CMX867 查看數據表（PDF） - CML Microsystems Plc

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CMX867

Low Power V.22 Modem

CML Microsystems Plc 

Low Power V.22 Modem

CMX867

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:

QAM, DPSK and FSK Tx modes (no guard tone)

Single tone transmit mode

DTMF transmit mode

VDD = 3.0V

-10dBm

-10dBm

-6 and -8 dBm

VDD = 5.0V

-5.5dBm

-5.5dBm

-1.5 and -3.5 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 CMX868 are proportional to VDD 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 / VDD 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. 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.

Further details of line interfacing can be found in the EV8680 and DE8681 User Manuals, available from

the CML website.

As an example, the following component values are appropriate for use with the MIDCOM 82111 line

transformer:

R11

See text

R12

100kΩ

R13

392Ω

R14

120k

R15

180k

C10

33nF

C11

100pF

C12

0.1µF

C13

47pF

© 2004 Consumer Microcircuits Limited

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