fx604 Consumer Microcircuits Limited, fx604 Datasheet - Page 12

• Manufacturer Part Number: fx604
• Description: V23 Compatible Modem
• Manufacturer: Consumer Microcircuits Limited

The signals on the telephone line are not suitable for direct connection to the FX604. A Line Interface circuit is
V.23 Compatible Modem
1.6
1.6.1
necessary to:
Notes:
1996 Consumer Microcircuits Limited
Application Notes
Line Interface
R2
R3
R4-R7
Provide high voltage and dc isolation
Attenuate the Tx signal present at the Rx input
Provide the low impedance drive necessary for the line
Filter the Tx and Rx signals
The components 'Z' between points B and C should match the line impedance.
Device A2 must be able to drive 'Z' and the line.
R2: For optimum results R2 should be set so that the gain is V
5.0V, rising to 150k
R3: The levels in dB (relative to a 775mV rms signal) at 'A', 'B' and 'C' in the line interface circuit are:
3.3V
5.0V
V
DD
See below
See below
100k
Resistors 1%, capacitors 10% unless otherwise stated.
-3.6dB
0dB
at V
'A'
DD
= 3.3V.
Figure 7 Line Interface Circuit
'A' = 20Log(V
'B' = 'A' + 20Log(100k /R3)
'C' = 'B' - 6
100k
150k
R3
12
DD
/5)
C5
C6
C7
-3.6dB
-3.5dB
'B'
22 F ( 20%)
100pF
330pF
DD
-9.6dB
-9.5dB
/5.0, i.e. R2 = 100k
'C'
D/604/3
FX604
at V
DD
=