Electronic Instrumentation
Analog and Digital Oscilloscope* In
this presentation definitions and examples
from Wikipedia, HowStaffWorks and some other sources
were used
Lecturer: Dr. Samuel Kosolapov
Items to be
defined/refreshed/discussed
• CRT Oscilloscope
• Idea of operation
• Calculation of deflection
• Block-Chart
• Need for Probe
• Digital Scope
• Arduino-based Scope
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CRT Oscilloscope with Deflection Plates
http://wps.aw.com/wps/media/objects/877/898586/topics/topic07.pdf
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CRT Oscilloscope with Deflection Plates
http://wps.aw.com/wps/media/objects/877/898586/topics/topic07.pdf
The initial speeds at which the electrons are
emitted from the cathode
are very small
in comparison to their final speeds,
so we assume that the initial speeds are zero
Then the speed Vx of the electrons
as they leave the electron gun is given by
Example: V1 = 2 kV
4
CRT Oscilloscope with Deflection Plates
http://wps.aw.com/wps/media/objects/877/898586/topics/topic07.pdf
Electrons enter the region
between the vertical-deflection plates
with CONSTANT speed Vx.
V2 a potential difference (voltage) between
Vertical deflection plates
 electric field magnitude E = V2/d
 A constant upward force with magnitude
eE then acts on the electrons,
 and their upward (y-component)
acceleration
Ay is
The horizontal component of velocity vx is constant.
The path of the electrons in the region between the plates is
a parabolic trajectory
5
CRT Oscilloscope with Deflection Plates
http://wps.aw.com/wps/media/objects/877/898586/topics/topic07.pdf
After the electrons leave the “plates” this region,
their paths again become straight lines,
and they strike the screen at a point “y”
We are going to prove that this distance “y”
is directly proportional
to the deflecting potential difference V2
The time t required for the electrons to travel
the length L of the plates is
During this time t, electrons they acquire
an upward velocity component Vy
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CRT Oscilloscope with Deflection Plates
http://wps.aw.com/wps/media/objects/877/898586/topics/topic07.pdf
When the electrons leave the deflecting field,
their velocity V makes an
angle THETA with the x-axis
Length L of the deflection plates is much smaller
than the distance D from the plates to the screen.
In this case the angle THETA ~ tan( THETA) = y/D
IMPORTANT: Y is propotional to “input voltage “V1”.
 Linear “electronic” Voltmeter (Very Fast !!!)
If there is also a field between the horizontal deflecting plates, the beam is also deflected in the horizontal direction.
The coordinates of the luminous spot on the screen are then proportional to the horizontal and vertical deflecting
voltages, respectively. This is the principle of the cathode-ray oscilloscope.
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CRT Oscilloscope with Deflection Plates.
Additional Details
8
CRT Oscilloscope with Deflection Plates
http://www.electrical4u.com/cathode-ray-oscilloscope-cro/
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CRT Oscilloscope with Deflection Plates. Focusing
http://www.electrical4u.com/cathode-ray-oscilloscope-cro/
Electrostatic Focusing:
TWO Parallel Plates
Electrostatic Focusing:
TWO Coaxial Cylinders
10
CRT Oscilloscope with deflection coils
11
CRT Oscilloscope Block chart. Basic Operation
http://www.doctronics.co.uk/scope.htm
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CRT Oscilloscope Block chart. Panel
http://www.doctronics.co.uk/scope.htm
13
CRT Oscilloscope. Need for Probe
http://www.rfcafe.com/references/electrical/NEETS-Modules/NEETS-Module-16-6-31-6-40.htm m
With probe:
1:10 !!!
Input resistance 10 M
Before using an attenuator probe for measurement
of high-frequency signals or for fast-rising waveforms,
you must adjust (Q: How ?)
the probe compensating capacitor (C1)
14
Digital Scope. Block-chart
http://www.ti.com/solution/oscilloscope
Digital scope
is actually
a full-fledged computer
15
Arduino Oscilloscope (Poor man’ Oscilloscope)
http://www.instructables.com/id/Arduino-Oscilloscope-poor-mans-Oscilloscope/
All the scope is “ADC of the Arduino board.
Input signal is send to PC to show samples in the graphic form
Frequency range is SMALL !!! < 10 kHz
By using some internal tricks (interrupts, circular
buffer and low level functions)
It is possible to arrive to ~ 50000 Samples/sec
Girino – Fast Arduino Oscilloscope
http://www.instructables.com/id/Girino-FastArduino-Oscilloscope/
16
Arduino Oscilloscope with LCD
(With full and long code)
http://www.semifluid.com/2013/05/28/arduino-fio-lcd-oscilloscope/
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Control Questions
• What have I learned ?
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Literature to read
1. TBD
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