Half Duplex GMSK Modem
FX579
1.4
External Components
External Components
Component
Value
1MW
Note 1
Note 3
100nF
100nF
Tolerance
±10%
± 5%
Component
Value
Note 2
Note 2
Note 3
Note 3
Note 1
Tolerance
±20%
±20%
R1
R2
X1
C1
C2
C3
C4
C5
C6
C7
±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
4800 bits/sec BT = 0.5
8000 bits/sec BT = 0.3
9600 bits/sec BT = 0.5
19 200 bits/sec BT = 0.5
32 000 bits/sec BT = 0.3
32 000 bits/sec BT = 0.5
38 400 bits/sec BT = 0.3
38 400 bits/sec BT = 0.5
180.0kW
91.0kW
91.0kW
47.0kW
91.0kW
47.0kW
47.0kW
47.0kW
47.0kW
470pF
470pF
470pF
470pF
120pF
220pF
150pF
180pF
120pF
Note that in all cases the value of R2 should not be less than 47.0kW 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 (VDD = 5V) and up to 5.0MHz
(VDD = 3V). The value chosen for C5 and C6 should be suitable for the applied VDD 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 2.0kW at 1MHz falling to 150W at the maximum frequency. Stray
capacitance on the Xtal/Clock circuit pins must be minimised.
Figure 2 Recommended External Components (SOIC Package)
ã 1996 Consumer Microcircuits Limited
6
D/579/4