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ML6674

ATM 155Mbps UTP Transceiver

Micro Linear Corporation 

FUNCTIONAL DESCRIPTION

The ML6674 transceiver is a physical media dependent

transceiver that allows the transmission and reception of

155 Mbps data over 100 meters over shielded twisted pair

cable or category 5 unshielded twisted pair cable.

The transmit section accepts NRZ data, 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.

The receive section accepts NRZ coded data after it

passes through an isolation transformer and band limiting

filter. 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. As the input signal amplitude

diminishes, the amount of equalization increases until it

reaches its maximum (Figure 1).

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. A 10pF capacitor can be connected between TPIN+

and TPIN– to improve Bit Error Rate.

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 is fed through the

loopback multiplexer onto the RXOUT± pins.

Figure 1 shows a typical gain vs frequency plot of the

adaptive equalizer for 0, 25, 50, 75 and 100 meter

category 5 cable lengths.

TRANSMISSION

PECL level scrambled NRZ data is received by the

ML6674 and the current driven transmitter then sent the

data to the filter/transformer module. The transmit

amplitude is controlled by one external resistor, RTSET.

IOUT

=

32 × 1.25V

RTSET

For ATM UTP applications the transmit amplitude is 1V

peak to peak. The termination at the transmitter output is

50ý. Therefore the transmit current IOUT = 1/50 = 20 mA.

Thus,

RTSET = 32 × 1.25V = 2kΩ

20mA

The transmitter may be disabled via the TXOFF pin. When

this pin is pulled low, the transmitter’s output goes to a

ML6674

high impedance state and no current flows through the

transformer.

ADAPTIVE EQUALIZATION

During transmission of data over UTP (unshielded twisted

pair), distortion and ISI are caused by dispersion in the

cable. Equalization is used to overcome this signal

corruption. However, the distortion is frequency

dependent and cable length dependent. Therefore, in most

practical cases, the TP port characteristic is unknown and

it is impractical to tune the equalizer specifically to each

individual port. Hence, adaptive equalizer is used in the

TP-PMD to ensue proper compensation of the received

signal.

By using adaptive equalizer, the receiver automatically

compensates different length of cable without over

equalizing or under equalizing the line. The ML6674

monitors the amplitude of the received signal to

determine the cable length and adjust the equalizer

accordingly. The input signal level is inversely

proportional to the cable length. Therefore, as the signal

level decreases, the amount of equalization is increased

to compensate for the line loss.

ML6672 COMPATIBILITY

The ML6674 implements the Baseline Wander correction

circuit, in addition to providing the functionality of the

existing ML6672 device. The ML6674 is plug-compatible

with the ML6672 with the following notes:

•

In the ML6674 design, the RTSET resistor must be

2.0kW

•

In the ML6674 design, the following passive

components used in the ML6672 design may be

eliminated from the ML6674 design

— RSET resistor

— RTH resistor

— CAP1 capacitor

— CAP2 capacitor

20

15

10

5

0 1 x 106

1 x 107

1 x 108

Figure 1. Equalization Range

1 x 109

5

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