CMX624D5 CML Microcircuits, CMX624D5 Datasheet - Page 16

• Manufacturer Part Number: CMX624D5
• Description: V.23/Bell 202 modem
• Manufacturer: CML Microcircuits

V23 / Bell 202 Modem
In the transmit direction, the level on the 4-wire line is determined by the level at the TOP 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.
The TOP pin signal level is proportional to V
(bit 2) of the FSK Mode Register.
Assuming that the Tx o/p level control bit is set to ‘1’ (giving a FSK signal level of -3dB wrt 775mVrms at the
TOP pin when V
For example, to generate a nominal Tx FSK line level of -9dBm, R13 should be 180k
falling to 120k
2-Wire Line Interface
Figure 8b shows an interface circuit suitable for connection to a 600
a relay may be driven from the RLYDRV pin. Note that when the CMX624 is powered from less than 5.0V,
buffer circuitry will be required to drive a 5V relay.
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 minimising 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
8a.
Tx FSK 4-wire line level = -(3 +6 +1) + 20 x LOG
2001 Consumer Microcircuits Limited
R11
R12
R13
R14
at 3.3V.
DD
= 5.0V) and that there is 1dB loss in the transformer, then:
See text
100k
See text
100k
Note ac and dc loads and line protection not shown for clarity.
Figure 8b 2-Wire Line Interface Circuit
Resistors 1%, capacitors 20%
DD
and is also affected by the setting of the Tx o/p level control bit
R15
R16
R17
10
16
(2 x R14 / R13) + 20 x LOG
600
120k
100k
2-wire line. The circuit also shows how
10
(V
C11
C12
C13
C14
DD
/ 5.0) dBm
when V
220pF
330pF
10nF
100nF
CMX624
D/624/6
DD
= 5.0V,