2.55).
wi (t)
I(t)
d
dt Q(t) Q (t)
d
dt I(t)
I 2(t) Q2(t)
MODULATION THEORY 83
I(t)
dt
d
X
Q(t) X
dt
d
??“
/
( )2
( )2
+ wi(t)
FIGURE 2.55 Alternative representation of the baseband frequency discriminator.
BPF
R(t)
Quadrature
Demodulator
LPF
LPF
~ cos(wct)
tan??“1
I(k)
Q(k)
Frequency Discriminator I(t)
Q(t)
dt
d wi(t)
FIGURE 2.54 Baseband frequency discriminator block diagram.
When BFSK does not satisfy the system requirements of your standard,M-ary FSK can be deployed.
Let??™s, for example, consider a 4-level FSK modulation scheme. The typical method described above,
which used filtered data to excite the VCO, is shown in Fig. 2.56. Note that the LPF input signal now
has an alphabet of size 4.
If we once again allow ourselves to have a simplified viewpoint of the modulation scheme, then
the transmit spectrum can be viewed as having four spectral lines, centered around the center carrier
frequency (see Fig. 2.57).
We can now present the complex envelope waveforms for a 4-level FSK system with a data rate
Rb 3.2 kbps and a frequency deviation fd 1.6 kHz. In this case, the modulation index is 1 1
84 CHAPTER TWO
Premodulation
Filter
m(t)
VCO PA
S(t)
???{+3, +1, ??’1, ??’3}
FIGURE 2.
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