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Model 181A/AR
4-17. If this procedure is repeated many times, the
Stored trace will eventually be erased.
The time re-
quired to accomplish
this erasure
is controlled
by
varying the duty cycle of the pulses applied to the stor-
age mesh (or by varying the pulse width if the pulse
repetition rate remains fixed).
l
l
4-18, Duringthetime the storage mesh is pulsed pos-
itive, floodelectrons are allowed through to the phos-
phor viewing screen.
Thusa light background glow is
visible when the CRT is used in the variable persis-
tence mode.
.
4-19., NORMAL.
4-20. If the storage mesh potential is reduced to -25
volts it acts as a control grid to flood gun electrons
andpreventsthemírom
reaching the phosphor.
How-
ever, it has little affect on write gun electrons, allow-
- ing many of them to reach the phosphor viewing screen.
Some of the write gun electrons strike the storage sur-
face, charging it positive toward zero volts wherever
they strike.
Thusthe CRT appearstoact as a conven-
tional CRT
without
variable persistence or storage.
However,
when the storage mesh is brought back to
-3.3 volts,
those areas of the storage surface which
have been struck by write gun electrons allow flood
electrons to be captured by the post accelerator field,
and thus display the pattern that was written on the
phosphor
while the storage
mesh
was
at -25 volts.
4-21. In order to view a stored trace for one minute
or more,
the storage mesh is held at a constant +3. 3
volts.
This
may be accomplished by reducing
the
width ofthe variable persistence erase pulses to zero
(corresponding to maximum persistence),
or by actu-
ally disconnecting the pulses from the storage mesh.
The write gun may be turned off if desired,
to prevent
additional writing on the storage surface.
4-22. The
mechanism
which
limits
viewing time
is
the "fade positive"
of the storage
surface
( entire
Screen illuminated).
This is caused by positive ions
reaching the storage surface and charging it positive.
The positive ions are generated by flood gun electrons
striking residual gas molecules in the CRT.
To ob-
tain an extended storage time,
the flood guns
should
02612-2
Figure 4-4.
Variable Persistence Accomplishment Through Pulsed Erasure.
Section IV
Paragraphs 4-17.to 4-30
STORAGE
MESH
POTENTIAL |
mp
igla-8-4
|
be turned off.
Thisisdone in the STORE
mode; how-
ever,
thefloodgunsare turned on occasionally to per-
mit viewing of the stored trace at à reduced intensity.
4-23.
OVER-ALL DESCRIPTION.
4-24.
GENERAL.
4-25. The Model 181A/AR Oscilloscope consists of à
low- voltage power supply, a calibrator,
four external
output isolation
amplifiers, a high voltage power sup-
ply,
a gate amplifier,
à horizontal
amplifier,
and
storage and variable persistence puise circuitry.
Fig-
ure 4-5 shows the functional relationship of these cir-
cuits.
The instrument may be operated as a general
purpose,
a variable persistence,
or a storage oscil-
loscope.
Circuitoperationin all three modes is des-
scribed in this section.
4-26. Three input signals--intensity,
horizontal de-
flection,
and vertieal deflection--are necessary to
obtain
a usable display on the CRT.
The circuitry for
the intensity. and horizontal deflection signals is ex-
plained in the foliowing paragraphs which are refer-
enced to Figure 4-5,
The vertical
deflection
signal
is coupled to the CRT from the Vertical
Plug-In.
4-27.
INTERNAL.
4-28.
Horizontal deflection signals may be obtained
from the Horizontal Plug-In by positioning the HORI-
.
ZONTAL DISPLAY switch to INT.
Thisapplies -100
voltstothe plug-in which produces an unbianking gate
and generates
an internal
sweep
signal.
4-29. The unblanking
gate is coupled from the Hori-
zontal Plug-Into the gateamplifier where it is summed
with the Z-Axis Input, chopped blanking signals,
and
the unblanking
signal from the storage pulse
circuit.
The resulting signal is amplified,
coupled through the
high voltage power
supply,
and applied to the CRT
write
gun grid to control
the display
intensity.
4-30. Each
unblanking
gate. signal developed by the
gate amplifier also generates analternate trigger sig-
nai.
This alternate trigger is directly coupled to the
Vertical Plug-In.
(The Operatingand Service Manual
for the Vertical
Plug-In unit employed should be re-
ferred to for the signal function. )
4-3
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