CMX869D2 Просмотр технического описания (PDF) - CML Microsystems Plc

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CMX869D2

Low Power V.32 bis Modem

CML Microsystems Plc 

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

130kΩ (see text)

C3

See Figure 2

R12

100kΩ

C10

33nF

R13

600Ω

C11

100pF

R14

15kΩ (see text)

C12

100nF, 250V (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:

QAM and FSK Tx modes (no guard tone)

Single tone transmit mode

DTMF transmit mode

VDD = 3.3V

-7.5 dBm

-7.5 dBm

-3.5 and -5.5 dBm

For a line impedance of 600Ω, 0dBm = 775mVrms. See also section 8.1.3

© 2003 CML Microsystems Plc

9

D/869/1

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