Lesson 2
•The Cathode Ray Tube
 The Cathode Ray Oscilloscope
Cathode Ray Oscilloscope Controls
Uses of C.R.O.
•Electric Flux
Electric Flux Through a Sphere
•Gauss’s Law
The Cathode Ray Tube
• Example 7 on an accelerated electron (See lesson 1)
describes a portion of a cathode ray tube (CRT). This
tube, is commonly used to obtain a visual display of
electronic information in oscilloscopes, radar systems,
television receivers, and computer monitors.
• The CRT is a vacuum tube in which a beam of
electrons is accelerated and deflected under the influence
of electric or magnetic fields. The electron beam is produced
by an assembly called an electron gun located in the neck of
the tube.
• These electrons, if left undisturbed, travel in a straight-line
path until they strike the front of the CRT, the “screen,’’
which is coated with a material that emits visible light
when bombarded with electrons.
The Cathode Ray Tube
The Cathode Ray Tube
The Cathode Ray Tube
• The CRT is composed of two main parts,
1.
2.
•
Electron Gun
Deflection System
Electron Gun
–
–
–
Electron gun provides a sharply focused electron beam directed
toward the fluorescent-coated screen.
The thermally heated cathode emits electrons in many
directions. The control grid provides an axial direction for the
electron beam and controls the number and speed of electrons
in the beam.
The momentum of the electrons determines the intensity, or
brightness, of the light emitted from the fluorescent coating due
to the electron bombardment. Because electrons are negatively
charged, a repulsion force is created by applying a negative voltage
to the control grid, to adjust their number and speed.
The Cathode Ray Tube
– A more negative voltage results in less number of electrons
in the beam and hence decreased brightness of the beam spot.
Since the electron beam consists of many electrons, the
beam tends to diverge. This is because the similar (negative)
charges on the electrons repulse each other. To compensate for
such repulsion forces, an adjustable electrostatic field is
created between two cylindrical anodes, called the focusing
anodes. The variable positive voltage on the second anode
cylinder is therefore used to adjust the focus or sharpness of the
bright spot.
• The Deflection System
– The deflection system consists of two pairs of parallel
plates, referred to as the vertical and horizontal deflection
plates. One of the plates in each set is permanently
connected to the ground (zero volt), whereas the other
plate of each set is connected to input signals or triggering
signal of the CRO.
Cathode Ray Oscilloscope
Time base
Display
Y-gain
Channel1
Channel 2
Cathode Ray Oscilloscope
• A typical analogue oscilloscope
Cathode Ray Oscilloscope Controls
• Y-Gain
– amplifies the Y-deflection
– small input voltages are amplified by built-in
amplifiers before applying to the Y-plates.
– Y- Gain = 0.5 V/div
• 0.5 volt will cause a vertical deflection of 1 division
Cathode Ray Oscilloscope Controls
Time Base
• is a saw-tooth voltage applied internally
across the X-plates.
volts
time
Cathode Ray Oscilloscope Controls
Time Base
• controls the speed at which the spot
sweeps across the screen horizontally from
left to right.
volts
spot on right side
of screen
spot at centre
of screen
0
Fly back
time
spot on left side
of screen
Time taken
for spot to
move across
the screen and
back
Cathode Ray Oscilloscope Controls
Time Base
volts
spot on right side
of screen
spot at centre
of screen
Fly back
0
time
spot on left side
of screen
Screen
Cathode Ray Oscilloscope Controls
Time Base
• it helps to display the actual waveform of
any a.c. applied across the Y-plates
• normally calibrated in
– s/cm
– ms/cm
– s/cm
• gives the time required for the spot to sweep 1 cm
horizontally across the screen.
Cathode Ray Oscilloscope Controls
Time Base: How It Works
volts
spot on right side
of screen
spot at centre
of screen
spot on left side
of screen
B
Fly back
0
A
time
C
Time taken for spot to move
across the screen and back
Uses of C.R.O.
1. Display waveforms of alternating p. d.
2. Measure potential difference
– d.c.
– a.c.
3. Measurement of Frequency
4. Measurement of Phase
Uses of C.R.O.
Displaying a Voltage Waveform
Peak-to-Peak
voltage
Time Period (ms)
To get the time period you need to
measure this distance and convert it
to time by multiplying by the time
base setting
Uses of C.R.O.
Displaying a Voltage Waveform
Uses of C.R.O.
Displaying a Voltage Waveform
• Set the time-base to a suitable frequency,
• Apply the input to the Y-plate
– a steady waveform of the input will be displayed
on the C.R.O.
Uses of C.R.O.
Measuring a Direct Current Voltage
•
•
•
•
switch off the time-base
a spot will be seen on the C.R.O. screen
d.c. to be measured is applied to the Y-plates
spot will either deflected upwards or
downwards
• deflection of the spot is proportional to the d.c.
voltage applied
Uses of C.R.O.
Measuring a Direct Current Voltage
• Set the VOLTS/DIV to 1
by adjusting the outer
dial.
• Turn the inner dial all
the way to the right,
which will put it in the
calibrated position.
• Switch the AC-GND-DC
switch for channel 1 to
DC.
Uses of C.R.O.
Measuring a Voltage as a Voltmeter
• it has nearly infinite resistance (between the
X- and Y-plates), therefore draws very little
current;
• it can be used to measure both d.c. and a.c.
voltages; and
• it has an immediate response.
Uses of C.R.O.
Measurement of Frequency
Time Per Division Dial
• The Time/Div dial on the
oscilloscope
controls
the
amount of time per centimeter
division.
•
•
•
•
A simple method of determining the frequency of a signal is to estimate its periodic time
from the trace on the screen of a CRT.
To calculate the frequency of the observed signal, one has to measure the period, i.e. the
time taken for 1 complete cycle, using the calibrated sweep scale. The period could be
calculated by
T = ( no. of squares in cm) x ( selected Time/cm scale )
Once the period T is known, the frequency is given by f (Hz)= 1/T(sec)
Uses of C.R.O.
Example: Measurement of Frequency and a. c. voltage Using a CRO
The total height of the wave
from peak to trough is 6.4 cm
Vpk to pk= 12.8 V
V0 = 6.4 V
6.4
cm
1 cycle occupies 2.8 cm
T = 1.40 ms = 1.40 10-3 s
Frequency = 1 1.40 10-3 s
= 714 Hz
2.8 cm
The time base controls are set at 5 ms/cm
The voltage gain is set at 2 V/cm
Uses of C.R.O.
•Measurement of Phase
•The calibrated time scales can be used to calculate the
phase shift between two sinusoidal signals of the same
frequency. If a dual trace or beam CRO is available to
display the two signals simultaneously ( one of the
signals is used for synchronization), both of the signals
will appear in proper time perspective and the amount of
time difference between the waveforms can be measured.
•This, in turn can be utilized to calculate the phase angle ,
between the two signals.
Phase shift in cm.
x 360
One period in cm.
Uses of C.R.O.
Measurement of Phase
Phase shift in cm
One period in cm
Phase shift in cm.
x 360
One period in cm.
Uses of C.R.O.
Lissajous’ Figures
• Lissajous’ figure can be displayed by applying two a.c. signals
simultaneously to the X-plates and Y-plates of an oscilloscope.
• As the frequency, amplitude and phase difference are altered,
different patterns are seen on the screen of the CRO.
• Lissajous’ figures are obtained with an oscilloscope when this is
operated in XY mode and when in both channel 1 and 2 voltages
are applied.
• If the ratio of the two frequencies of the voltages is just equal to a
rational number, standing figures appear on the oscilloscope
(Lissajous’ figures); if the frequency ratio, however, deviates
slightly from a rational number, these figures are moving.
• In this way it is possible to make small frequency differences
visible and measurable. Unknown frequencies can be
determined, if these are applied to one channel of the
oscilloscope and are superimposed with a voltage of known, but
adjustable frequency from a function generator applied to the
other channel.
Uses of C.R.O.
• Use of Lissajous Patterns to Calculate Phase Shift
– Lissajous patterns are obtained on the scope
simultaneously by applying the two sinusoidal inputs
to be compared at the vertical and horizontal
channels.
– The phase shift is then determined using measured
values taken from resulting Lissajous pattern. This
pattern on the CRT screen may be either a straight line
or a circle or an ellipse depending on the amount of
phase shift.
Uses of C.R.O.
Lissajous’ Figures
Same amplitude but different frequencies