ML6672

5

FUNCTIONAL DESCRIPTION

The ML6672 transceiver is a physical media dependent

transceiver that allows the transmission and reception of

155 Mbps data 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 cable length and

adjusts the equalizer accordingly. As the input signal

amplitude diminishes, the amount of equalization increases

until it reaches its maximum of an equivalent 100 meters of

category 5 cable. A parallel 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 link detection status. The link detection

threshold has a fixed relationship to the overall

equalization level which is currently 25% of the

transmitted amplitude. For the link status to be true, a

minimum level signal must be received. 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

ML6672 and the current driven transmitter then sent the

data to the filter/transformer module. The transmit

amplitude is controlled by one external resistor, RTSET.

I

V

RTSET

´

64

125

.

For ATM UTP applications the transmit amplitude is 1V

peak to peak. The termination at the transmitter output is

50

Therefore, RTSET = (64 x 1.25/20)k

W = 4kW

The transmitter may be disabled via the TXOFF pin. When

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

through the transformer.

Figure 1. Equalization Range

20

15

10

5

0

1 x 106

1 x 107

1 x 108

1 x 109

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 loop 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

compensate different length of cable without over

equalizing or under equalizing the line. The ML6672

monitors the energy 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.

RECEIVE CIRCUIT

After the data is received and equalized, it is then sent to

the clock recovery circuit via the RXOUT pins. A resistor

RTH is used to control the internal level of equalization.

V

RTH

RSET

´´

16 125

.

VAMP is the transmit voltage amplitude and is equal to 1V

and RSET = 5k

W. Therefore, RTH = 1 x 5/(16 x 1.25) kW =

250

W.

CAP1 and CAP2 are capacitors used to set the time

constant for adaptation of the equalizer loop and should

be 0.33µF.