Draft 6/5/00
Block Diagram Description
2-19
Copyright 2000 by LSI Logic Corporation. All rights reserved.
2.2.7 Twisted-Pair Transmitters
This section describes the operation of the 10 and 100 Mbits/s TP
transmitters.
2.2.7.1 100 Mbits/s TP Transmitter
The TP transmitter consists of an MLT3 encoder, waveform generator,
and line driver.
The MLT3 encoder converts the NRZI data from the scrambler into a
three-level code required by IEEE 802.3. MLT3 coding uses three levels,
converting ones to transitions between the three levels, and zeros to no
transitions or changes in level.
The purpose of the waveform generator is to shape the transmit output
pulse. The waveform generator takes the MLT3 three level encoded
waveform and uses an array of switched current sources to control the
shape of the twisted-pair output signal. The waveform generator consists
of switched current sources, a clock generator, filter, and logic. The
switched current sources control the rise and fall time as well as signal
level to meet IEEE 802.3 requirements. The output of the switched
current sources goes through a second order low-pass filter that
“smooths” the current output and removes any high-frequency
components. In this way, the waveform generator preshapes the output
waveform transmitted onto the twisted-pair cable such that the waveform
meets the pulse template requirements outlined in IEEE 802.3. The
waveform generator eliminates the need for any external filters on the TP
transmit output.
The line driver converts the shaped and smoothed waveform to a current
output that can drive greater than 100 meters of category 5 unshielded
twisted-pair cable or 150-ohm shielded twisted-pair cable.
2.2.7.2 10 Mbits/s TP Transmitter
Even though the 10 Mbits/s transmitter operation is much different than
that of 100 Mbits/s, it also consists of a waveform generator and line
driver (see
Figure 2.1
).
The waveform generator, which consists of a ROM, DAC, clock
generator, and filter, shapes the output transmit pulse. The DAC