screen and
earthed.
Cathode Ray Oscilloscope
1.
2.
The CRO is used to show how the voltage varies with
time.
Thermionic Emission: It is the escape of electrons from
the surface of a heated metal.
Filament/Cathode
4.
earth.
The X-shift and the Y-shift knobs at the front panel are
used to position the trace at the centre of the screen.
2 other parameters to get a proper waveform are:
a. Y-gain(Y-sensitivity): It amplifies the Y-deflection.
b. Time-base: Controls the speed at which the electron
beam shifts from left to right (done by altering the
frequency of the time base).
Anode
a.
3.
Metallic substances have numerous electrons on
their surface.
b. When heated, these electrons gain enough energy
to break away from the surface.
c. This liberation of electrons or irons from substances
that are highly heated is termed thermionic
emission.
d. The number of thermions emitted increases rapidly
as temperature rises.
e. Thermionic emission’s most important practical
application in electronics is in the electron tube, the
mechanism by which electrons are emitted from
the cathode.
Parts of the CRO:
c.
d.
e.
Main Part
Electron
Gun
Component
Filament
Cathode
Grid
Focussing Anode
Accelerating Anode
Deflecting
System
X- plates
Y-plates
Fluorescent
Screen
Screen coated with
a fluorescent
material
Graphite is coated
over fluorescent
Function
Heating up the cathode
Emitting electrons by
thermionic emission
To control the amount
of electrons
To attract and focus the
electrons into a beam
To accelerate the
electrons to a higher
velocity.
To deflect the beam of
electrons horizontally.
To deflect the beam of
electrons vertically.
Change the kinetic
energy of the electrons
to light energy.
Electrons that hit the
screen are sent to the
Notes/static&current_electricity/mili@chms
f.
Brightness and focus knobs are used to adjust the
trace.
Use X-shift and Y-shift knobs to position trace at
centre of screen.
Voltage applied across the Y-plates gives different
displays on the screen:
 With no voltage across Yplates, a light spot rests at
the centre of the screen.

With a d.c. voltage, the
light spot is deflected
upwards. As voltage
increases, deflection
increases.

With a 50 Hz a.c. voltage,
spots appear as a vertical
line.
 A gain of 2 V/cm
 the spot is deflected 1 cm for every 2 V
applied across the Y-plates.
A built-in voltage, called the time base is applied
across the X-plates, to make the light spot sweep
across the screen at a steady speed, then 'fly back'
rapidly to the start.
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g.
 A time
base of 5 ms/cm
 the electron
beam takes 5 ms
to sweep 1 cm
across the screen.
Time base on + a.c. voltage applied across Y-plates =
a waveform of the a.c. voltage on the screen.
EXTRA NOTES:
Notes/static&current_electricity/mili@chms
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