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Improved Virtual Cathode for Flat Cathode Ray Tube Display

IP.com Disclosure Number: IPCOM000119142D
Original Publication Date: 1997-Dec-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 4 page(s) / 137K

Publishing Venue

IBM

Related People

Knox, AR: AUTHOR [+2]

Abstract

Matrix driven flat Cathode Ray Tube (CRT) displays, such as the Magnetic Matrix Display (MMD), require the use of an area cathode to provide a uniform source of electrons to each pixel aperture. Thermionic cathodes are excellent sources of electrons, but because they must be heated to about 750 degrees C, it is not currently possible to use one cathode source for each pixel. This would require too much heater power and also would produce so much heat that materials in the display (e.g., the magnet in an MMD) would exceed their rated operating temperature.

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Improved Virtual Cathode for Flat Cathode Ray Tube Display

      Matrix driven flat Cathode Ray Tube (CRT) displays, such as the
Magnetic Matrix Display (MMD), require the use of an area cathode to
provide a uniform source of electrons to each pixel aperture.
Thermionic cathodes are excellent sources of electrons, but because
they must be heated to about 750 degrees C, it is not currently
possible to use one  cathode source for each pixel.  This would
require too much heater power  and also would produce so much heat
that materials in the display (e.g.,  the magnet in an MMD) would
exceed their rated operating temperature.

      Field emission electron sources such as MIMs, PFEs and FEDs do
not require heating but are non space charge limited and suffer from
problems of uniformity and instability that require some form of
smoothing to make their use practical.

      Remote virtual cathode designs exist for thermionic cathodes
that form a uniform planar space charge cloud remote from the hot
filaments and this technique can be extended to utilize the MIM, PFE,
or FED cold cathodes.  A virtual cathode, when used with cold cathode
sources, has the particular advantage of providing in built
smoothing, so eliminating problems of uniformity and instability.

      A problem with all forms of virtual cathode is how to
compensate for emission variations with time.  In the case of
thermionic cathodes, the oxide coated filaments degrade with time and
the same is  true of cold cathode sources.  In the latter case, there
is also the problem that there is no relationship between the
emission current and  the current forming parameter (extraction
voltage in the case of PFEs and  FED, and sandwich current in the
case of MIMs).

      A typical virtual cathode, shown in Fig. 1, uses an extraction
grid to produce a nearly parallel flow of electrons from the cathode.
If the voltages on the extraction and control grids are correctly
chosen, then a remote virtual cathode with a uniform volume of
electrons at a uniform potential in a dense space charge limited
cloud is formed.

      The control grids are arranged to be at, or slightly lower
than, the cathode voltage (identical to the screen/anode grid
potential arrangement in the beam power valve), so that the electrons
are slowed  and then reversed near the control grids.  This slowing
causes an increase in the electron density and, hence, a remote
virtual cathode and a potential minimum.  If the extraction grid had
a high enough transmission, then most electrons will reach this point
and will then be reflected back and forward until absorbed by the
extractor grid. In  operation in a CRT, the control grids will be
taken slightly positive at a pixel which is switched on and, hence,
current will be extracted from the remove virtual cathode and
directed towards the phosphor screen.

      The equations of electron flow will be governed by the
Child-Langmuir law and negl...