V22 and Bell 212A Modem

CMX644A

© 2000 Consumer Microcircuits Limited

21

D/644A/6

1.6

Application Notes

1.6.1

Line Interface

A line interface circuit is needed to provide dc isolation between the modem and the line, to perform line

impedance termination, and to set the correct transmit and receive signal levels.

4-Wire Line Interface

Figure 6a shows an interface circuit for use with a 600

Ω 4-wire line. The line terminations are provided by R10

and R15, while R11 and R13 should be selected to give the desired transmit and receive levels.

When VDD = 5.0V, the gain of the receive input amplifier (R12/R11) should be 6dB (times 2.0) plus whatever

additional gain is required to compensate for the loss of the input transformer. At other values of VDD the

amplifier gain should be multiplied by the ratio VDD/ 5.0.

Thus for R12 = 100k

Ω:

R11 = 100k

Ω ∗ (5.0/VDD) / (Input transformer loss * 2.0)

where the ‘Input transformer loss’ = (Rx level on 4-wire line) / (level at point A of Figure 6a).

Assuming a transformer loss of about 1dB, R11 should be 47k

Ω at VDD = 5.0V, and 68kΩ at 3.3V. The value

of the resistor R11 is optimised for the carrier detect level. Increasing the input gain (by reducing the value of

R11) will improve modem sensitivity.

Note Relay circuit, ac and dc loads and line protection are not shown for clarity.

R10

600

Ω

R14

100k

Ω

C10

100nF

R11

See text

R15

600

Ω

C11

330pF

R12

100k

Ω

C12

330pF

R13

See text

C13

100nF

Resistors

±1%, capacitors ±20%.

Figure 6a 4-Wire Line Interface Circuit