The premodulation filter is inserted in order to reduce
the instantaneous phase changes, thus reducing the out-of-band emissions. Sometimes, this filter is
called a splatter control filter.
A simplified way of viewing the transmit spectrum is to consider BFSK as consisting of two spectral
tones (see Fig. 2.49).
80 CHAPTER TWO
fc fc + fd fc ??“ fd
Hz
|S( f )|
FIGURE 2.49 Simplified view of BFSK modulation scheme.
Premodulation
Filter
m(t)
VCO PA
S(t)
???{+1,??’1}
FIGURE 2.48 Conventional FM generation method for BFSK.
x m(t)
2pkf
cos( )
sin( )
Quadrature
Modulator
s(t)
I(t)
Q(t)
LPF dt ??«
t
??’???
FIGURE 2.50 Baseband FM modulator.
So, for positive values of the information signal m(t), the transmitted frequency is shifted upward
by an amount of fd Hz, where fd frequency deviation. Also for negative values of the information
signal, the transmitted frequency is shifted downward by fd.
A question remains as to how we choose the appropriate frequency deviation for an application.
What is commonly accepted is to introduce a variable that represents the normalized frequency deviation
of the modulation scheme; the variable is called the modulation index and is defined as
(2.33)
We will shortly discuss the fact that there are certain values of that have more benefits over others.
Pages:
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151