V.23 Compatible Modem

FX604

© 1996 Consumer Microcircuits Limited

12

D/604/3

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

See below

C5

22

µF (±20%)

R3

See below

C6

100pF

R4-R7

100k

Ω

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.

•

5.0V, rising to 150k

•

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

'B' = 'A' + 20Log(100k

Ω/R3)

'C' = 'B' - 6

'A'

R3

'B'

'C'

3.3V

-3.6dB

100k

Ω

-3.6dB

-9.6dB

5.0V

0dB

150k

Ω

-3.5dB

-9.5dB