V23 / Bell 202 Modem
CMX624
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
control bit (bit 2) of the FSK Mode Register.
and is also affected by the setting of the Tx o/p level
DD
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 = 5.0V) and that there is 1dB loss in the transformer, then:
DD
Tx FSK 4-wire line level = -(3 +6 +1) + 20 x LOG10(2 x R14 / R13) + 20 x LOG10(V / 5.0) dBm
DD
For example, to generate a nominal Tx FSK line level of -9dBm, R13 should be 180kΩ when V = 5.0V,
DD
falling to 120kΩ at 3.3V.
2-Wire Line Interface
Figure 8b 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 CMX624 is powered from less than
5.0V, buffer circuitry will be required to drive a 5V relay.
Note ac and dc loads and line protection not shown for clarity.
R11
R12
R13
R14
See text
100kΩ
See text
R15
R16
R17
C11
C12
C13
C14
220pF
330pF
10nF
600Ω
120kΩ
100kΩ
100nF
100kΩ
Resistors ±1%, capacitors ±20%
Figure 8b 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 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.
2003 CML Microsystems Plc
16
D/624/7
CMLMICRO [ CML MICROCIRCUITS ]
