CMX878D1 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'CMX878D1' PDF : 61 Pages View PDF
Line Powered Modem plus DAA
CMX878
1.4.2.3 Loop Current Control, AC Impedance and Modulation
When the CMX878 is in Standby mode, the GYON and ICTRL output pins will be at Vss, hence Q4, Q5
and M2 will be turned off and will draw no current from the telephone line.
In the off-hook mode, the regulated supply will be turned on, the GYON output will be at VDD and the
ICTRL output will be high impedance, hence M2 will be turned on, providing drive to the quasi-darlington
Q4/Q5. The DC loop current taken from the line is then controlled by the network R16, D8, R19, R20,
R14 and R15. Transistor Q6, together with the voltage divider R17, R18 and decoupling capacitor C11,
act as an optional DC current limiter which limits to approx. 60mA with the recommended components
(as required in TBR21). With selection of the appropriate external component values the resulting DC
line voltage vs. current characteristic can be made to meet the requirements of TBR21 or EIA-470-A.
The AC impedance presented to the line in the off-hook mode is controlled by the network C10, R15,
R14. With selection of the component values for the appropriate market it will either give a good match
to the TBR21 (European) nominal impedance as shown below…
…or 600Ω for American systems.
The analogue signal output of the CMX878 modem appears at the MOD pin and modulates the line
voltage via C13, R22, R21, Q5 and Q4. C14 attenuates any high frequency noise that may be present at
the MOD output pin.
When the regulated supply is enabled, the CMX878 may also be set into a ‘line probing’ mode, in which it
draws a programmable current from the telephone line. This can be useful in characterising the line. In
this mode the GYON output is at VSS and the line current is determined by the output of the DAC
appearing at the ICTRL output pin. Note that in this mode the AC impedance presented to the line and
the transmit signal levels will not be correct.
1.4.2.4 Rx Hybrid
The AC signal input to the CMX878 is taken from the line through C16 and R25 to the input amplifier pin
RXIN, the level of the receive signal appearing at the RXFB pin being determined by the ratio of the
resistors R25 and R24. C15 and R23 provide a receive hybrid function to reduce the level of transmitted
signal appearing at the RXFB pin (the CMX878 requires a minimum of 7dB rejection, however with
careful component selection it will be possible to greatly exceed this).
1.4.2.5 Transmit Levels and Receive Thresholds
Transmit levels and receive thresholds are proportional to AVDD. 0dBm = 775mVrms.
With the circuit in Figure 4a (which regulates AVDD to 3.3V), the Tx Mode Register set for a Tx Level
Control gain of 0dB and a matched line, the nominal transmit line levels will be:
QAM, DPSK and FSK Tx modes (no guard tone)
Single tone transmit mode
DTMF transmit mode
-10dBm
-10dBm
-6 and -8 dBm
With the Rx Mode Register set for a Rx Level Control gain of 0dB, the nominal receiver thresholds will be
as stated in the Operating Characteristics.
© 2002 Consumer Microcircuits Limited
12
D/878/2
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