PRODUCT SPECIFICATION
RC2211
6
Applications
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 deÞned as follows: R0 and C0 set
the PLL center frequency, R1 sets the system bandwidth, and
C1 sets the loop Þlter time constant and the loop damping
factor. CF and RF form a one pole post-detection Þlter for the
FSK data output. The resistor RB (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 Connectbn 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 Þve key circuit components: R0,
R1, C0, C1 and CF. For a given set of FSK mark and space
frequencies, F1 and F2, these parameters can be calculated as
follows:
FSK Band
Component Values
300 Baud
C0 = 0.039
mF, CF = 0.005 mF
F1 = 1070 Hz
C1 = 0.01
mF, R0 = 18 kW
F2 = 1270 Hz
R1 = 100 k
W
300 Baud
C0 = 0.022
mF, CF = 0.005 mF
F1 = 2025 Hz
C1 = 0.0047
mF, R0 = 18 kW
F2 = 2225 Hz
R1 = 200 k
W
1200 Baud
C0 = 0.027
mF, CF = 0.0022 mF
F1 = 1200 Hz
C1 = 0.01
mF, R0 = 18 kW
F2 = 2200 Hz
R1 = 30 k
W
1
2
3
4
5
6
7
14
13
12
11
10
9
8
RC2211
65-2211-05
C0
VCO
Fine Tune
R0
RX
5K
R1
C1
0.1
µF
RF
100K
510K
CF
FSK Data
Output
5.1K
+VS
0.1
µF
0.1
µF
+VS
FSK
Input
RL
RB
1.
Calculate PLL center frequency, F0
2.
Choose a value of timing resistor R0 to be in the range
of 10 k
W to 100 kW. This choice is arbitrary. The recom-
mended value is R0 = 20 k
W. The Þnal value of R0 ios
normally Þnetuned with the series potentiometer, RX.
3.
Calculate value of C0 from Design Equation No. 1 or
from Typical Performance Characteristics:
C0 = 1/R0F0
4.
Calculate R1 to give a
Df equal to the markspace
deviation:
R1 = R0 [F0/(F1 - F2)]
5.
Calculate C1 to set loop damping. (See Design Equation
No. 4)
Normally,
z » 1/2 is recommended
Then: C1 = C0/4 for
z = 1/2
6.
Calculate Data Filter Capacitance, CF:
For RF = 100 k
W, RB = 510 kW, the recommended value
of CF is:
Note: All calculated component values except RO can be
rounded off to the nearest standard value, and R0 can
be varied to fine-tune center frequency through a series
potentiometer, RX (see Figure 2).
Design Example
75 Baud FSK demodulator with mark space frequencies of
1110/1170 Hz:
Step 1: Calculate F0:
F0=(1110+1170)(1/2)= 1140Hz
Step 2: Choose R0 = 20 k
W (18 kW Þxed resistor in series
with 5 k
W potentiometer)
Step 3: Calculate C0 from VCO Frequency vs. Timing
Capacitor: C9 = 0.044
mF
Step 4: Calculate R1: R1 = R0 (1140/60) = 380 k
W
Step 5: Calculate C1: C1 = C0/4 = 0.011
mF
Note: All values except R0 can be rounded off to nearest
standard value.
fF
0
F
1
F
2
+
2
------------------
=
C
F in mF
()
3
Baud Rate
-------------------------
=
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