dp83223 National Semiconductor Corporation, dp83223 Datasheet - Page 6

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

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,
R
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
REF
, sets up a reference current which determines the
V
OUTpk-pkdiff
I
TXO
= 20.48
= I
R
TXO *
REF
Z
cable
2
6
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).
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.
Binary
MLT-3
0 MHz
Figure 3-1. Example of Binary vs. MLT-3
Figure 3-2. 1V Binary Power Spectrum
1
1
0
0
1
-1
0
0
125MHz
0
0
1
1
1
1
0
0
250MHz
1
-1
1
-1