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Model 1331A
Theory
SECTION IV
PRINCIPLES OF OPERATION
4-1.
INTRODUCTION.
4-2. This
section contains a
simplified explanation of
the principles of
variable
persistence
and
storage.
It
a lso contains
functional
descriptions of Model
1331A keyed to
an
overall block diagram and to
block
diagrams
of circuit groups.
A detailed
ex-
planation of circuit
functions, keyed to the
schematics,
is
provided after the block
diagram discussion.
4-3.
VARIABLE PERSISTENCE AND
STOR-
AGE.
4-4.
The
following simplified explanations are
pro-
vided
to give
the reader
a
better understanding
of
the
overall operation of
the instrument.
4-5.
THE STORAGE
CRT.
4-6.
The
storage CRT (figure
4-1)
contains a con-
ventional electrostatic
deflection
electron gun and an
aluminized phosphor viewing surface. The conven-
tional electron gun
in
a storage
tube is
called the
write gun.
Additionally,
the storage CRT contains a
second electron gun (called the flood gun),
flood
beam shaping and accelerator
grids,
collimator, a
collector
mesh,
and a storage
mesh. These
storage
and variable persistence elements are
located between
the
write
gun and
the
viewing
phosphor.
4-7.
The flood gun
is located just
outside the
write
gun's
horizontal deflection plates. Its function is to
emit
a
cloud of
electrons
from its
cathode.
The
electron
cloud (flood electrons)
is
shaped and accelerated
toward
the storage
mesh
by the
collimator and
the
collector
mesh. The
collimator
is
a conductive
coating
on the
inside
of
the
funnel
section
of
the CRT glass.
The collector
mesh is
an electrically
transparent
metallic mesh located just behind the
storage
mesh.
4-8.
The
action of
the
storage
mesh makes
storage
and variable
persistence
possible.
It
is an electrically
transparent
metallic mesh
covered
with
a special
coating consisting of a
dielectric
material. Storage
action
takes place
on
the
outside surface of
the
coat-
ing.
This surface
is
called the
storage
surface.
The
dielectric material
causes capacitive coupling
between
the stora
ge
mesh
and
the
storage surface.
In
opera-
tion,
the
storage
mesh is held
at about +4
volts.
Beca
use
of
the
capacitive coupling,
the
storage
sur-
face
drops to
about -10
volts. In this
condition,
the
flood
electrons are
repelled
and collected
by the
collector
mesh.
4-9. The discussion in the previous
paragraph
as-
sumed
that
th
e
write gun was turned off.
If
the
write gun is now turned on,
the
electron
beam
(wh
ose
energy
is much
h
igher than the flood electrons)
strikes
the storage surface with
such
force
th
at elec-
trons already on the
storage
surface
are
knocked
off (secondary
emission).
The
secondary
emission
electrons are collected
by the
collector
mesh.
Wh
er-
ever
the
write gun electron
beam
strikes
the storage
surface,
the potential
at
that point rises due to the
loss of
electrons.
Because these points have become
more positive, flood
gun electrons
pass through and
strike
the phosphor
viewing screen.
In
this
manner,
the
pattern
traced
by the write
gun
beam is written
(and stored)
on the
storage surface.
4-10.
SECONDARY EMISSION
RATIO.
4-11.
Figure 4-2 is the
secondary emission
ratio
curve of
the
storage surface.
The
secondary
emission
ratio is the basis
for
storage
of information
on the
storage surface and for erasure of
information from
the storage surface.
The point where the number of
electrons
leaving
the storage surface equals
the num-
ber
arriving
is
called a crossover
point. At
a cross-
over point the secondary emission
ratio
=
1.
When
more electrons are
leaving than
arriving,
the potential
of the storage surface rises;
when more
are arriving
than
leaving,
the
potential decreases.
4-1
2.
ERASE
CYCLE.
4-13. Figure 4-3
graphically shows the variations
of
the potentials on the
storage
surface during the
erase cycle.
Prior to initiation
of the
cycle,
the storage
mesh is held
at +4
volts. The unwritten
areas
of th
e
storage
surface
are
at -10 volts and the
written
areas are
near
0
volt. When the ERASE
push
button
is pressed, the
storage
mesh
and
the storage surface
are
brought to the
same
potential
as
the collector
mesh,
+ 158 volts.
When
the
ERASE pushbutton is
released both
storage
mesh
and storage
surface are
dropped
to approximately-12.5
volts. During
an
erase
timing
period
of approximately 800
milliseconds, an
RC
charging action
brings the
storage
mesh up to
+
14
volts. Capacitive coupling
between the
storage
mesh
and
the
storage surface causes
the
storage
sur-
face
to
follow and rise
to
0
volt. At the
end
of the
erase timing
period,
the storage
mesh potential is
returned to
+4 volts and
the
storage surface
to
-
10 volts.
All
these actions are caused
by waveforms
applied
to the storage mesh by the
pulse
circuits in
Model 1331A.
4-1
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