dp83223 National Semiconductor Corporation, dp83223 Datasheet - Page 7

• Manufacturer Part Number: dp83223
• Description: Dp83223 Twister High Speed Networking Transceiver Device
• Manufacturer: National Semiconductor Corporation

Revision A
3.0 Functional Description
3.3 Transition Time Control
The DP83223 TWISTER incorporates a unique feature
which virtually eliminates the need for external filtering of
the transmitted signal. The transition times of the TXO
output signals are digitally synthesized resulting in closely
matched and controlled rise and fall times (Refer to Fig-
ure 3-4). These controlled transition times, in conjunction
with the associated magnetics, result in typical rise and
fall times of greater than 3ns. These transition times lie
within the range specified in the X3.263 Twisted Pair PMD
standard.
0 MHz
Figure 3-4. Controlled TXO Transitions
Figure 3-3. 2V MLT-3 Power Spectrum
125MHz
(Continued)
250MHz
7
3.4 Adaptive Equalization
When transmitting data at high speeds over copper
twisted pair cable, frequency dependent attenuation
becomes a concern. In Twisted Pair Fast Ethernet or
FDDI signalling the frequency content of the transmitted
signal can vary greatly during normal operation based pri-
marily on 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. In a fixed equalization system, the
selection of long cable lengths for a given implementation,
requires significant compensation which will over-com-
pensate for shorter, lower attenuation lengths. Conversely,
the selection of short or intermediate cable lengths requir-
ing less compensation will cause serious under-compen-
sation
compensation (equalization) must be adaptive to ensure
proper conditioning of the received signal independent of
cable length.
The combination of choosing MLT-3 as the signal encod-
ing scheme, in conjunction with a requirement for adaptive
equalization, demands that compensation occur at the
receive end of the transmission network. In order to imple-
ment receiver adaptive equalization, a known relationship
between transmit output amplitude and a receive input
reference must be specified and controlled. Nominal
transmit output amplitude, as specified by the ANSI
X3.263 TP-PMD document, is 2.0V peak to peak differen-
tial. The DP83223 TWISTER incorporates a fixed nominal
receive input reference. Given these two parameters, the
adaptive equalization can determine the approximate
cable length via signal attenuation at certain frequencies
and actively compensate for cable length variations.
Since the DP83223 TWISTER transceiver’s receive input
reference is fixed at approximately 1.45V and the transmit
amplitude is fixed at 2.0V, an attenuation factor is
required. This attenuation is accomplished by a simple
resistive voltage divider placed at the RXI+/- inputs, which
also serves as the forward termination for the transmis-
sion line (Refer to Figure 4-3). Because the voltage
divider attenuates any noise along with the received sig-
nal, the Signal-to-Noise ratio is not decreased. An addi-
tional benefit is gained by including the voltage divider,
where any insertion loss caused by the media coupling
magnetics can be compensated for by adjusting the volt-
age divider ratio. This will ensure appropriate signal trans-
fer and optimal adaptive equalization.
For additional information regarding the interdependen-
cies between the adaptive equalizer and the receive
attenuation/termination circuit, refer to the NSC applica-
tion note entitled “DP83223 Adaptive Equalizer Consider-
ations”.
3.5 Jitter Performance
This section briefly presents the typical jitter performance
exhibited by the DP83223 TWISTER transceiver. The
DP83223 TWISTER was subjected to the near worst-
case condition of 110 Meters of Category 5 cable con-
nected to two lengths of 10Meter cable via two 110
punchdown blocks (totalling 130 Meters). The DP83223
for
longer
length
cables.
Therefore,
the