile radio systems, for direct-satellite broadcasting,
and for cordless and cellular telephone systems, just to name a few common
applications. PM by itself and in combination with AM is used extensively
in modern data-communications systems. Angle modulation has
a very important advantage over AM in its ability to provide increased immunity
to noise. Angle-modulation systems typically require a larger bandwidth
than AM systems, a necessary trade-off for its improved resistance
to noise.
An Introduction to the Development of FM
This chapter will introduce the reader to the second analog form of modulation.
This type of modulation scheme is known as angle modulation.
Angle modulation can be further subdivided into two distinct types:
frequency modulation (FM) and phase modulation (PM).
The history and evolution of angle modulation basically revolves
around one man, Major Edwin Armstrong. Armstrong, a radio pioneer
who invented first the regenerative and then the superheterodyne receiver
in the 1910s, worked on the principles of frequency and phase modulation
starting in the 1920s. It was not until the 1930s, however, that he
finally completed work on a practical technique for wideband frequencymodulation
broadcasting. For further information, visit a Web site devoted
Frequency-Modulation Theory
We will start our discussion of angle modulation by first examining
frequency modulation. The classic definition of FM is that the instantaneous
output frequency of a transmitter is varied in accordance with the
modulating signal. Recall that we can write an equation for a sine wave as
follows:
e(t) = EP sin(ωt + φ) 4.1
While amplitude modulation is achieved by varying EP, frequency modulation
is realized by varying ω in accordance with the modulating signal
or message. Notice that one can also vary φ to obtain another form of angle
modulation known as phase modulation (PM). Later we will examine the
relationship between FM and PM. See Figure 4-1 for a time display of a
typical FM signal.
Definitions
An important concept in the understanding of FM is that of frequency
deviation. The amount of frequency deviation a signal experiences is a
measure of the change in transmitter output frequency from the rest frequency
of the transmitter. The rest frequency of a transmitter is defined as
the output frequency with no modulating signal applied. For a transmitter
with linear modulation characteristics, the frequency deviation of the carrier
is directly proportional to the amplitude of the applied modulating signal.
Thus an FM transmitter is said to have a modulation sensitivity,
represented by a constant, kf, of so many kHz/V,
kf = frequency deviation/V = kf kHz/V
142 CHAPTER 4
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FIGURE 4-1 A
typical FM signal
shown with the
modulating signal