DP83223 datasheet(7/14 Pages) NSC

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3.0 Functional Description (Continued)

Revision A

Figure 3-3. 2V MLT-3 Power Spectrum

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.

Figure 3-4. Controlled TXO Transitions

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

for

longer

length

cables.

Therefore,

the

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

0 MHz

125MHz

250MHz

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