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AM, FM, and Digital Modulated Systems Chap. 5
Oscillator
Frequency=f 1
Electronic
switch
Oscillator
Frequency=f 2
FSK
output
Binary data input
m(t)
Control
line
(a) Discontinuous-Phase FSK
Binary data input
m(t)
Frequency
modulator
(carrier
frequency=f c )
FSK
output
(b) Continuous-Phase FSK
Figure 5–23
Generation of FSK.
Since FSK transmitters are not usually built this way, we will turn to the second type, shown
in Fig. 5–23b.
The continuous-phase FSK signal is generated by feeding the data signal into a frequency
modulator, as shown in Fig. 5–23b. This FSK signal is represented (see Sec. 5–6) by
or
where
t
s(t) = A c cos cv c t + D f m(l) dl d
L
s(t) = Re{g(t)e jv ct }
g(t) = A c e ju(t)
t
u(t) = D f m(l) dl for FSK
L
-q
(5–81a)
(5–81b)
(5–81c)
and m(t) is a baseband digital signal. Although m(t) is discontinuous at the switching time, the
phase function u(t) is continuous because u(t) is proportional to the integral of m(t). If the
serial data input waveform is binary, such as a polar baseband signal, the resulting FSK signal
-q