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PRODUCT SPECIFICATION
RC2211A
7
Applications Discussion
FSK Decoding
Figure 2 shows the basic circuit connection for FSK decod-
ing. With reference to Figures 1 and 2, the functions of
external components are defined as follows: R
0
and C
0
set
the PLL center frequency, R
1
sets the system bandwidth, and
C
1
sets the loop filter time constant and the loop damping
factor. C
F
and R
F
form a one pole post-detection filter for the
FSK data output. The resistor R
B
(510 k
W
) from pin 7 to pin
8 introduces positive feedback across FSK comparator to
facilitate rapid transition between output logic states.
Recommended component values for some of the most
commonly used FSK bauds are given in Table 1.
Figure 2. Circuit Connection for FSK Decoding
Table 1. Recommended Component Values
for Commonly Used FSK Bands
(see Circuit of Figure 2)
Design Instructions
The circuit of Figure 2 can be tailored for any FSK decoding
application by the choice of five key circuit components: R
0
,
R
1
, C
0
, C
1
and C
F
. For a given set of FSK mark and space
frequencies, F
1
and F
2
, these parameters can be calculated as
follows:
1.
Calculate PLL center frequency, F
0
2.
Choose a value of timing resistor R
0
to be in the range
of 10 k
W
to 100 k
W
. This choice is arbitrary. The recom-
mended value is R
0
= 20 k
W
. The final value of R
0
ios
normally finetuned with the series potentiometer, R
X
.
3.
Calculate value of C
0
from Design Equation No. 1 or
from Typical Performance Characteristics:
C
0
= 1/R
0
F
0
4.
Calculate R
1
to give a
D
f equal to the markspace
deviation:
R
1
= R
0
[F
0
/(F
1
- F
2
)]
5.
Calculate C
1
to set loop damping. (See Design Equation
No. 4)
Normally,
z
1/2 is recommended
Then: C
1
= C
0
/4 for
z
= 1/2
6.
Calculate Data Filter Capacitance, C
F
:
For R
F
= 100 k
W
, R
B
= 510 k
W
, the recommended value
of C
F
is:
Note:
All calculated component values except R
0
can be
rounded off to the nearest standard value, and R
0
can
be varied to fine-tune center frequency through a
series potentiometer, R
X
(see Figure 2).
Design Example
75 Baud FSK demodulator with mark space frequencies of
1110/1170 Hz:
Step 1: Calculate F
0
:
F
0
=(1110+1170)(1/2)= 1140Hz
Step 2: Choose R
0
= 20 k
W
(18 k
W
fixed resistor in series
with 5 k
W
potentiometer)
Step 3: Calculate C
0
from VCO Frequency vs. Timing
Capacitor: C
9
= 0.044
m
F
Step 4: Calculate R
1
: R
1
= R
0
(1140/60) = 380 k
W
Step 5: Calculate C
1
: C
1
= C
0
/4 = 0.011
m
F
Note:
All values except R
0
can be rounded off to nearest
standard value.
FSK Band
300 Baud
F
1
= 1070 Hz
F
2
= 1270 Hz
300 Baud
F
1
= 2025 Hz
F
2
= 2225 Hz
1200 Baud
F
1
= 1200 Hz
F
2
= 2200 Hz
Component Values
C
0
= 0.039
m
F, C
F
= 0.005
m
F
C
1
= 0.01
m
F, R
0
= 18 k
W
R
1
= 100 k
W
C
0
= 0.022
m
F, C
F
= 0.005
m
F
C
1
= 0.0047
m
F, R
0
= 18 k
W
R
1
= 200 k
W
C
0
= 0.027
m
F, C
F
= 0.0022
m
F
C
1
= 0.01
m
F, R
0
= 18 k
W
R
1
= 30 k
W
1
2
3
4
5
6
7
14
13
12
11
10
9
8
RC2211A
65-2211-05
C0
VCO
Fine Tune
R0
R
X
5K
R1
C1
0.1
μ
F
R
F
100K
510K
C
F
FSK Data
Output
5.1K
+V
S
0.1
μ
F
0.1
μ
F
+V
S
FSK
Input
R
L
R
B
fF
0
F
-----------------
F
2
+
2
=
C
F
in
m
F
(
)
Baud Rate
----------3
=