DP83848C_07 datasheet(28/82 Pages) NSC

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4.2.2.1 Digital Adaptive Equalization and Gain Control

When transmitting data at high speeds over copper twisted

pair cable, frequency dependent attenuation becomes a

concern. In high-speed twisted pair signalling, the fre-

quency content of the transmitted signal can vary greatly

during normal operation based primarily on the random-

ness of the scrambled data stream. This variation in signal

attenuation caused by frequency variations must be com-

pensated to ensure the integrity of the transmission.

In order to ensure quality transmission when employing

MLT-3 encoding, the compensation must be able to adapt

to various cable lengths and cable types depending on the

installed environment. The selection of long cable lengths

for a given implementation, requires significant compensa-

tion which will over-compensate for shorter, less attenuat-

ing

lengths.

Conversely,

the

selection

short

intermediate cable lengths requiring less compensation will

cause serious under-compensation for longer length

cables. The compensation or equalization must be adap-

tive to ensure proper conditioning of the received signal

independent of the cable length.

The DP83848C utilizes an extremely robust equalization

scheme referred as ‘Digital Adaptive Equalization.’

The Digital Equalizer removes ISI (inter symbol interfer-

ence) from the receive data stream by continuously adapt-

ing to provide a filter with the inverse frequency response

of the channel. Equalization is combined with an adaptive

gain control stage. This enables the receive 'eye pattern' to

be opened sufficiently to allow very reliable data recovery.

The curves given in Figure 8 illustrate attenuation at certain

frequencies for given cable lengths. This is derived from

the worst case frequency vs. attenuation figures as speci-

fied in the EIA/TIA Bulletin TSB-36. These curves indicate

the significant variations in signal attenuation that must be

compensated for by the receive adaptive equalization cir-

cuit.

Figure 8. EIA/TIA Attenuation vs. Frequency for 0, 50,

100, 130 & 150 meters of CAT 5 cable

DP83848C Datasheet(PDF) 28 Page - National Semiconductor (TI)

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