6

Revision A

3.0 Functional Description

3.1 Overview

The DP83223 transceiver consists of the major functional

blocks shown in the “Block Diagram”. The Transmit sec-

tion consists of an ECL input buffer for PMRD+/- and the

Programmable Current Output Driver. The Programmable

Current Output Driver can be configured to convert the

incoming binary datastream to a current sourced MLT-3

encoded datastream.

The transmit amplitude of the signal presented at the TXO

output pins can be controlled by varying the value of resis-

tance between TXREF and GND. This TXREF resister,

final output current at TXO+/- as described by:

The following equation yields the differential peak-peak

transmit voltage for a given characteristic cable imped-

ance:

The transition times at the TXO+/- outputs are of special

interest. These matched rise and fall times are digitally

synthesized to reduce EMI emissions at the media inter-

face and on the media. The controlled transition times

also significantly reduce the design complexity and cost

by minimizing external filtering.

The Receive section consists of the following functional

blocks: a differential input Equalization Amplifier with Sig-

nal Detect circuitry, signal Comparators with Control

Logic, Loopback Multiplexer Logic, and differential ECL

output drivers for PMID and Signal Detect.

In adaptive or full equalization mode, as selected by the

EQSEL input pin, the receive data is first equalized and

then amplified for signal detection. If the receive equalizer

is turned off, the data is then only amplified for signal

detection.

The Comparator and Control Logic Block performs sev-

eral functions. Primarily, the comparators quantize and

convert incoming MLT-3 into binary. The control logic

receives input from CDET and ENCSEL enabling final sig-

nal detect indication and control of data conversion/regen-

eration.

The Loopback Mux logic performs the function of routing

the transmit data at the PMRD+/- inputs to either the

PMID+/- pins (loopback enabled) or directly to the TXO+/-

current outputs (normal operation).

Finally, ECL output drivers are used to drive both PMID+/-

receive data and SD+/- Signal Detect data to the appro-

priate Physical Layer device.

3.2 MLT-3 Encoding

The decision to incorporate MLT-3 (Multi-Level-Transmit /

3 levels) signal encoding into the ANSI X3.263 Twisted

Pair Standard is based solely on the issue of Electro-Mag-

netic Compatibility (EMC). Scrambling the datastream, via

the Stream Cipher function, reduces EMI emissions at key

frequencies by approximately 20dB. Although 20dB is sig-

nificant, it may be insufficient to pass the FCC Class B

radiation limit (with margin) for NRZI signalling at the stan-

dard transmit amplitude of 2.0V. The inclusion of MLT-3

encoding theoretically provides an additional 3dB reduc-

tion in EMI emissions depending on the measurement

technique and system design/layout.

The effect of MLT-3 encoding is the reduction of energy on

the media in the critical frequency range of 20MHz to

100MHz. This is achieved by trading line frequency for

line voltage complexity. When a binary data stream is

MLT-3 encoded, the result is a shift, in part, of some fre-

quency components of the signal.

See Figure 3-1, the second ‘high’ pulse in the binary

waveform is transformed by an inversion to the ‘-1’ level in

the MLT-3 waveform. This inversion corresponds to a cer-

tain decrease in energy from the original binary frequency

component. The decrease in energy at the critical fre-

quency of 62.5MHz is appreciable (3dB to 6dB).

Figure 3-1. Example of Binary vs. MLT-3

The power spectrum plots in Figure 3-2 and Figure 3-3

provide a clear comparison between scrambled binary

and scrambled MLT-3 respectively. It should be noted that

FCC Class B limits relate to radiated emissions and not

the direct power spectrum. The plots in Figure 3-2 and

Figure 3-3 are intended strictly for use as a general com-

parison between Binary and MLT-3 and should not be

interpreted as absolute EMI performance indicators.

Figure 3-2. 1V Binary Power Spectrum

2

0

1010

1

0

1

Binary

MLT-3

11

0

10-1

0

1

0

1-1

-1

0 MHz

125MHz

250MHz