XR-2211 Datasheet, PDF (10/24 Pages) Exar Corporation – FSK Demodulator/ Tone Decoder

XR-2211
APPLICATIONS INFORMATION
FSK Decoding
Figure 10 shows the basic circuit connection for FSK decoding. With reference to Figure 3 and Figure 10, the functions
of external components are defined as follows: R₀ and C₀ set the PLL center frequency, R₁ sets the system bandwidth,
and C₁ 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 from pin 7 to pin 8 introduces positive feedback across the FSK comparator to
facilitate rapid transition between output logic states.
Design Instructions:
The circuit of Figure 10 can be tailored for any FSK decoding application by the choice of five key circuit components: R₀,
R₁, C₀, C₁ and C_F. For a given set of FSK mark and space frequencies, f_O and f₁, these parameters can be calculated as
follows:
(All resistance in W’s, all frequency in Hz and all capacitance in farads, unless otherwise specified)
a) Calculate PLL center frequency, f_O:
f_O + ^ǸF₁·F₂
b) Choose value of timing resistor R₀, to be in the range of 10KW to 100KW. This choice is arbitrary. The recommended
value is R₀ = 20KW. The final value of R₀ is normally fine-tuned with the series potentiometer, R_X.
R_O
+
R_O
)
R_X
2
c) Calculate value of C0 from design equation (1) or from Figure 7:
C_O +
1
R₀ ·
f₀
d) Calculate R1 to give the desired tracking bandwidth (See design equation 5).
R₁ +
R₀·f₀
(f₁–f₂)
·2
e) Calculate C1 to set loop damping. (See design equation 4):
Normally, j = 0.5 is recommended.
C₁
+
1250·C₀
R₁ · j²
Rev. 3.01
10