TLF11894

National Semiconductor

Application Note 902

Todd Vafiades

July 1993

Twisted Pair FDDI

Magnetics Overview and

Recommendations

10

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 X3T95 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

20

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 andor 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

30

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

40

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

41

Insertion Loss

This is the loss introduced by the insertion of the magnetics

and can be generally expressed as

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

42

Return Loss

This is a measure of the match between the two impedanc-

es on either side of a junction point defined by

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

RRD-B30M105Printed in U S A