SNLS059E datasheet(26/110 Pages) TI2

26 / 110 page

3.0 Functional Description (Continued)

Subject to change without notice.

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3.10.3 Digital Adaptive Equalization

When transmitting data at high speeds over copper twisted

pair cable, frequency dependent attenuation becomes a

concern. In high-speed twisted pair signalling, the

frequency content of the transmitted signal can vary greatly

during

normal

operation

based

primarily

the

randomness of the scrambled data stream. This variation

in signal attenuation caused by frequency variations must

be compensated for 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

given

implementation,

requires

significant

compensation which will over-compensate for shorter, less

attenuating lengths. Conversely, the selection of short or

intermediate cable lengths requiring less compensation will

cause serious under-compensation for longer length

cables. Therefore, the compensation or equalization must

be adaptive to ensure proper conditioning of the received

signal independent of the cable length.

The DP83815 utilizes an extremely robust equalization

scheme

referred

herein

‘Digital

Adaptive

Equalization’. Traditional designs use a pseudo adaptive

equalization scheme that determines the approximate

cable length by monitoring signal attenuation at certain

frequencies. This attenuation value was compared to the

internal receive input reference voltage. This comparison

would indicate the amount of equalization to use. Although

this scheme is used successfully on the DP83223V twister,

it is sensitive to transformer mismatch, resistor variation

and process induced offset. The DP83223V also required

an external attenuation network to help match the incoming

signal amplitude to the internal reference.

The Digital Equalizer removes ISI (Inter Symbol

Interference) from the receive data stream by continuously

adapting to provide a filter with the inverse frequency

response of the channel. When used in conjunction with a

gain stage, this enables the receive 'eye pattern' to be

opened sufficiently to allow very reliable data recovery.

Traditionally 'adaptive' equalizers selected 1 of N filters in

an attempt to match the cables characteristics. This

approach will typically leave holes at certain cable lengths,

where the performance of the equalizer is not optimized.

The DP83815 equalizer is truly adaptive.

The curves given in Figure 3-10 illustrate attenuation at

certain frequencies for given cable lengths. This is derived

from the worst case frequency vs. attenuation figures as

specified 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 circuit.

Figure 3-11 represents a scrambled IDLE transmitted over

zero meters of cable as measured at the AII (Active Input

Interface) of the receiver. Figure 3-12 and Figure 3-13

represent the signal degradation over 50 and 100 meters

of category V cable respectively, also measured at the AII.

These plots show the extreme degradation of signal

integrity and indicate the requirement for a robust adaptive

equalizer.

Figure 3-9 100BASE-TX BLW Event Diagram

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