CMX644AD2 View Datasheet(PDF) - MX-COM Inc
Part Name
Description
MFG CO.
'CMX644AD2' PDF : 31 Pages View PDF
Bell 212A / V.22 Modem with Call Progress and DTMF
20
CMX644A Preliminary Information
5 Application Notes
5.1 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.
5.1.1 4-Wire Line Interface
Figure 10 shows an interface circuit for use with a 600Ω 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 = 100kΩ:
R11 =
100kΩ
x
(Input
(5.0/VDD )
transformer loss
x
2.0)
Where the ‘Input transformer loss’ = (Rx level on 4-wire line) / (level at point A of Figure 10).
Assuming a transformer loss of about 1dB, R11 should be 47kΩ at VDD = 5.0V, and 68kΩ at 3.3V. The value
of the resistor R11 is optimized for the carrier detect level. Increasing the input gain (by reducing the value of
R11) will improve modem sensitivity.
Rx
4-Wire
Line
Tx
RXAMPOUT
A
1:1
C10
R10
R12
R11
VBIAS
C11
RX-
RX+
TX
C13
R13
TXIN-
-
+
CMX644A
R14
C12
1:1
R15
TX+
VBIAS
TX-
VBIAS
Figure 10: 4-Wire Line Interface Circuit
Note: The relay circuit, AC and DC loads and line protection are not shown for clarity.
R10
600Ω
R14
100kΩ
C10
R11
See text
R15
600Ω
C11
R12
100kΩ
C12
R13
See text
C13
100nF
330pF
330pF
100nF
Resistors ±1%, capacitors ±20%.
Table 25: 4-Wire Line Interface Circuit
In the transmit direction, the level on the 4-wire line is determined by the level at the TX pin, the gain of the
Output Buffer Amplifier, a loss of nominally 6dB due to the line termination resistor R15, and the loss in the
transformer.
2000 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 20480197.006
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.
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