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
21
CMX644A Preliminary Information
The TX pin signal level is proportional to VDD. It is also affected by the setting of the transmitter
programmable gain block.
Assuming that the Tx programmable gain block is set to -2dB (giving a PSK signal level of -4dB with respect
to 775mVRMS at the TX pin when VDD = 5.0V) and that there is 1dB loss in the transformer, then:
Tx PSK 4 - wire line level =
-
(4
+
6
+
1)
+
20
x
LOG10
(2
x
R14
R13
)
+
20
x
LOG10
(
VDD
5.0
)
dBm
For example, to generate a nominal Tx PSK line level of -10dBm, R13 should be 180kΩ when
VDD = 5.0V, falling to 120kΩ at 3.3V.
5.1.2 2-Wire Line Interface
Figure 11 shows an interface circuit suitable for connection to a 600Ω 2-wire line. The circuit also shows how
a relay may be driven from the RLYDRV pin. Note that when the CMX644A is powered from less than 5.0V,
buffer circuitry will be required to drive a 5V relay.
RXAMPOUT
2-Wire
Line
R12
R11
C11
RX- -
RX+ +
RLYDRV
TX
R16 R17
C14
R13
CMX644A
TXIN-
C13
1:1
R15
R14
C12
TX+
VBIAS
TX-
VBIAS
Figure 11: 2-Wire Line Interface Circuit
Note: AC and DC loads and line protection are not shown for clarity
R11
See text
R15
R12
100kΩ
R16
R13
See text
R17
R14
100kΩ
600Ω
120kΩ
100kΩ
C11 330pF
C12 330pF
C13 10nF
C14 100nF
Resistors ±1%, capacitors ±20% unless otherwise specified
Table 26: 2-Wire Line Interface Circuit
This circuit includes a 2-wire to 4-wire hybrid circuit, formed by R11, R15, R16, R17, C13 and the impedance
of the line itself, which ensures that the modem receive input and transmit output paths are both coupled
efficiently to the line, while minimizing coupling from the modem’s transmit signal into the receive input.
The values of R11 and R13 should be calculated in the same way as for the 4-wire interface circuit of Figure
10.
Note: The component values R17 and C13 depend on the transformer characteristics and should be adjusted
to achieve a flat frequency response measured at the RXO pin.
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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