AN-902 Datasheet, PDF (1/10 Pages) National Semiconductor (TI) – Twisted Pair FDDI Magnetics Overview and Recommendations

Twisted Pair FDDI
Magnetics Overview and
Recommendations
National Semiconductor
Application Note 902
Todd Vafiades
July 1993
1 0 INTRODUCTION
The use of twisted pair cable for high speed LAN signalling
necessitates the inclusion of transmit and receive magnet-
ics 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 transmis-
sion signal Several important factors must be considered
when choosing the magnetics for FDDI twisted pair PMDs
This application note discusses key performance parame-
ters 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 recom-
mendations for a typical PMD transceiver implementation
employing the National Semiconductor DP83223 TWISTER
transceiver and suggests the use of some readily available
magnetics
2 0 WHAT ARE MAGNETICS
In the case of Twisted pair FDDI signal transmission the
term ‘‘magnetics’’ refers to the one-to-one isolation trans-
formers and common mode choke transformers which cou-
ple 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 integ-
rity of the transmitted and or received FDDI data stream
These additional filter elements may or may not be de-
scribed 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 cou-
pling magnetics
3 0 WHY ARE MAGNETICS REQUIRED
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 transmit-
ter 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
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 tran-
sition 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 tran-
sient or continuous noise injected onto the media from out-
side sources This note contains example schematics detail-
ing components and interconnection
4 0 KEY PARAMETERS
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 ap-
plication note briefly examines several important parame-
ters which contribute to the effectiveness of a given mag-
netics design
4 1 Insertion Loss
This is the loss introduced by the insertion of the magnetics
and can be generally expressed as
I_L(dB) e 20 Log
V_IN
V_OUT
where V_IN is the voltage across the input of the magnetics
while V_OUT represents the voltage across the output of the
magnetics in an appropriately configured system Some fac-
tors 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 set-
ting specified transmit amplitudes for standard compliant
Twisted Pair FDDI signalling
4 2 Return Loss
This is a measure of the match between the two impedanc-
es on either side of a junction point defined by
Z1 a Z2
R_L(dB) e 20 Log _{Z1 b Z2}
where Z1 and Z2 are the complex impedances of the two
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 signifi-
cantly 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 imped-
ances
C1995 National Semiconductor Corporation TL F 11894
RRD-B30M105 Printed in U S A