Low Power V.22 bis Modem
CMX868
The transmit line signal level is determined by the voltage swing between the TXA and TXAN pins, less
6dB due to the line termination resistor R13, and less the loss in the line coupling transformer.
Allowing for 1dB loss in the transformer, then with the Tx Mode Register set for a Tx Level Control gain
of 0dB the nominal transmit line levels will be:
V
DD
= 3.0V
-10dBm
-10dBm
-6 and -8 dBm
V
DD
= 5.0V
-5.5dBm
-5.5dBm
-1.5 and -3.5 dBm
QAM, DPSK and FSK Tx modes (no guard tone)
Single tone transmit mode
DTMF transmit mode
For a line impedance of 600Ω, 0dBm = 775mVrms. See also section 1.7.1.3
In the receive direction, the signal detection thresholds within the CMX868 are proportional to V
DD
and
are affected by the Rx Gain Control gain setting in the Rx Mode Register. The signal level into the
CMX868 is affected by the line coupling transformer loss and the values of R11 and R12 of Figure 4a.
Assuming 1dB transformer loss, the Rx Gain Control programmed to 0dB and R12 = 100kΩ, then for
correct operation (see section 1.7.1.3) the value of R11 should be equal to 500 / V
DD
kΩ i.e. 160kΩ at
3.0V, falling to 100kΩ at 5.0V. For best Rx performance, it is recommended that the transformer
coupling arrangement should provide at least 7dB trans-hybrid loss. Further details of line interfacing can
be found in the EV8680 and DE8681 User Manuals, available from the CML website.
4-Wire Line Interface
Figure 4b shows a simplified interface for use with a 600Ω 4-wire line. The line terminations are provided
by R10 and R13, high frequency noise is attenuated by C11 while R11 and R12 set the receive signal
level into the modem.
Transmit and receive line level settings and the value of R11 are as for the 2-wire circuit.
R10, 13
R11
R12
600Ω
See text
100kΩ
C3
C11
C12
Resistors
±5%,
capacitors
±20%
See Figure 2
100pF
33nF
Figure 4b 4-Wire Line Interface Circuit
©
2002 Consumer Microcircuits Limited
8
D/868/6
CMLMICRO [ CML MICROCIRCUITS ]
