FSK Modem and DTMF Codec
CMX865
4-Wire Line Interface
Figure 4b shows a simplified interface for use with a 600Ω 4-wire line. The line terminations are provided
by R10 and R13, the values of which are dependent on the choice of transformer: see notes above. High
frequency noise is attenuated by C11 while R11 and R12 set the receive signal level into the modem.
Transmit and receive line level settings and the value of R11 are as for the 2-wire circuit.
R10, 13
R11
R12
See text
See text
100kΩ
C3
C11
C12
See Figure 2
100pF
33nF
Resistors ±5%, capacitors ±20%
Figure 4b 4-Wire Line Interface Circuit
© 2005 CML Microsystems Plc
9
D/865/3
CMLMICRO [ CML MICROCIRCUITS ]
