NJM2211 datasheet(7/10 Pages) NJRC

7 / 10 page

NJM2211

-7-

Ver.2003-12-09

■ APPLICATIONS

FSK Decoding

Figure 2 shows the basic circuit connection for FSK decoding. With reference to Figures 1 and 2, 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. CF and RF from a one pole post-detection filter for

the FSK data output. The resistor RB (=510kΩ) 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.

Design Instructions

The circuit of Figure 2 can be tailored for any FSK decoding application by the choice of five 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 :

1. Calculate PLL center frequency, f0

2. Chose a value of timing resistor R0 to be in the range of 10kΩ to 100kΩ. This choice is arbitary. The recommended

value is R0

≅ 20kΩ. The final value of R0 is normally fine-tuned 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 ∆f equal to the mark-space deviation :

R1=R0 [f0 / (f1 - f2)]

5. Calculate C1 to set loop damping. (See Design Equation No.4.)

Normally, ξ ≈ 1 / 2 is recommended

Then :C1=C0 / 4 for ξ =1 / 2

6. Calculate Data Filter Capacitance, CF :

For RF=100kΩ. RB=510kΩ, the recommended value of CF is :

Rate

Band

)

(

Note : All calculated component values except R0 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).

Table 1. Recommended Value for FSK

(Ref. Fig. 2)

FSK Band

Component Values

300 Band

C0=0.039µF CF=0.005µF

F1 =1070Hz

C1=0.01µF R0=18kΩ

f2 =1270Hz

R1=100kΩ

300 Band

C0=0.022µF CF=0.005µF

f1=2025Hz

C1=0.0047µF R0=18kΩ

f2=2225Hz

R1=200kΩ

1200 Band

C0=0.027µF CF=0.0022µF

f1=1200Hz

C1=0.01µF R0=18kΩ

f2=2200Hz

R1=30kΩ

Figure 2. FSK Decoding

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