FX614 Datasheet, PDF (12/17 Pages) CML Microcircuits – Optional Line Equalisation

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
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 V_DD/5.0, i.e. R2 = 100kΩ at V_DD =
5.0V, rising to 150kΩ at V_DD = 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(100kΩ/R3)
"
'C' = 'B' - 6
Example:
VDD
'A'
R3
'B'
'C'
3.3V
-3.6dB
100kΩ
-3.6dB
-9.6dB
5.0V
0dB
150kΩ
-3.5dB
-9.5dB
© 1997 Consumer Microcircuits Limited
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