Digital Signal Processing Reference
In-Depth Information
PSD of S
H
(
t
)
PSD of S
L
(t)
Resultant PSD
1.5
1.5
1
1
a b
c d
0.5
0.5
0
0
-
5
0
5
-
5
0
5
1.5
1.5
1
1
0.5
0.5
0
0
-
5
0
5
-
5
0
5
Fig. 6.13
PSD of BFSK: (
a
)
f
H
−
f
L
<
/
T
B
,(
b
)
f
H
−
f
L
=
/
T
B
,(
c
)
f
H
−
f
L
>
/
T
B
,
2
2
2
(
d
)
f
H
−
f
L
>>
2
/
T
B
6.3.3 FSK Demodulator
FSK signal can be detected using both coherent and non-coherent arrangements.
As the binary FSK signal is generated using two carrier frequencies, the
coherent
detector
consists of two correlators that are individually turned to two different car-
rier frequencies to represent symbol 1 and 0. A correlator is the combination of a
multiplier followed by an integrator (LPF). The same BFSK signal is multiplied by
two different carrier signals and the passed separately through the LPF. The out-
puts of the LPF must be inverted type, i.e., if one is upper going, the other must be
lower going. Next, the outputs are fed to the decision deice which is essentially a
comparator. If the output produced in the upper path is more than the output pro-
duced in the lower path, the magnitude comparator will decide in favor of symbol 1.
Similarly, if the output of the upper path is less than that of the lower path, the
comparator will decide in favor of symbol 0. This type of digital communication
receivers are also called as correlator receiver (Fig.
6.14
).
As in case of ASK, binary FSK also can be demodulated
non-coherently
using
envelope detector. The received BFSK signal is applied to the bank of two band
pass filters (parallel BPF combination) with central pass frequencies
f
H
and
f
L
. Each
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