Download Post Deflection Acceleration

Post Deflection
Acceleration
- Baher Abu-Sbaa’
- Mohammed Naser
Post Deflection Acceleration:
For maximum brightness, the electrons
should be accelerated to its greatest
velocity. However, if the electron
velocity is very high when passing
through the deflection plates, the
deflecting voltages will have a
reduced influence and the deflection
sensitivity will be poor.
It is for this reason that the post
deflection acceleration is provided;
that is the electrons are accelerated
again after they pass between the
deflecting plates.
We have two ways to do this :
- Mesh scan expansion
- Meshless scan expansion
Use of expansion
mesh
A metallic mesh is balanced in the electron
beam, and acts as a magnifying lens that
causes the deflection to be further increased,
which improves the deflection
sensitivity.
With this technique, deflection sensitivity can
remain on the order of 5 to 50 V/cm
even though the total electron beam
acceleration is more than 10,000 V.
Disadvantages !
The mesh tends to defocus the electron beam
and make the spot broader than it would be
without the mesh interfering with the beam.
Second, the mesh conducts some of the electron
beam away from the screen. This results in a
reduced beam current and thus reduced spot
intensity.
The problem is not unique to the mesh, is that the
electron beam tends to be defocused in the
vicinity of the deflection plates owing to repulsion
from charge distributions within the tube.
Meshless scan
expansion
Several recent advances in cathode ray tube
design have eliminated the mesh and alleviated
these problems, thus producing a highperformance electron gun for use in
high-frequency cathode ray tubes.
The electron beam is generated from a
conventional heated cathode surrounded by the
control grid.
The accelerating anode and two focus electrodes
follow and provide focus, as well as the first
accelerating voltage.
These focus electrodes different from the
cylindrical elements used in the
conventional tube in that they are
constructed from individual metal wafers
with non-cylindrical holes in the center.
This allows for a different focusing
characteristic in the horizontal plane and
the vertical plane, typically divergent in
one plane being convergent in the other.
The holes in the center of the metal wafers
can be formed with greater precision than
in a formed cylinder, and thus greater
tolerances can be achieved at a lower cost.
After the two focusing electrodes, the beam
passes through the vertical deflection plates.
The beam at this point is not fully focused,
which decreases the amount of beam
distortion due to the internal charge
distributions.
The beam will be further focused after deflection to provide: a
fine spot.
After vertical deflection, the beam passes through a scan
expansion lens that increases the amount of beam bending in
the vertical plane.
The beam is then deflected in the horizontal direction and passed
through another
electron lens, which provides additional focusing.
The beam is accelerated to the final velocity by a quadrupole
lens, which provides not
only an increase in electron velocity, but adds to the scan angle
(scan expansion,
which is similar to the mesh ) without distorting or defocusing the
electron beam.
The result of this design is increased deflection
sensitivity, typically 2.3 V/cm for the
vertical deflection and 3.7 V/cm in the horizontal
direction.
The difference between the vertical and horizontal
deflection sensitivities is due to the
fact that the vertical deflection occurs at a lower
beam velocity.
Because the horizontal deflection of the oscilloscope involves only
a time linear
sweep, while the vertical deflection requires complex waveforms,
the more sensitive deflection should be reserved for the vertical
direction.
Using the mesh-less electron gun, 100-MHz plus oscilloscopes can
be constructed
with integrated circuits using only 40 or 50 V or even less for
deflection.
The mesh-less tube is being considerably shorter, results in smaller
and lighter
oscilloscopes for laboratory and portable use