Browse Prior Art Database

Technique for Reducing Low Frequency Electric Field Emissions from Monitors

IP.com Disclosure Number: IPCOM000115505D
Original Publication Date: 1995-May-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 120K

Publishing Venue

IBM

Related People

Eagle, DJ: AUTHOR [+2]

Abstract

Disclosed is a method that may be used to significantly reduce the level of low frequency electric field emissions from monitors by using a signal derived from the AC power inlet and generating a signal which can be used to create a cancellation field using an antenna. It is envisaged that this would be adopted as part of an 'ultra-low emissions' solution, allowing a monitor's emissions to be reduced well below the levels currently required by the TCO91 standard.

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Technique for Reducing Low Frequency Electric Field Emissions from
Monitors

      Disclosed is a method that may be used to significantly reduce
the level of low frequency electric field emissions from monitors by
using a signal derived from the AC power inlet and generating a
signal which can be used to create a cancellation field using an
antenna.  It is envisaged that this would be adopted as part of an
'ultra-low emissions' solution, allowing a monitor's emissions to be
reduced well below the levels currently required by the TCO91
standard.

      There has been considerable interest over the last ten years in
ways of reducing the levels of electric and magnetic field emissions
from monitors and displays.  This has resulted from studies which
indicated that there may be a link between such emissions and various
medical problems.  Whilst to this day there is no conclusive proof of
such a link, many employers have taken note of their employees'
concerns and will only purchase low emission monitors.  Such monitors
originally tended to comply with a Swedish standard now commonly
known as MPR1 which was published in 1987.  This was extended in 1990
and the new standard subsequently became known as MPR2.  In 1991, the
Swedish Confederation of Professional Employees published their own
guidelines for emissions which are considerably tougher than the MPR2
standard.  These guidelines are known as 'TCO91'.

      Meeting either the MPR2 or TCO91 emission standards is now seen
as a marketing requirement for many countries for all but 'home use'
monitors.  IBM has responded by producing a wide range of MPR2
compliant monitors and a smaller range of TCO91 compliant monitors.
The MPR2 standard can be met reasonably easily by the addition of
shielding and selection of a particular type of CRT yoke design.
Meeting TCO91 is much more difficult and at the time of writing has
only been successfully achieved by either using an expensive
conductive screen overlay or else by use of an active cancellation
technique.  It is likely that future standards will require even
lower emission levels and this will present new challenges.  One of
these challenges; how to further reduce the emission level of low
frequency fields, is the subject of this disclosure.

      There are two major sources of low frequency electric field
emissions connected with the monitor.  Firstly, there is the AC power
cord.  In Europe, this will typically have 220V rms on one of its
conductors whilst the other power conductor will be grounded.  There
is hence potential for a large low frequency field to be emitted by
the power cable and this is in fact observed - when measuring to the
TCO91 limits, the field from a standard power cord is typically half
the total field allowed.  The other major source of emissions is the
monitor's degauss coil.  The degauss coil is typically permanently
connected to AC power 'neutral'.  This is usually connected to earth
and so...