FSK Modem and DTMF Codec
CMX865
Resistor R13 is used to match the ac impedance of the interface to the line. With an ideal transformer this
resistor would be equal to the desired impedance (e.g. 600Ω); however in practice with a real transformer,
R13 should be set such that the interface as a whole presents the desired impedance. Line transformer
manufacturers normally provide guidance in this regard.
The transmit line signal level is determined by the voltage swing between the TXA and TXAN pins, less
6dB due to the line termination, 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:
VDD = 3.3V
Tx modem modes
Single tone transmit mode
DTMF transmit mode
-9.2dBm
-9.2dBm
-5.2 and –7.2 dBm
For a line impedance of 600Ω, 0dBm = 775mVrms. See also section 7.1.3
In the receive direction, the signal detection thresholds within the CMX865 are proportional to VDD and
are affected by the Rx Gain Control gain setting in the Rx Mode Register. The signal level into the
CMX865 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 7.1.3) the value of R11 should be equal to 500 / VDD kΩ i.e. 150kΩ at 3.3V.
For best Rx performance it is recommended that the transformer coupling arrangement should provide at
least 7dB trans-hybrid loss. This is achieved by minimising the amount of the transmitted signal presented
to the receiver at RXAFB. A mis-match between the transformer impedance and R13 will result in a
proportion of the transmitted signal being fed to the receiver op-amp circuit via R11. The effect of this can
be significantly nulled by careful selection of the potential divider components R14 and R15 to provide a
cancellation signal at RXA. (Note: with an ideal transformer, R13 would be set equal to the line
impedance, and R14 would be set equal to R15.
As an example, the following component values are appropriate for use with the MIDCOM 82111 line
transformer:
R11
R12
R13
See text
100kΩ
392Ω
C10
C11
33nF
100pF
© 2005 CML Microsystems Plc
8
D/865/3
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