CML Microcircuits CMX866, Manual, Page: 10 / 53

V.22 bis Modem with AT Commands CMX866
©
2008 CML Microsystems Plc 10 D/866/5
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 DSP Tx Mode Register set for a Tx Level Control gain of
0dB (S25 register set to 'xxxxx111') the nominal transmit line levels will be:
AV
DD
= 3.0V AV
DD
= 5.0V
QAM, DPSK and FSK Tx modes (no guard tone) -10dBm -5.5dBm
Single tone transmit mode -10dBm -5.5dBm
DTMF transmit mode -6 and -8 dBm -1.5 and -3.5 dBm
For a line impedance of 600
Ω
, 0dBm = 775mVrms. See also section 1.7.1.3
In the receive direction, the signal detection thresholds within the CMX866 are proportional to AV
DD
and
are affected by the Rx Gain Control gain setting in the DSP Rx Mode Register (as indicated by the value
held in the S26 register). The signal level into the CMX866 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 (S26 register set to 'xxxxx111')
and R12 = 100k
Ω
, then for correct operation (see section 1.7.1.3) the value of R11 should be equal to:
500 / AV
DD
k
Ω
i.e. 160k
Ω
at 3.0V, falling to 100k
Ω
at 5.0V
When the RDRVN pin is high impedance (= on-hook = inactive) the auxiliary receive input pin RXBN is
internally connected to the RXAN pin. This can be used to increase the Rx gain (required for Type 1
Caller Line Identification reception) by reducing the effective value of R11, as shown in Figures 11 and 12.
When the RDRVN pin is low (= off-hook = active) the RXBN pin is not connected.
4-Wire Line Interface
Figure 4b shows an interface for use with a 600
Ω
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
600
Ω C3 See Figure 2
R11 See text C11 100pF
R12
100k
Ω
C12 33nF
Resistors
±
5%, Capacitors
±
20%
Figure 4b 4-Wire Line Interface Circuit