CMX850L8 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'CMX850L8' PDF : 103 Pages View PDF
Communications Controller
CMX850
R10
R11, 14
R12
R13
R15
180kΩ
See text
100kΩ
600Ω
120kΩ
C17
100nF
C18
47pF (USA)
C19
wire link (USA)
C23
100pF
C24
33nF
Resistors ±5%, capacitors ±20%
Figure 4a 2-Wire Line Interface Circuit
The transmit line signal level is determined by the voltage swing between the TXP and TXN 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:
QAM, DPSK and FSK Tx modes (no guard tone)
Single tone transmit mode
DTMF transmit mode
VDD = 3.3V
-9.2dBm
-9.2dBm
-5.2 and -7.2 dBm
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 modem are proportional to VDD and are
affected by the Rx Gain Control gain setting in the Rx Mode Register. The signal level into the modem 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 / VDD kΩ, i.e. 150kΩ at VDD
= 3.3V.
4-Wire Line Interface
Figure 4b shows a simplified interface for use with a 600Ω 4-wire line. The line terminations are provided by
R13 and R16, high frequency noise is attenuated by C23 while R11 and R12 set the receive signal level into
the modem. R14 connects in parallel with R11 when enabled in the Analogue Signal Path Register. This is
for the purpose of increasing gain, which is necessary to compensate for signal attenuation during on-hook
CLI detection.
© 2003 CML Microsystems Plc
15
D/850/6
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