April 21, 2010
Prof. Dr. Mahmoud El-Gammal
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Chapter 3
Introduction
to the
Oscilloscope
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Prof. Dr. Mahmoud El-Gammal
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What is an oscilloscope?
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Prof. Dr. Mahmoud El-Gammal
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Sometimes you want to see how signals
change with time
Electrical signals are invisible,
unless you know how to look at them.
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Prof. Dr. Mahmoud El-Gammal
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Major Components of CRO
1.
Cathode ray tube (CRT), which is the heart of
the instrument.
2. Vertical amplifier.
3. Horizontal amplifier.
4. Sweep generator.
5. Trigger circuit.
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Prof. Dr. Mahmoud El-Gammal
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Cathode Ray Tube (CRT)
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Cathode Ray Tube (CRT)
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CRT
 Cathode ray tube essentially consists of an
electron gun for producing a stream of electrons,
focusing and accelerating anodes for producing a
narrow and sharply focused electron beam,
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Prof. Dr. Mahmoud El-Gammal
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 ELECTRON GUN
 Consists of cathode, control grid,
focusing anode and accelerating
anode.
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Prof. Dr. Mahmoud El-Gammal
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CATHODE RAY TUBE (CRT) CONT’D
 CONTROL GRID
 Regulates the number of electrons that reach the anode
and hence control the brightness of the spot on the screen.
 FOCUSING ANODE
 Ensures that the electrons leaving the cathode in slightly
different directions are focused down to a narrow beam
and all arrive at the same spot on the screen.
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Prof. Dr. Mahmoud El-Gammal
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Electrostatic Focusing System of a CRT
Equipotential
Surface
1
Direction of
Electric Force
V2N > V1N
V1T =V2T
 2<  1
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2
V2T
V2N
V1T
V1N
Prof. Dr. Mahmoud El-Gammal
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Electrostatic Focusing System of a CRT
Electric Field
Lines
Equipotential
Surface
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Prof. Dr. Mahmoud El-Gammal
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CRT
 Cathode
ray
tube
consists
of
horizontal and vertical deflection
plates for controlling the beam path
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Deflection of Electron Beam in CRT (Theory of
Electrostatic Deflection)
L
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Theory of Electrostatic Deflection
 L : length of the deflecting plates in meter;
 d : distance between the deflecting plates in meter;
 Vd : potential difference between the deflecting plates in volt;
 E : electric field intensity = (Vd/d) in volt/meter, (uniform electric
field);
 u : electron beam initial velocity in m/sec along X-axis (u = vx);
 t : transit time = the time required for the electrons to pass
between the plates, t = (L/u) in sec;
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Prof. Dr. Mahmoud El-Gammal
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Theory of Electrostatic Deflection
 m: the mass of an electron in kg = 9.109 × 10-31
 e: charge of an electron in coulomb = 1.602 × 10-19
 e/m = 1.77 × 1011 coulomb / kg;
 Va: the accelerating voltage through the electron gun.
 u = √ [(2eVa)/m] in m/sec
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Prof. Dr. Mahmoud El-Gammal
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The Deflection Sensitivity of a CRT
Period
electron
region
plates :
during
which
remains
between
L
t
u
an
in
the
the
two
L
Y
d
Acceleration in y-axis :
eE e Vd
 
m m d
eVd  L
v y  0  ay  t 
mdu
Y vy vy
tan   

S vx
u
ay 
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Deflection
Sensitivity (m/V)
Y
e LS
LS


2
Vd m  d  u
2d  Va
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Signal on the CRT
CRT display
Time
Vertical deflection
voltage
Time
Horizontal deflection
voltage
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Control panel of an oscilloscope
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Basic setting
 Vertical system
 attenuation or amplification of signal (volts/div)
 Horizontal system
 The Time base (sec/div)
 Trigger system
 To stabilize a repeating signal and to trigger on a single event
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Prof. Dr. Mahmoud El-Gammal
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Measurements of Oscilloscope
 Voltage Measurements
 Period and Frequency Measurements
 Phase Measurements or Time Delay
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Voltage Measurement
 The vertical scale is calibrated in either volts per
division or milivolts per division.
 Using the scale setting of the scope and the signal
measured off the face of the scope, then it can
measured peak-to-peak voltage for an ac signal
Vp-p = (vertical p-p division) × (volts/div)
OR
Vp-p = (no. of vertical division) × (volts/div)
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Prof. Dr. Mahmoud El-Gammal
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Voltage Measurement
2.5
Voltage Peak-to-Peak (a
Vp-p= (V/Div) x No. of vert. div.
3.8
Vp-p
Vp
= 100 mV/div x (3.8 x 2)
= 0.76 V
3.8
T
b) Voltage Peak
TD
Vp = (V/Div) x No. of vert. div.
A
10
B
= 100 mV/div x (3.8)
= 0.38 V
(Volt/Div : 100mV/Div, Time/Div : 0.5ms/Div)
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Prof. Dr. Mahmoud El-Gammal
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Period & Frequency Measurement
 PERIOD
 Horizontal scale of the scope can be used to measure time in
second, milisecond or nanosecond.
 The interval of a pulse from start to end is the period of the pulse.
Period = (horizontal p-p division) x (time/div)
 FREQUENCY
 The measurement of a repetitive waveform period can be used to
calculate the signal frequency.
F= 1/T
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Prof. Dr. Mahmoud El-Gammal
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Period & Frequency Measurement
Period, T (a
2.5
T = (Time/Div) x (no.
div/cycle)
3.8
3.8
Vp-p
Vp
T
TD
A
= 5ms
b) Frequency, f
10
B
(Time/Div : 0.5ms/Div)
April 21, 2010
= 0.5ms/div x 10
Prof. Dr. Mahmoud El-Gammal
f = 1/T
= 1/5ms
= 200 Hz
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Phase Shift (Phase Different)
 The time interval between pulses is called pulse delay.
 The pulse delay is measured between the midpoint at
the start of each pulse
Phase difference, Ө = (phase difference in division) x (degrees/div)
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Prof. Dr. Mahmoud El-Gammal
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Phase Shift (Phase Different)
= 10 div 1 cycle
2.5
= 2 div
3.8
Vp-p
TD
Therefore,
Vp
1 cycle : 10 div = 360o
3.8
T
TD
A
1 div = 360o / 10 = 36o
2 div = 2 x 36o = 72o
10
B
(Time/Div : 0.5ms/Div)
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Prof. Dr. Mahmoud El-Gammal
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Lissajous Patterns
 FREQUENCY MEASUREMENT
 The alternative way of using oscilloscope to measure
frequency.
 In order to generate a Lissajous pattern a known reference
frequency sine wave is applied to one of the deflection
plates of the oscilloscope and the unknown sinusoidal
signal to the other deflection plates
 A Lissajous pattern is produced on the screen according to
the frequency ration between the two signal:
Fy
Number of Positive y  Peaks in pattern

Fx Number of Positive x  Peaks in pattern
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Prof. Dr. Mahmoud El-Gammal
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Lissajous Patterns
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Lissajous Patterns
 PHASE ANGLE MEASUREMENT
 Oscilloscope can be used in the X-Y mode to determine the
phase angle between two signals.
 This useful technique is limited to small frequency.
 The formula for phase angle measurement:
Sin θ = Y1/Y2 = X1/X2
Where
θ = phase angle in degree
Y1 = the distance from X-axis to the point where the
Lissajous pattern crosses Y-axis
Y2 = the maximum vertical distance on the Lissajous
X1 = the distance from Y-axis to the point where the
Lissajous pattern crosses X-axis
Y2 = the maximum vertical distance on the Lissajous
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Prof. Dr. Mahmoud El-Gammal
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Lissajous Patterns
θ- phase angle in degree
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Yo-Y axis intercept
Prof. Dr. Mahmoud El-Gammal
Ym-maximum vertical deflection
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Lissajous Patterns
θ- phase angle in degree
April 21, 2010
Yo-Y axis intercept
Prof. Dr. Mahmoud El-Gammal
Ym-maximum vertical deflection
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