Datasheet

2.0 Functional Description (Continued)
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attenuation caused by frequency variations must be com-
pensated 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 a given implementation, requires significant compensa-
tion which will over-compensate for shorter, less attenuat-
ing 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 DP83843 utilizes an extremely robust equalization
scheme referred to herein as ‘Digital Adaptive Equaliza-
tion.’ Existing designs use an adaptive equalization scheme
that determines the approximate cable length by monitor-
ing signal attenuation at certain frequencies. This attenua-
tion value was compared to the internal receive input
reference voltage. This comparison would indicate that
amount of equalization to use. Although this scheme is
used successfully on the DP83223V TWISTER, it is sensi-
tive 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.
Digital Adaptive Equalization is based on an advanced dig-
itally controlled signal tracking technique. This method
uses peak tracking with digital over-sampling and digitally
controlled feedback loops to regenerate the receive signal.
This technique does not depend on input amplitude varia-
tions to set the equalization factor. As a result it maintains
constant jitter performance for any cable length up to 150
meters of CAT-5. Digital Adaptive Equalization allows for
very high tolerance to signal amplitude variations.
The curves given in Figure 6 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 sig-
nificant variations in signal attenuation that must be com-
pensated for by the receive adaptive equalization circuit.
Figure 7 represents a scrambled IDLE transmitted over
zero meters of cable as measured at the AII (Active Input
Interface) of the receiver. Figure 8 and Figure 9 represent
the signal degradation over 50 and 100 Meters of CAT-5
cable respectively, also measured at the AII. These plots
show the extreme degradation of signal integrity and indi-
cate the requirement for a robust adaptive equalizer.
The DP83843 provides the added flexibility of controlling
the type of receive equalization required for a given imple-
mentation. This is done through TW_EQSEL (bits [13:12]
of the PHYCTRL register, address 19h). While digital adap-
tive equalization is the preferred method of cable compen-
sation for 100BASE-TX, the ability to switch the equalizer
completely off or to a fixed maximum is provided. This fea-
ture is intended as a test mode only and, if enabled, will
inhibit normal performance of the DP83843.
2.3.4 MLT-3 to NRZI Decoder
The DP83843 decodes the MLT-3 information from the Dig-
ital Adaptive Equalizer block to binary NRZI data. The rela-
tionship of binary to MLT-3 data is shown in Figure 4.
2.3.5 Clock Recovery Module
The Clock Recovery Module (CRM) accepts 125 Mb/s
NRZI data from the MLT-3 to NRZI decoder. The CRM locks
onto the 125 Mb/s data stream and extracts a 125 MHz ref-
erence clock. The extracted and synchronized clock and
data are used as required by the synchronous receive
operations as generally depicted in Figure 5.
The CRM is implemented using an advanced digital Phase
Locked Loop (PLL) architecture that replaces sensitive
analog circuits. Using digital PLL circuitry allows the
DP83843 to be manufactured and specified to tighter toler-
ances.
For further information relating to the 100BASE-X clock
recovery module, refer to Section 4.3.
Figure 1. EIA/TIA Attenuation vs Frequency for 0, 50,
100 meters of CAT-5 cable
Figure 2. MLT-3 Signal Measured at AII after 0 meters of
CAT-5 cable
100M
50M
0M
t
enuation (dB)
4.00
6.00
8.00
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12.00
14.00
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