LXT9784 データシートの表示（PDF） - Intel
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LXT9784 Datasheet PDF : 14 Pages
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LXT9784 Octal 10/100 Transceiver Hardware Integrity Function Overview
2.0
Transmission Lines and Return Wave Theory
All transmission lines have two coexisting waves propagating through the media at the same time.
The main wave is the forward wave propagating from the transmitter to the receiver. The second
wave is a return wave created by an imperfect line or load and propagates from the load to the
source (or transmitter).
2.1
Cable Terminations and Return Loss
A perfectly terminated line is defined as a line with no attenuation and an impedance that is equal
to the source’s impedance and with a load that is equal to the line impedance as well. For a
perfectly terminated line the return wave is zero. In that case the load receives all the forward wave
energy.
In an open line, meaning there is no load connected to its end, the return wave amplitude is equal to
the forward wave amplitude. The same applies for a short circuit line.
The reflection coefficient is defined as:
TL =
Reflected wave
Forward wave
=
V- =
V+
ZL - Z0
ZL + Z0
Where ZL is the load impedance
Z0 is the cable impedance
The return loss in (dB) is defined as:
RL(db) = 20log10 |1/TL| = 20log 10 ZL + Z0
ZL - Z0
Figure 1 illustrates the forward and return waves within a cable system. Table 1 describes the
Return Loss as a function of Load Impedance for cable with characteristic impedance of 100Ω
(typical CAT5 UTP cable impedance).
Table 1. Typical CAT5 Return Loss
Load Impedance (Ω)
∞ (open load)
300
192
138
100 (perfectly terminated
load)
73
Return Loss
(dB)
0
6
10
16
∞
16
V-/V+
1
0.5
0.316
0.158
0
-0.158
Application Note
7
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