Philips PM 3266 - Page 94

3. 1 .
5
.
Storage
system
3.1.5.
1.
Introduction
The PM
3266
oscilloscope
is
equipped
with
a
special
type
of storage
tube
with
an
extremely
high
writing
speed.
The
electrode
configuration
of this
tube is shown
in
the block
diagram.
The high
writing
speed
is
achieved
by
a
triple-mesh
storage
system:
—
a
collector
mesh
(G10)
—
a fast
storage
mesh
(G11
)
—
a front
mesh
(G12)
The flood
gun system
in
the c.r.t.
uses
three collimator
electrodes
Cl,
C2 and
C3.
In addition
to the
storage
system,
the c.r.t.
has a scan
magnifier
(G7), which
is
located
between
the vertical
and
horizontal
deflection
plates.
This
magnifier
gives
a x1 .8 increase
of vertical
gain.
The
scan
magnifier gives
a
divergence
of
the electron
beam
in the vertical
direction and
a convergence
in the
horizontal
direction.
To correct
for
this, the c.r.t. has
two focusing
electrodes:
G3 for
horizontal
focus and
G5 for
vertical
focus.
The
focusing
function
is also
correctively
influenced by
the setting of the INTENS
control, so that
the
spot
on
the c.r.t.
screen
remains
weill-focused over
the whole range
of the intensity.
The block
diagram
description of
the c.r.t.
display section can
be conveniently
divided into
the
following
sections:
—
storage
electrode
systems with
applied waveforms in the WRITE,
FAST,
STORE and MEMORY
OFF
modes;
—
block diagram description of
storage logic
and storage
amplifiers;
—
block
diagram
description of
the focus
and Z-amplifier section.
3. 1.5.2.
Storage Electrode
System
The storage
cathode-ray is of
the triple-mesh
type. A
fast
mesh has been added between
the collector mesh
and storage
mesh of the conventional storage tube. This
fast mesh is characterised
by a fast writing speed and
short storage
time. In the FAST
writing speed mode, the information is written
into
the fast mesh and is
transferred
to
the
front
mesh immediately after the end of
the time-base
sweep. This information transfer is
aided by
the flood beams
and by pulsing the front storage mesh.
After
this "image transfer",
the fast storage
mesh potential is increased to approximately
that of the collector
and the fast
information
is
visible
on the screen with the normal storage
time.
The voltage waveforms
applied
to the storage electrodes in the WRITE MODE, FAST
MODE, STORE MODE
and NON
STORE
MODE
are now
described.
The voltages that are
mentioned
are typical
values;
the precise
values
are referred to in
the section
on
Checking
and
Adjusting.
3. 1.5.3. Write Mode
As
the fast
storage mesh (G11) is not used
in this mode, it is connected
to approximately the same potential
as the collector
mesh, as shown in Fig. 3.2.
This figure
also indicates the voltage
waveforms that are applied
to
the
other electrodes.
During
the erase cycle, the front
mesh
(G12)
is first connected to
a potential of 500 V for 100 ms (the first
erase pulse) and then
returned
to
approximately
0 V
for
500 ms.
The erase
cycle is
then
completed
by applying the
front
mesh to
approximately
10 V for 600 ms (the second
erase pulse). After
this, the front
mesh
is
again returned to 0 V and can then be written afresh.
During the
erase cycle the collimators Cl and
C3
and the flood gun accelerator
are pulsed in
a
positive direction.
The cycle described is a static erase
cycle.
It starts after the
release of the
ERASE
pushbutton.
In the
MAX WRITE
mode the 0 V levels are increased to a slightly positive value.
However,
in the VARIABLE PERSISTENCE position an
adjustable rapidly vanishing trace is required. To
achieve this, dynamic erase pulses
are
applied
to
the front mesh as shown in Fig.
3.2. These
pulses
have an
upper level of about
-h
8 V,
a
frequency of about 1 00 Hz and an adjustable duty cycle. This duty cycle is
adjustable
between
0
and
30 % by means of the
PERSISTENCE control;
at 0 % the trace persists for some
time (unless the ERASE pushbutton is operated) and
at
30
%
the trace rapidly
disappears.
A time-base blocking signal is also shown in Fig. 3.2.
The time-base is
blocked
from the beginning
of the
first
erase
pulse until 100 ms after the end of the second erase pulse. During this period
the time-base does not
respond
to
trigger
signals.