Bell 202 Compatible Modem
FX614
1.6
Application Notes
1.6.1 Line Interface
The signals on the telephone line are not suitable for direct connection to the FX614. 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
100kW
C5
C6
C7
22mF (±20%)
100pF
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 = 100kW at VDD
5.0V, rising to 150kW 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:
Level at
'A' = 20Log(VDD/5)
'B' = 'A' + 20Log(100kW/R3)
'C' = 'B' - 6
"
"
Example:
VDD
3.3V
5.0V
'A'
-3.6dB
0dB
R3
100kW
150kW
'B'
-3.6dB
-3.5dB
'C'
-9.6dB
-9.5dB
ã 1997 Consumer Microcircuits Limited
12
D/614/4
CMLMICRO [ CML MICROCIRCUITS ]
