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FX604

V23 Compatible Modem

CML Microsystems Plc 

V.23 Compatible Modem

FX604

1.6 Application Notes

1.6.1 Line Interface

The signals on the telephone line are not suitable for direct connection to the FX604. A Line Interface circuit is

necessary to:

• 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

R2

R3

R4-R7

See below

See below

100kΩ

C5

22µF (±20%)

C6

100pF

C7

330pF

Resistors ±1%, capacitors ±10% unless otherwise stated.

Figure 7 Line Interface Circuit

Notes:

•

•

•

•

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 VDD/5.0, i.e. R2 = 100kΩ at VDD =

5.0V, rising to 150kΩ at VDD = 3.3V.

R3: The levels in dB (relative to a 775mV rms signal) at 'A', 'B' and 'C' in the line interface circuit are:

'A' = 20Log(VDD/5)

'B' = 'A' + 20Log(100kΩ/R3)

'C' = 'B' - 6

VDD

3.3V

5.0V

'A'

-3.6dB

0dB

R3

100kΩ

150kΩ

'B'

-3.6dB

-3.5dB

'C'

-9.6dB

-9.5dB

© 1996 Consumer Microcircuits Limited

12

D/604/3

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