Low Power V.32 bis Modem
CMX869
4.3
Line Interface
A line interface circuit is needed to provide dc isolation and to terminate the line. The CMX869 is
connected to the line when the line relay is closed. The relay may be driven from the RDRVN output, as
shown in figure 4a. Control of the output level of the RDRVN pin is described in section 6.2. The diagrams
of Figure 4a and Figure 4b are functional representations only and should not be used in product designs.
A reference design will be available separately.
2-Wire Line Interface
Figure 4a shows a simplified interface for use with a 600Ω 2-wire line. The line termination impedance is
provided by the transformer, R13 and C10, high frequency noise is attenuated by C10 and C11, while R11
and R12 set the receive signal level into the modem. For clarity the 2-wire line protection circuits have not
been shown.
R11
R12
R13
R14
C3
See Figure 2
33nF
100pF
130kΩ (see text)
100kΩ
600Ω
C10
C11
C12
100nF, 250V (see text)
15kΩ (see text)
Resistors ±5%, capacitors ±20%
Figure 4a 2-Wire Line Interface Circuit
The transmit line signal level is determined by the voltage swing between the TXA and TXAN pins, less
6dB due to the line termination resistor R13, and less the loss in the line coupling transformer. Allowing for
1dB loss in the transformer, with the Tx Mode Register set for a Tx Level Control gain of 0dB the nominal
transmit line levels will be:
VDD = 3.3V
QAM and FSK Tx modes (no guard tone)
Single tone transmit mode
-7.5 dBm
-7.5 dBm
DTMF transmit mode
-3.5 and -5.5 dBm
For a line impedance of 600Ω, 0dBm = 775mVrms. See also section 8.1.3
© 2004 CML Microsystems Plc
9
D/869/4
CMLMICRO [ CML MICROCIRCUITS ]
