V23 / Bell 202 Modem
CMX624
1.6
Application Notes
1.6.1 Line Interface
A line interface circuit is needed to provide dc isolation between the modem and the line, to perform line
impedance termination, and to set the correct transmit and receive signal levels.
4-Wire Line Interface
Figure 8a shows an interface circuit for use with a 600Ω 4-wire line. The line terminations are provided by
R10 and R15, while R11 and R13 should be selected to give the desired transmit and receive levels.
The gain of the receive input amplifier (R12 / R11) should be set to compensate for the loss of the input
transformer and the supply voltage.
Assuming transformer loss of about 1dB, R11 should be 91kΩ at 5.0V, or 130kΩ at 3.0V.
Note relay circuit, ac and dc loads and line protection not shown for clarity.
R10
R11
R12
R13
R14
R15
See text
100kΩ
600Ω
C10
C11
C12
C13
100nF
220pF
330pF
100nF
600Ω
See text
100kΩ
Resistors ±1%, capacitors ±20%.
Figure 8a 4-Wire Line Interface Circuit
2003 CML Microsystems Plc
15
D/624/7
CMLMICRO [ CML MICROCIRCUITS ]
