FX579 Datasheet, PDF (6/23 Pages) CML Microcircuits – HALF DUPLEX GMSK MODEM

Half Duplex GMSK Modem
1.4 External Components
FX579
External Components
Component
R1
R2
X1
C1
C2
Value
1MΩ
Note 1
Note 3
100nF
100nF
Tolerance
±10%
± 5%
±20%
±20%
Component
C3
C4
C5
C6
C7
Value
Note 2
Note 2
Note 3
Note 3
Note 1
Tolerance
±20%
±20%
±5%
Notes:
1. The RC network formed by R2 and C7 is required between the TXOP pin and the input to the modulator. This network, which can
form part of any dc level shifting and gain adjustment circuitry, forms an important part of the transmit signal filtering. The ground
connection to the capacitor C7, should be positioned to give maximum attenuation of high frequency noise into the modulator.
The component values should be chosen so that the product of the resistance (Ohms) and the capacitance (Farads) is:
BT of 0.3 = 0.34/bit rate (bits/sec)
BT of 0.5 = 0.21/bit rate (bits/sec)
with suitable values for common bit rates being:
R2
C7
4000 bits/sec BT = 0.3
180.0kΩ
470pF
4800 bits/sec BT = 0.5
91.0kΩ
470pF
8000 bits/sec BT = 0.3
91.0kΩ
470pF
9600 bits/sec BT = 0.5
47.0kΩ
470pF
19 200 bits/sec BT = 0.5
91.0kΩ
120pF
32 000 bits/sec BT = 0.3
47.0kΩ
220pF
32 000 bits/sec BT = 0.5
47.0kΩ
150pF
38 400 bits/sec BT = 0.3
47.0kΩ
180pF
38 400 bits/sec BT = 0.5
47.0kΩ
120pF
Note that in all cases the value of R2 should not be less than 47.0kΩ and that the calculated value of C7 includes calculated
parasitic capacitances.
2. C3 and C4 should both be 15.0nF for a data rate of 8kbits/sec, and inversely proportional to the data rate for other data rates, e.g.
30.0nF at 4kbits/sec, 3.0nF at 40kbits/sec.
3. The FX579 can operate correctly with Xtal/Clock frequencies between 1.0MHz and 6.5MHz (V_DD = 5V) and up to 5.0MHz
(V_DD = 3V). The value chosen for C5 and C6 should be suitable for the applied V_DD and the frequency of X1 and include stray
capacitance. As a guide: At 5 Volts, C5 = C6 = 33.0pF at 1.0MHz falling to 18pF at 6.5MHz
At 3 Volts, C5 = C6 = 33.0pF at 1.0MHz falling to 18pF at 5MHz
The equivalent series resistance of X1 should be less than 2.0kΩ at 1MHz falling to 150Ω at the maximum frequency. Stray
capacitance on the Xtal/Clock circuit pins must be minimised.
Figure 2 Recommended External Components (SOIC Package)
© 1996 Consumer Microcircuits Limited
6
D/579/4