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LXT9883/9863 — Advanced 10/100 Unmanaged Repeater

Datasheet

Document #: 249115

Revision #: 003

Rev. Date: 08/07/01

4.2.7

Twisted-Pair Interface

The LXT98x3 transmitter uses standard 1:1 magnetics for both receive and transmit. Nonetheless,

system designers should take precautions to minimize parasitic shunt capacitance and meet return

loss specifications. These steps include:

• Place magnetics as close as possible to the LXT98x3.

• Keep transmit pair traces short.

• Do not route transmit pair adjacent to a ground plane. Eliminate planes under the transmit

traces completely. Otherwise, keep planes 3-4 layers away.

• Improve EMI performance by filtering the output center tap supply. A single ferrite bead may

be used in the center tap supply to all ports. All ports draw a combined total of > 1000 mA, so

the bead should be rated at > 1500 mA.

• Place the 270pF 5% capacitors at TPIP and TPIN to improve the signal-to-noise immunity at

the receiver.

In addition, follow all the standard guidelines for a twisted-pair interface:

• Route the signal pairs differentially, close together. Allow nothing to come between them.

• Keep distances as short as possible; both traces should have the same length.

• Avoid vias and layer changes.

• Keep the transmit and receive pairs apart to avoid cross-talk.

• To provide maximum isolation, place entire receive termination network on one side and

transmit on the other side of the PCB.

• Bypass common-mode noise to ground on the in-board side of the magnetics using 0.01

µF

capacitors.

• Keep termination circuits grouped closely together and on the same side of the board.

• Always put termination circuits close to the source end of any circuit.

4.2.7.1

Magnetics Information

The LXT98x3 requires a 1:1 ratio for the receive transformers and a 1:1 ratio for the transmit

transformers. The transformer isolation voltage should be rated at 2kV to protect the circuitry from

static voltages across the connectors and cables. Refer to Table 17 for magnetics specifications.

Table 17. LXT98x3 Magnetics Specifications

Parameter

Min

Nom

Max

Units

Test Condition

Rx turns ratio

–

1 : 1

––

Tx turns ratio

–

1 : 1

––

Insertion loss

0.0

–

1.1

80 MHz

Primary inductance

350

––

µH

Transformer isolation

–

Differential to common mode rejection

-40

––

.1 to 60 MHz

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