XEV90 查看數據表（PDF） - Xecom
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XEV90 Datasheet PDF : 10 Pages
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2/4 Wire Convertor
R2
Transmit
R3
R1
10K
11.5K
4558
R4 40K
Receive
R6
320 ohms
20K R5
XEV90
T1
T2
4558
2/4 Wire Conversion:
Full Duplex communications over a two-wire telephone line
requires that transmit and receive signal share the available
bandwidth. The two-to-four wire convertor separates these
signals at the host interface. Most modem analog front end
chips incorporate an internal 2/4 wire convertor making it
unnecessary to provide one in the DAA.
If you are using the XEV90 for an application other than a
modem, such as voice processing, or your modem analog front
end does not provide the 2/4 wire convertor, you will need to
provide a discrete 2/4 wire convertor. The schematic on this
page shows a simple 2/4 wire convertor circuit.
recommended value for the impedance matching resistor, R6,
is used variations from line to line alter the impedance match.
The value of R3 can be changed to improve the Transhybrid
Loss.
The 2/4 wire convertor also amplifies the transmit and receive
signals to compensate for the insertion loss of the DAA. This
circuit provides 6 dB gain of both the transmit and receive
signals. The values of R1 and R2 set the transmit gain. The
values of R4 and R5 set the receive gain.
The performance of the 2/4wire convertor is measured by its
Transhybrid Loss. The Transhybrid Loss shows how much the
2/4 wire convertor attenuates the transmit signal on the received
data line. The circuit above provides a typical Transhybrid Loss
of 20 dB.
The Transhybrid Loss will vary with the quality of the
impedance match to the telephone line. Even when the
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