ML6673
FUNCTIONAL DESCRIPTION
The ML6673 MLT-3 transceiver is a physical media
dependent transceiver that allows the transmission and
reception of 125 Mbaud data over shielded twisted pair
cable or category 5 unshielded twisted pair cable. It
provides a standard Physical Media Dependent (PMD)
interface compatible with many FDDI chip sets.
The transmit section accepts NRZI data, converting it to a
three level MLT-3 code and sending the information on a
two pin current driven transmitter. The transmitted output
passes through an external low pass filter and transformer
before entering the connectors to the STP or UTP cable.
The output amplitude of the transmitted signal is
programmable through the external RTSET resistor.
I
OUT
=
64
×
1.25V
RTSET
The adaptive control block governs both the equalization
level as well as the signal detection status. Signal detect is
asserted when the equalizer control loop settles or when
loop back is asserted. When the input signal is small, the
equalization will be at its maximum.
After the signal has been equalized, it passes into the MLT-3
to NRZI converter where it is converted back to NRZI
and fed through the loopback multiplexer onto the
RXOUT± pins.
Figure 1 shows a timing diagram of NRZI data and the
equivalent MLT-3 data. The MLT-3 data shows the output
current I
OUT
for one side of the transmitter, either TPOUT+
or TPOUT–. The other transmit output pin will be the
complement. Whenever there is a change in level in
NRZI, MLT-3 will change levels too. The maximum
fundamental frequency of MLT-3 is half of the maximum
fundamental of NRZI.
Figure 2 shows a typical gain vs frequency plot of the
adaptive equalizer for 0, 25, 50, 75 and 100 meter
category 5 cable lengths.
ML6671 COMPATIBILITY
The ML6673 implements the Baseline Wander correction
circuit, in addition to providing the functionality of the
existing ML6671 device. The ML6673 is plug-compatible
with the ML6671 with the following note:
• In the ML6673 design, the following passive
components may be eliminated
— RSET resistor
— RTH resistor
— CAP1 capacitor
— CAP2 capacitor
For 100BASE-TX UTP application, the transmit amplitude
is 2VP-P differential achieved by setting RTSET = 2kΩ (1%).
The receive section accepts MLT-3 coded data after
passing through an isolation transformer and band
limiting filter. Before the data can be converted from MLT-
3 back to NRZI, the adaptive equalizer is used to
compensate for the amplitude and phase distortion
incurred from the cable. The adaptive control section
determines the signal amplitude (and therefore the cable
length) and adjusts the equalizer accordingly.
The receiver also includes the Baseline Wander correction
circuitry. The circuit will compensate and track the DC
baseline wander caused by DC imbalance of the received
data. It will tolerate the test pattern as specified in the
ANSI X3T12 TP-PMD specification. A parallel 10pF
capacitor can be connected between TPIN+ and TPIN–
to improve Bit Error Rate.
MLT-3 DATA
CURRENT (mA)
20
IOUT
15
IOUT/2
10
0
8 16 24 32 40 48 56 64 72 80 88 96 104
(nsec)
5
NRZI DATA
1
0
0
1 x 106
1 x 107
1 x 108
1 x 109
Figure 1. MLT-3 Encoding
Figure 2. Equalization Range
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
