XRT73LC00 Datasheet, PDF (29/53 Pages) Exar Corporation – E3/DS3/STS-1 LINE INTERFACE UNIT

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PRELIMINARY
XRT73LC00
E3/DS3/STS-1 LINE INTERFACE UNIT
REV. P1.0.1
2.4.2 Disabling the Transmit Line Build-Out Cir-
cuit
If the Transmit Line Build-Out circuit is disabled, then
the XRT73LC00 outputs partially-shaped pulses onto
the line via the TTIP and TRING output pins.
Disable the Transmit Line Build-Out circuit in the
XRT73LC00 by doing the following:
• If the XRT73LC00 is operating in the Hardware
Mode, set the TXLEV input pin (pin 1) to “High”
• If the XRT73LC00 is operating in the HOST Mode,
set the TXLEV bit-field to “1” as illustrated below.
COMMAND REGISTER CR1 (ADDRESS = 0X01)
D4
D3
D2
D1
D0
TXOFF TAOS TXCLKINV TXLEV TXBIN
0
X
X
1
X
2.4.3 Design Guideline for Setting the Transmit
Line Build-Out Circuit
The setting ofTXLEV input pin or bit-field should be
based upon the overall cable length between the
Transmitting Terminal and the Digital Cross Connect
system where the pulse template measurements are
made.
If the cable length between the Transmitting Terminal
and the DSX-3 or STSX-1 is less than 225 feet, it is
advisable to enable the Transmit Line Build-Out circuit
by setting the TXLEV input pin or bit-field to "0".
NOTE: In this case the XRT73LC00 outputs shaped (e.g.,
not square-wave) pulses onto the line via the TTIP and
TRING output pins. The shape of this output pulse is such
that it complies with the pulse template requirements even
when subjected to cable loss ranging from 0 to 225 feet.
If the cable length between the Transmitting
Terminal and the DSX-3 or STSX-1 is greater than
225 feet, it is advisable to disable the Transmit Line
Build-Out circuit by setting the TXLEV input pin or bit-
field to "1".
NOTE: In this case the XRT73LC00 outputs partially-
shaped pulses onto the line via the TTIP and TRING output
pins. The cable loss that these pulses experience over long
cable lengths (e.g., greater than 225 feet) causes these
pulses to be properly shaped and comply with the appropri-
ate pulse template requirement.
2.4.4 The Transmit Line Build-Out Circuit and
E3 Applications
The ITU-T G.703 Pulse Template Requirements for
E3 states that the E3 transmit output pulse should be
measured at the Secondary Side of the Transmit Out-
put Transformer for Pulse Template compliance.
There is no Digital Cross Connect System pulse tem-
plate requirement for E3 and the Transmit Line Build-
Out circuit in the XRT73LC00 is disabled whenever it
is operating in the E3 Mode.
2.5 INTERFACING THE TRANSMIT SECTION OF THE
XRT73LC00 TO THE LINE
The E3, DS3 and SONET STS-1 specification docu-
ments all state that line signals transmitted over coax-
ial cable are to be terminated with 75 Ohms. There-
fore, interface the Transmit Section of the
XRT73LC00, as illustrated in Figure 16 which shows
two 31.6 Ohm resistors in series with the primary side
of the transformer. These two 31.6Ohm resistors
closely match the 75Ohm load termination resistor
thereby minimizing Transmit Return Loss.
FIGURE 16. RECOMMENDED SCHEMATIC FOR INTERFACING THE TRANSMIT SECTION OF THE XRT73LC00 TO THE
LINE
TTIP
R1
31.6Ω
BNC
T1
1:1
TRING
R2
31.6Ω
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