參數(shù)資料
型號(hào): LXT9883HC
英文描述: LAN HUB CONTROLLER
中文描述: 局域網(wǎng)集線器控制器
文件頁(yè)數(shù): 50/96頁(yè)
文件大?。?/td> 1309K
代理商: LXT9883HC
LXT980/980A Dual-Speed, 5-Port Fast Ethernet Repeater
50
Datasheet
3.1.5
The RBIAS Pin
The LXT980 requires a 22.1 k
,
1% resistor directly connected between the RBIAS pin and
ground. Place the RBIAS resistor as close to the RBIAS pin as possible. Run an etch directly from
the pin to the resistor, and sink the other side of the resistor to ground. Surround the RBIAS trace
with ground; do not run high-speed signals next to RBIAS.
3.1.6
The Twisted-Pair Interface
Because the LXT980 transmitter uses 2:1 magnetics, system designers must take extra precautions
to minimize parasitic shunt capacitance in order to meet return loss specifications. These steps
include:
Use compensating inductor in the output stage (
Figure 21
).
Place magnetics as close as possible to the LXT980.
Keep transmit pair traces short.
Do not route transmit pair adjacent to a ground plane. If possible, eliminate planes under the
transmit traces completely. Otherwise, keep planes 3-4 layers away.
Some magnetic vendors are producing magnetics with higher than average return loss
performance. Use of these improved magnetics increases the return loss budget available to
the system designer.
Improve EMI performance by filtering the output center tap. A single ferrite bead may be used
to supply center tap current to all four ports.
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 as much as possible.
Keep the transmit and receive pairs apart to avoid cross-talk.
If possible, place entire receive termination network on one side and transmit on the other.
Keep termination circuits close together and on the same side of the board.
Always put termination circuits close to the source end of any circuit.
Bypass common-mode noise to ground on the in-board side of the magnetics using 0.01
μ
F
capacitors.
3.1.7
The Fiber Interface
The fiber interface consists of a pseudo-ECL (PECL) transmit and receive pair to an external fiber
optic transceiver. The transmit pair should be AC coupled to the transceiver, and biased to 3.7V
with a 50
equivalent impedance. The receive pair can be DC-coupled, and should be biased to
3.0V with a 50
equivalent impedance.
Figure 20 on page 55
shows the correct bias networks to
achieve these requirements.
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