NSC AN-902

National Semiconductor
Application Note 902
Todd Vafiades
July 1993
The use of twisted pair cable for high speed LAN signalling
necessitates the inclusion of transmit and receive magnetics to couple the transmission signal to and from the copper
media. The choice of magnetics in a given implementation
can have a significant effect on the integrity of the transmission signal. Several important factors must be considered
when choosing the magnetics for FDDI twisted pair PMDs.
This application note discusses key performance parameters of magnetics suitable for use within a PMD designed for
ANSI X3T9.5 FDDI Twisted Pair Draft Proposal compliance.
Although magnetics are required for both shielded and
unshielded twisted pair media, this note focuses specifically
on magnetics suitable for FDDI signalling over Category 5
Unshielded Twisted Pair. This note includes layout recommendations for a typical PMD transceiver implementation
employing the National Semiconductor DP83223 TWISTER
transceiver and suggests the use of some readily available
When using the DP83223 Twisted Pair Transceiver, it is not
necessary to employ complex multiple pole LC filters which
are commonly found in many 10BASE-T, Token Ring and
FDDI implementations. Due to the controlled output transition times of the DP83223, simple networks which include
only the termination resistors, isolation transformers and
common mode chokes may be all that are required. Some
designers may choose to add simple filtering at the receive
end of a system in order to reduce the susceptibility to transient or continuous noise injected onto the media from outside sources. This note contains example schematics detailing components and interconnection.
In the case of Twisted pair FDDI signal transmission, the
term ‘‘magnetics’’ refers to the one-to-one isolation transformers and common mode choke transformers which couple the signal to and from the twisted pair media. These
elements couple the serial data stream from one FDDI node
to the twisted pair media and again from the twisted pair
media to another FDDI node. It is also possible that these
transformers may coexist with other filter elements, such as
resistors or capacitors, which attempt to enhance the integrity of the transmitted and/or received FDDI data stream.
These additional filter elements may or may not be described as part of the magnetics depending on the individual
vendor’s perspective. As a point of clarification, ferrite
beads or inductors, sometimes used to decouple sensitive
power and ground pins from potential noise sources on
transceiver ICs, may also be referred to as magnetics. This
application note is only intended to report on the media coupling magnetics.
In most electrical signal transmission systems, the data
moving between two nodes is AC coupled in order to isolate
potential system ground differences between the transmitter and receiver which could interfere with proper signal
transfer. A one-to-one isolation transformer is a convenient
component for use in signal transfer for several reasons: DC
current blocking (system ground differences are isolated
from one another), end stations protection from static
charges that may build up on the cable, inherent differential
signal coupling and common mode rejection.
Magnetics play an essential role in ensuring signal integrity
within a transmission system. Parameters such as Insertion
Loss, Crosstalk and Transition Time contribute greatly to
the performance of the magnetics within a system. This application note briefly examines several important parameters which contribute to the effectiveness of a given magnetics design.
4.1 Insertion Loss
This is the loss introduced by the insertion of the magnetics
and can be generally expressed as:
is the voltage across the input of the magnetics
IL(dB) e 20 Log
where VIN
while VOUT represents the voltage across the output of the
magnetics in an appropriately configured system. Some factors which may contribute to loss include: DC resistance of
the windings, variation from a true one-to-one (primary to
secondary) winding relationship resulting in a ‘‘step-down’’
effect, core loss as well as the inherent loss of additional
filtering. It is important to consider insertion loss when setting specified transmit amplitudes for standard compliant
Twisted Pair FDDI signalling.
Twisted Pair FDDI Magnetics Overview and Recommendations
Twisted Pair FDDI
Magnetics Overview and
4.2 Return Loss
This is a measure of the match between the two impedances on either side of a junction point, defined by:
À Z1 Z2 À
where Z1 and Z2 are the complex impedances of the two
RL(dB) e 20 Log
Z1 a Z2
halves of the circuit. If an impedance mismatch does exist,
signal reflections will measurably decrease the performance
of a given system. The effects of Return Loss are significantly reduced by the controlled output transition times of
the DP83223. These controlled transition times basically
eliminate the need for additional filtering which can increase
the potential for a mismatch in transmit and receive impedances.
C1995 National Semiconductor Corporation
RRD-B30M105/Printed in U. S. A.
ing in baseline wander. An increase in baseline wander contributes directly to increased jitter. In general, the higher the
OCL (open circuit inductance), the lower the low frequency
pole for the magnetics bandpass region and the less severe
the Baseline Wander.
Common Mode Rejection
This is the ability of the magnetics, either transmit or receive, to reject common mode energy which may exist in
the transmission signal. Also, the ability of the magnetics to
not impart any common mode energy to the signal. Common Mode Energy can be described as some potential existing equally (in phase) on each side of a differential pair
with respect to some fixed potential such as ground. As an
example, some twisted pair conductors are routed through
typical office locations which contain significant ambient
energy. This can inject as much as 30V AC (in some cases
even higher) of common mode potential to the twisted pair.
If this common mode voltage is not blocked, the line receiver, which may be powered by a single 5V rail, will fail to
receive a signal that is well outside of its specified operating
4.8 Conducted Power Spectrum
This is the power spectrum of a properly terminated PMD
transmitter (including the magnetics) as measured by direct
connection into a spectrum analyzer. This spectrum analysis is a convenient method of comparing the results of different signalling techniques. The degree of randomness
within the data stream as well as the differences between
binary and MLT-3 are easily compared via conducted emissions.
4.9 Radiated Emissions
This is the radiated power spectrum of a properly terminated
PMD transmitter (including the magnetics) as measured by
a near field antenna within a strictly controlled environment.
Although this application note does not report on the radiated emissions results of the recommended magnetics it remains a very important parameter. It is the responsibility of
the systems vendor to ensure that the performance lies
within mandated limits set forth by the various and appropriate regulatory agencies.
4.4 Crosstalk
This is the amount of energy coupled from the transmit
channel to the receive channel within the magnetics. The
effects of this type of crosstalk are virtually eliminated due
to the physical isolation between transmit and receive magnetics as shown in subsequent connection diagrams.
4.5 Output Transition Time
This is the standard ‘‘rise and fall time’’ as measured from
10% to 90% of full amplitude. With MLT-3, it is important to
measure both rise times and both fall times of the three
level signal. Again, due to the controlled output transition
time of the DP83223, additional wave shaping filters required by some implementations are unnecessary.
4.10 EMI Susceptibility
This is a measure of the tolerance of a working TP-PMD
receiver to a controlled ambient field of radiation imposed
on the twisted pair cable carrying the scrambled FDDI line
code. The receive-end magnetics can be supplemented
with some degree of high frequency filtering to afford greater immunity to susceptibility.
4.6 Overshoot
Given a square wave, overshoot may be defined as the
amount of energy above or below the intended final high or
low voltage level(s) as expressed in percent. Overshoot
may result from unintentional emphasis of some high frequency harmonics and or transitions coincident with reflections. Due to the controlled transmit transition times of the
DP83223, the potential for overshoot is reduced by the inherent decrease in high frequency energy of the transmitted
transition times.
This application note highlights specific magnetics from four
vendors. It is important to understand that this note does
not suggest preference to any one vendor or magnetics solution. The results herein are made available strictly as a
means of objective comparison intended to assist the system designer in making the best possible choice for a given
implementation. Due to the relative immaturity of Twisted
Pair FDDI, this application note reports on only a limited
number of magnetics solutions. Future updates or addendums to this application note will include a larger selection
of magnetics suggested for use with National Semiconductor PMD solutions. The four magnetics solutions are listed,
in alphabetical order, by company name followed by product
number. (Contact information for each of the vendors is located at the end of this applications note.)
Bel FuseÐÝ0556-3899-04
Pulse EngineeringÐÝPE-65620M
Please contact each individual magnetics vendor for the latest product information and part numbers.
4.7 Baseline Wander
In an AC coupled digital transmission system, baseline wander is the variation in the DC content of the transmitted
datastream at any point in time. This phenomenon is dependent on the digital content of a given data stream and the
low frequency cutoff of the magnetics. The scrambled FDDI
line code generated by a twisted pair FDDI PMD can result
in run lengths (no transitions) of up to 480 ns. If the magnetics low frequency pole is not sufficiently low to allow, without attenuation, a 480 ns static condition, then the attenuation at the critical frequencies will result in a ‘‘droop’’ or ‘‘tilt’’
of the waveform during the run length. This droop will effectively offset the baseline reference of the datastream result-
Transition (ns)
6.1 Insertion Loss
Insertion loss is measured in two steps. First, the magnetics
under test are replaced by shorting wires which DC couple
the transmitted signal to the digitizing oscilloscope and the
transmit waveform is calibrated to exactly 2V peak-peak differential. Second, the magnetics under test are reinserted
and a second peak-peak differential measurement is performed. The Insertion Loss resulting from scrambled FDDI
code is tabulated below.
Insertion Loss
Bel Fuse
Scrambled FDDI
b 0.26
b 0.67
b 0.26
b 0.35
Output Transition Time
The rise and fall times of a transmitted signal are a direct
indication of the bandwidth of the transmit channel. The
transition time specification depends somewhat on results
of EMI radiation testing and other performance tests. Slower transition times can be achieved using different magnetics components. To test the rise and fall times of the magnetics, the input of the magnetics were presented with the
2.0 ns transition times generated by the DP83223 TWISTER. The output of each magnetics solution was then measured to determine the transition time performance.
This section summarizes pertinent data as measured from
some of the key parameters mentioned previously. All tests
were performed using the same specially designed evaluation platform. This platform consists of a multi-layer ‘‘ODL
replacement’’ emulation board fitted with a DP83223 transceiver in order to duplicate, as closely as possible, the performance of a true TP-PMD application. Each of the four
magnetics solutions were tested against the same DP83223
transceiver in the same environment to ensure comparable
conditions. All tests were performed identically on each of
the magnetics solutions for both binary and MLT-3 encoded
data transmission unless otherwise noted. Again, it is very
important to understand that any data reported herein is
preliminary and is provided for reference. Each magnetics
vendor should be contacted for the latest performance information.
Bel Fuse Coilcraft Pulse Valor
Binary Rise
Binary Fall
MLT-3 Rise (b1 to 0)
MLT-3 Rise (0 to 1)
MLT-3 Fall (1 to 0)
MLT-3 Fall (0 to b1)
(Refer to Figures 1 and 2 .)
6.6 Overshoot
Overshoot, especially in MLT-3 mode, will decrease the
noise margin of the transmitted signal. Serious overshoot
may also contribute to unwanted bit errors in the received
signal. The overshoot at the output of the magnetics is minimized because the input signal to the magnetics includes
the controlled transition times generated by the DP83223
TWISTER. Overshoot of less than 2% can be considered
6.2 Return Loss
Although this parameter was not measured, the return loss
due to the magnetics alone should be minimal because
complex filtering is not required. Potential return loss may
be inferred by examining the magnetics vendor’s manufacturing tolerances.
6.3 Common Mode Rejection
This parameter was not tested. Refer to each vendor’s datasheet for performance specifications.
Overshoot (%)
Bel Fuse
k 2.0
k 2.0
k 2.0
k 2.0
k 2.0
k 2.0
k 2.0
k 2.0
(Refer to Figures 3 and 4 .)
6.7 Baseline Wander
The effects of baseline wander can be directly inferred by
measuring the magnetics droop characteristic over a worst
case run length period of 480 ns for scrambled FDDI code.
The baseline wander is arrived at by doubling the percentage droop exhibited by a given magnetics solution.
6.4 Crosstalk
The virtual absence of Interchannel crosstalk between the
transmit and receive magnetics is due to sufficient physical
separation of the components as specified by National
Semiconductor. There will be some degree of crosstalk that
occurs between the transmit and receive channel outside of
the magnetics which will most likely occur within the media
connector and within the media itself. This effect can be
minimized by observing good high speed layout practices
and will not be increased by the use of the magnetics solutions outlined herein.
Wander (%)
Bel Fuse
480 ns Width
(Refer to Figure 5 .)
Conducted Power Spectrum
Conducted Power
@ 31.25
@ 62.50
b 47.0
b 53.0
b 53.0
b 62.0
Ultimately, the most important performance factor of Twisted Pair FDDI signaling is long term, error free data transmission. To ensure that each of the four magnetics solutions
tested herein will support error free transmission, 16 separate bit error rate (BER) tests were performed. Each of the
four magnetics were tested against themselves and each
other at both the transmit and receive ends of the transmission system. Specifically, each test was performed using
130 meters of Category 5 cable with scrambled code set at
2.0V peak-to-peak differential transmit voltage. These tests
were performed for both Binary and MLT-3 signal encoding.
Each of the 16 BER tests passed proving acceptable interoperability in terms of the magnetics to the TP-PMD standard BER limit of k10E-12.
Several additional electrical parameters exist for each of the
magnetics solutions presented herein. Although these parameters are not included in this analysis, they are nonetheless important and may help to further inform the system
designer regarding performance. Each of the magnetics
vendors publish a list of these specifications, tolerances and
test conditions included where applicable, to accompany
their solutions. It is best to refer to these figures for a more
comprehensive understanding of performance. Some of the
standard parameters associated with magnetics include:
Ð Turns Ratio
Ð OCL (open circuit inductance)
Ð LL (leakage inductance)
Ð Cw/w (interwinding capacitance)
Ð DCR (DC resistance)
Ð HI POT (high voltage tolerance)
Ð CMR (common mode rejection)
The conducted Power Spectrum offers a convenient method of understanding and comparing the differential power
spectrum of a given set of magnetics. Due to the similarities
between the conducted spectra of each of the magnetics
tested herein, only typical measurements are presented. Of
specific interest are the differences between the binary and
MLT-3 encoded conducted power spectrum for scrambled
line code. Although MLT-3 suffers from 6 dB lower noise
immunity than binary given equal transmit amplitudes,
MLT-3 does exhibit an improvement in the reduction of differential conducted power at key frequencies.
(Refer to Figures 6 and 7 .)
6.9 Radiated Emissions
Currently, no data is provided for this parameter. However,
preliminary data will be available soon. Please contact National Semiconductor for information pertaining to the Radiated Emissions of suggested PMD implementations.
6.10 EMI Susceptibility
Currently, no data is provided for this parameter. However,
preliminary data will be available soon. Please contact National Semiconductor for information pertaining to the EMI
Susceptibility of suggested PMD implementations.
TL/F/11894 – 2
FIGURE 1. Typical Binary Transitions
FIGURE 2. Typical MLT-3 Transitions
TL/F/11894 – 3
TL/F/11894 – 4
FIGURE 3. Typical Binary Overshoot
FIGURE 4. Typical MLT-3 Overshoot
TL/F/11894 – 5
FIGURE 5. Typical Binary Droop
TL/F/11894 – 6
TL/F/11894 – 7
FIGURE 6. Typical Binary Conducted Power Spectrum
FIGURE 7. Typical MLT-3 Conducted Power Spectrum
Resistors R1, R2 and R3 form a voltage divider in which
the receive signal, as presented to the DP83223, is attenuated relative to the full receive amplitude. This amplitude
reduction is a good method of ensuring maximum operational headroom of the embedded adaptive equalizer and associated circuitry within the DP83223. In addition, this attenuation can be adjusted to accommodate for magnetics insertion loss.
Resistors R4 and R5 form the back termination for the
transmit signal path. These resistors are terminated directly
to the TXGND (Transmit Ground) plane. Since the DP83223
TWISTER allows these back termination resistors to be referenced to ground, the noise coupled to the transmitted signal is less than those implementations which reference the
output to VCC.
Resistors R6 through R9 provide the two unused twisted
pairs within the 4-pair bundle with 100X differential termination.
Resistors R10 through R13 provide good common mode
termination for each of the four twisted pairs within the bundle. More specifically, R10 and R11 terminate the two unused twisted pairs while R12 and R13 terminate the two
active twisted pairs. R12 is connected between the primary
center tap of the receive transformer and the common
mode common point while R13 is connected between the
secondary center tap of the transmit transformer and the
common mode common point. Within some magnetics the
transmit channel isolation transformer primary and receive
channel isolation transformer secondary center taps are
pinned out. The example shown in Figure 8 assumes these
pins float.
Although the common mode termination design presented
here is a viable option, other designs may potentially provide improved performance as well. An additional point of
clarification: to date, the ANSI subcommittee on Twisted
Pair FDDI has not yet defined common mode termination of
any kind. However, data has been presented that indicates
a significant enhancement in EMC performance for Category 5 cable fitted with common mode termination.
This section focuses on suggested interconnection and layout of the magnetics solution within the PMD. Due to the
high speed nature of Twisted Pair FDDI, careful layout practices are advised. Maintaining a 50X signal impedance and
keeping high speed signal traces as short as possible are
important design factors. The following design example
highlights several key areas of concern and also suggests
possibilities for improved overall system performance.
Figure 8 illustrates a typical magnetics layout using the National Semiconductor DP83223 Twisted Pair Transceiver.
This layout example assumes the use of four planes to accommodate the required power and signal routing as described in the cross sectional view provided in the Legend
(Figure 9 ). Additionally, the Legend provides component
type and values as well as identification of various signal
paths and power planes. Circuit details of the layout follow:
Capacitor C1 optionally helps to ensure that high frequency
energy outside of the intended passband across R3 will be
Capacitors C2, C3 and C4 provide power supply decoupling for each of the designated power planes. C2 decouples noise from TXVCC to TXGND. C3 decouples noise from
RXVCC to RXGND. Finally, C4 decouples noise from
Ferrite Beads FB1 through FB4 provide good isolation between unique supply islands and planes. FB1 isolates the
RXGND (Receive Ground) island from the ECLGND plane.
FB2 isolates the RXVCC (Receive Power) island from the
ECLVCC plane. FB3 isolates the TXGND (Transmit Ground)
island from the ECLGND plane. And F4 isolates the TXVCC
(Transmit Power) island from the ECLVCC plane. While
many implementations employ standard inductors of various
values for power supply isolation, National Semiconductor
recommends the use of Ferrite beads for improved isolation
and enhanced performance. Ferrite beads provide damping
of high frequency noise while not creating problems caused
by high Q inductors.
TL/F/11894 – 8
FIGURE 8. Typical Magnetics Layout Using DP83223 Transceiver
TL/F/11894 – 9
FIGURE 9. Legend
Pulse Engineering: Product ÝPE-65620M
Package type information includes: package encasement,
footprint and pinout for each of the three vendor’s products.
For precise mechanical information on each of the magnetics, please refer to the appropriate vendor’s datasheet. The
order of description is alphabetical by vendor name. Please
contact each vendor for the latest package information.
Bel Fuse: Product Ý0556-3899-04
One PE-65620M required for transmit channel
One PE-65620M required for receive channel
Plastic/surface mount/16-pin DIP/50 mil pin spacing/
300 mil device width
One 0556-3899-04 required for transmit channel
One 0556-3899-04 required for receive channel
Through-hole/6-pin SIP/100 mil pin spacing
TL/F/11894 – 14
TL/F/11894 – 15
Undesignated pins are no-connects
Valor: Product ÝST6021
One ST6021 required for transmit channel
One ST6021 required for receive channel
Plastic/surface mount/16-pin DIP/50 mil pin spacing/
300 mil device width
TL/F/11894 – 11
Undesignated pins are no-connects
Coilcraft: Product ÝQ3950-D
One Q3950-C required for transmit channel
One Q3950-C required for receive channel
Through-hole/6-pin SIP/100 mil pin spacing
TL/F/11894 – 16
TL/F/11894 – 17
Undesignated pins are no-connects
TL/F/11894 – 13
Bel Fuse, Inc.
5362 W. 78th St.
Indianapolis, IN 46268-4147
(317) 876-0044
1. National Semiconductor DP83223 device specification.
National Standard Rev. 0.3 dated 2/17/93.
3. Bell, David A., Solid State Pulse Circuits , Reston Publishing, (1981).
4. Various, Reference Data For Radio Engineers , Sams,
Coilcraft, Inc.
1102 Silver Lake Rd.
Cary, Illinois 60013
(708) 639-6400
Pulse Engineering, Inc.
P.O. Box 12235
San Diego, CA 92112
(619) 674-8100
Valor Electronics, Inc.
9715 Business Park Ave.
San Diego, CA 92131
(619) 537-2619
Twisted Pair FDDI Magnetics Overview and Recommendations
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