Learning Event 2:
DESCRIBE THE VARIOUS CONTROLS ON AN OSCILLOSCOPE
1.
If you understand the principle of operation of one oscilloscope you can
apply the same techniques to other oscilloscopes, since all scopes operate on the
same principle and have about the same controls.
By discussing the controls in
general you will have sufficient information to operate most types of
oscilloscopes.
a. The two basic controls affecting the readability of the scope display are
the beam intensity and focusing controls.
These two controls are considered
together because they interact to an extent that adjusting one requires adjusting
the other.
b. The intensity control is used to adjust the spot to the brightness
desired. When the spot is still, it becomes brighter, larger, and out of focus as
the intensity control is rotated toward maximum intensity.
Further rotation of
this control produces secondary emission, causing a halo around the spot. When the
halo appears, the intensity control must be immediately decreased to eliminate the
halo before the screen is burned.
(1) The halo from an excessively bright spot disappears to some extent when
the electron beam is subjected to the deflection fields because the energy in the
electron beam is distributed over a much greater area.
(2) However, the spot will produce a wide trace, tending to obliterate any
available fine detail.
c. The focus control is used to produce a round spot with a clearly defined
edge.
A stationary spot becomes smaller and sharper when you rotate the focus
control from minimum toward maximum value. As you rotate this control beyond the
focal point, an out-of-focus spot is again produced.
d. A poorly focused spot can appear elliptical instead of round.
When the
elliptical spot is set into motion under the influence of deflecting fields, it is
noticeable as a line of variable thickness.
The spot in motion produces a thin
line only at the peaks of sine wave, while the positive and negative-going portions
of the sine wave are considerably thickened.
e. Depending upon the velocity of the spot, an increase in spot intensity may
be required because the rapidly moving spot does not remain in one position long
enough to fully excite the phosphor screen of the cathode-ray tube.
You may
observe this effect in Figure 2-4, when viewing square waves with extremely rapid
rise and decay times.
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