Datasheet

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DP83848M
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 frequency content of the transmitted signal can vary
greatly during normal operation based primarily on the
randomness of the scrambled data stream. This variation
in signal attenuation caused by frequency variations must
be compensated to ensure the integrity of the transmis-
sion.
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 compen-
sation which will over-compensate for shorter, less attenu-
ating lengths. Conversely, the selection of short or
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 DP83848M 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 recov-
ery.
The curves given in Figure 8 illustrate attenuation at cer-
tain 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 equal-
ization circuit.
Figure 8. EIA/TIA Attenuation vs. Frequency for 0, 50,
100, 130 & 150 meters of CAT 5 cable