Section Iv Principles Of Operation - HP 181A Operating And Service Manual

Model 181A/AR Section IV
Paragraphs 4-1to 4-11
| SECTION IV
PRINCIPLES OF OPERATION
4-1. INTRODUCTION.
4-2. This section provides circuit theory analysis of
the Model 181A/AROscilloscope. Since variable per-
sistence and storage techniques may be somewhat un-
familiar to the reader, basic theory of operation will
be explained first to aid in grasping these concepts.
An over-all block diagram is explained next, followed
by a detailed description of the individual circuits.
4-3. VAR PERSISTENCE & STORAGE.
4-4. STORAGE PRINCIPLES.
4-5. The Model 181A/AR Storage CRT consists mainly
of a conventional electron gun with deflection plates
(write gun), an aluminized phosphor viewing screen,
a pair of flood guns operated in parallel, flood beam
shaping and accelerating grids, aflood beam collima-
tor, a collector mesh, and a storage mesh as shown
in Figure 4-1. `
4-6. The write gun functions as a conventional elec-
trostatic deflection gun, delivering high velocity elec-
trons to selected points on the phosphor viewing screen.
The elements which provide storage and variable per-
sistence are located between the write gun andthe phos-
phor.
4-1. The flood guns are physically located just out-
side the horizontal deflection plates. A cloud of elec-
trons is emitted by each flood gun cathode. These
clouds are combined, shaped, and accelerated by two
control grids. It shouldbe noted that under certain con-
ditions the twoelectronclouds will appear as light areas
onthe viewing screen when the instrument is first turned
on. The combinedcloud is further shapedandacceler-
ated by the collimator ( a coating on the inside of the
-funnel section of the glass). The positive voltage on
the collimator is adjusted so that the flood-gun elec-
tron cloud just fills the CRT viewing screen. The
cloud is further accelerated toward the storage mesh
and viewing screen by the collector mesh. After pas-
sing through the collector mesh, the flood electrons
are further controlled by potentials on the storage
mesh and suríace. l
POST
ACCELERATOR
STORAGE
FLOOD GUNS MESH
PHOSPHOR
VIEWING
SCREEN
DNE o
WRITE GUN' . ^,
AQUADAG
COLLIMATOR STORAGE
COLLECTOR SURFACE
MESH ` 18i A?
Figure 4-1. Simplified CRT Construction.
02612-2
4-8. The storage mesh is located between the collector
meshandthe phosphor. The back side of this meshis
coated with a layer of non-conductive material. The
storage of information takes place on the surface of
this non-conductive material (storage surface).
4-0, The basis for storage of information onthe non-
conductive material is the secondary emission ratio
curve shown in Figure 4-2. Thiscurve shows the ratio
of the number of electrons leaving the surface to the
„energy of the electrons striking the surface. At an
energy of about 40 electron/voits (ev) the number of
electrons leaving the surface is equal to the number
arriving. The point where the secondary emission
ratio is equal to unity is called "first crossover." If
the surface is bombarded with electrons with more than
40 ev of energy, the surface potential rises because
more electrons are leavingthanarriving. If the sur-
face is bombarded with electrons with less than 40ev
of energy, the surface potential decreases because
fewer electrons are leaving than arriving.
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STORAGE MESH POTENTIAL IN VOLTS
EISE IBIA--A- 8 |
Figure 4-2. Secondary Emission Ratio.
4-10. When the ERASE push button is pressed, the
storage mesh is changed to the same potential as the
collector mesh (+156v). The storage surface is also
changed to nearly this same potential by capacitive
coupling. Since the surface is then being bombarded
by electrons with energies much higher than first cross-
over energy, the entire storage potential becomes
equal to +156 volts. The surface potential cannot in-
crease beyond +156 volts because the collector mesh
would then repel the emitted electrons back to the
storage surface, tending to decrease the surface po-
tential. l
4-11. When the ERASE push button is released, (see
Figure 4-3) the storage mesh is now changed to «3. 3
volts and the storage surface follows to the same po-
tential by capacitive coupling. The surface potential
4-1