LXT9784 データシートの表示（PDF） - Intel

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LXT9784

Octal 10/100 Transceiver Hardware Integrity Function Overview

Intel 

LXT9784 Octal 10/100 Transceiver Hardware Integrity Function Overview

Table 1. Typical CAT5 Return Loss

Load Impedance (Ω)

52

33.3

0 (short load)

Return Loss

(dB)

10

6

0

V-/V+

-0.316

-0.5

-1

Figure 1. Forward and Return Waves

R

Cable Impedance Z0

Load Impedance ZL

Forward Wave

Return Wave

Load

RJ45

The System return loss performance is determined by three elements:

• The transmitter return loss

• The cable characteristic impedance and return loss

• The receiver return loss

IEEE 802.3 specifies that the receiver and transmitter return loss must meet the following criteria:

.

Table 2. Tx and Rx Return Loss

Frequency

(MHz)

2-30

30-60

60-100

I/O Minimum Return Loss (dB)

16

16 to 10 by 16-20log(f/30 MHz)

10

There are additional factors that may affect the accuracy of the return loss measurement which are:

• Connectors and patch panels

• CAT5 UTP cable impedance can vary ± 15Ω

All this means that setting a threshold of 6db return loss in a system return loss measurement

should cover transmitter, receiver connector and cable impedance variances and should trigger a

cable problem or anomaly indication. In other words, a good indication for a problem or an

anomaly in a cable is when the return wave is equal to or greater than one-half of the incident wave.

8

Application Note

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