XR-2211 Datasheet, PDF (16/24 Pages) Exar Corporation – FSK Demodulator/ Tone Decoder

XR-2211
Design Instructions:
The circuit of Figure 12 can be optimized for any tone detection application by the choice of the 5 key circuit components:
R₀, R₁, C₀, C₁ and C_D. For a given input, the tone frequency, f_S, these parameters are calculated as follows:
(All resistance in W’s, all frequency in Hz and all capacitance in farads, unless otherwise specified)
a) Choose value of timing resistor R₀ to be in the range of 10KW to 50KW. This choice is dictated by the max./min.
current that the internal voltage reference can deliver. The recommended value is R₀ = 20KW. The final value of R₀
is normally fine-tuned with the series potentiometer, R_X.
b) Calculate value of C₀ from design equation (1) or from Figure 7 f_S = f_O:
C_O
+
1
R₀·fs
c) Calculate R₁ to set the bandwidth +Df (See design equation 5):
R₁
+
R₀·f₀·2
Df
Note: The total detection bandwidth covers the frequency range of f_O +Df
d) Calculate value of C₁ for a given loop damping factor:
Normally, j = 0.5 is recommended.
C₁
+
1250·C₀
R₁·j²
Increasing C₁ improves the out-of-band signal rejection, but increases the PLL capture time.
e) Calculate value of the filter capacitor C_D . To avoid chatter at the logic output, with R_D = 470KW, C_D must be:
C_D
§
16
Df
C in mF
Increasing C_D slows down the logic output response time.
Design Examples:
Tone detector with a detection band of + 100Hz:
a) Choose value of timing resistor R₀ to be in the range of 10KW to 50KW. This choice is dictated by the max./min.
current that the internal voltage reference can deliver. The recommended value is R₀ = 20 KW. The final value of R₀
is normally fine-tuned with the series potentiometer, R_X.
b) Calculate value of C₀ from design equation (1) or from Figure 6 f_S = f_O:
C₀
+
1
R₀·f_S
+
20,
1
000·1,
000
+
50nF
Rev. 3.01
16