ML6673 Просмотр технического описания (PDF) - Micro Linear Corporation

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ML6673

Fast Ethernet/FDDI TP-PMD Transceiver

Micro Linear Corporation 

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.

IOUT

=

64 × 1.25V

RTSET

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.

ML6673

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 IOUT 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

IOUT

IOUT/2

MLT-3 DATA

0 8 16 24 32 40 48 56 64 72 80 88 96 104

(nsec)

1

NRZI DATA

0

Figure 1. MLT-3 Encoding

20

15

10

5

0 1 x 106

1 x 107

1 x 108

1 x 109

Figure 2. Equalization Range

5

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