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| 型號： | 21176 |
| 英文描述： | Designing 100BASE-TX? 86.8KB (PDF) System with the QFEX Family ? |
| 中文描述： | 設(shè)計100BASE - TX的？ 86.8KB（PDF格式）系統(tǒng)？與QFEX家庭 |
| 文件頁數(shù)： | 8/15頁 |
| 文件大?。?/td> | 86K |
| 代理商： | 21176 |
8
Designing 100BASE-TX Systems with the QFEX Family
without CIM capability. The CIM function is imple-
mented on a per port basis and can be turned off if the
repeater has its own CIM function.
PMD
The PMD sublayer lies below the PMA and was origi-
nally derived from the FDDI standard. Thus, it has been
defined to support 125-Mbps fiber and shielded and
unshielded twisted pair media. The PMD basically
translates the NRZI-coded data to and from signals
suitable for the specified medium, which is in this case
twisted pair. 100BASE-TX data must be transmitted
using at least Category 5 UTP. The QFEXr device can
transmit data to MLT-3 and fiberoptic transceivers over
the TX
±
, RX
±
, and SDI
±
pins.
MLT-3 Transmission
For twisted pair, data translation is accomplished most
often by using MLT-3 (Multilevel Transmission - 3
Level). MLT-3 is very similar to NRZI, except that it adds
a third level of voltage to represent the data. Thus,
where NRZI data falls between 0V and a predeter-
mined limit of V, MLT-3 data can be any of 3 levels: 0,
+V and -V. MLT-3 uses a fundamental frequency one-
half that of NRZI. With that, MLT-3 achieves data trans-
mission at the same data rate but with lower spectral
energy (e.g., lower EMI emissions) than NRZI. Often,
MLT-3 is further combined with scrambling to eliminate
repetitive patterns in the data that causes more peaks
in the EMI spectrum.
When designing for MLT-3, the following several con-
siderations must be attended to, due to the type of
media being used:
— Adaptive Equalization
— Baseline restoration
— Transmit Conditioning
Adaptive equalization is a key feature in MLT-3. Data
transmitted over twisted pair wire can suffer much at-
tenuation (signal loss) or experience varying amounts
of attenuation, due to different frequency components
of the signal itself, as well as cable length. When the
signal is received, the device has to reconstruct the
original signal and somehow overcome this loss. Adap-
tive equalization enables the MLT-3 device to compen-
sate for this loss and for varying cable lengths by
analyzing the incoming signal and adjusting the equal-
izer accordingly.
Baseline restoration is another key feature required
for MLT-3. A problem with data transmission is base-
line wander, which is described as DC drifts in the in-
coming signal baseline wander that may occur due to
data pattern dependent DC shifts and the inherent low
frequency bandwidth of the channel and AC coupling
transformers. If not corrected, the droop component of
the transformers dominates and “drags” the signal
amplitude down below baseline so as to cause data
errors or link failure. Baseline restoration circuitry
compensates for this wander by comparing the incom-
ing signal with a reconstructed reference. The differ-
ence is filtered and used to affect low frequency
compensation in order to maintain the equalized sig-
nal at the reference level.
Transmit conditioning includes providing the correct
transmit amplitude at the TX outputs, as well as gener-
ating output waveforms synchronized in timing with
minimal jitter effects.
Figure 8 shows the QFEXr connection to MLT-3 and
magnetics components for 100BASE-TX applications.
The QFEXr device has a DIS_MLT3 pin that can be
used to enable the MLT-3 interface whether needed or
not. Although the QFEXr device does not implement
MLT-3, AMD has an alliance with Pulse Engineering to
provide MLT-3 support for the QFEXr device. Refer to
the
References
section for more information. Other
MLT-3 vendors include National Semiconductor, Micro
Linear, and GEC Plessey.
PMD Magnetics
As in 10BASE-T, magnetics solutions are still needed
to provide isolation between the cable and the PHY
layer. Available magnetics modules also support FDDI,
ATM, and 100BASE-TX over unshielded (UTP Cate-
gory 5) and shielded twisted pair (STP). The modules
are mainly used to provide isolation for data transmis-
sion from the cables. Most modules are comprised of
any combination of filters, transformers, and chokes for
common mode rejection, but filters and chokes are op-
tional and can be obtained separately. They provide
high voltage isolation (ESD protection @ 2 KV), wide
bandwidth, and fast rise times.
An important point to note is to follow termination
guidelines as recommended by the magnetics manu-
facturer. This is interpreted as a combination of resis-
tors and decoupling capacitors and proper grounding
on non-data lines (including centertaps) used to re-
duce common mode noise, that provides significant
reduction in EMI and noise susceptibility. Center tap
connections can typically provide a 10 dB improve-
ment in common mode rejection. Often, the magnet-
ics module can be the savior for a 100-Mbps system
to pass FCC tests, which is much more sensitive than
a 10-Mbps system.
The magnetics modules connect to the RJ-45 interface
and to the transceiver device through the RD-to-RX
and TD-to-TX signals. Proper termination and imped-
ance matching to MLT-3 devices is often provided in ap-
plication notes of the transceiver vendor. Their
guidelines need to be followed carefully to obtain the
best performance from cable to PHY.
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