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Cathode Ray Oscilloscope(CRO)
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Cathode Ray Oscilloscope(CRO)
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Very useful and versatile laboratory instrument (diagnostic
lab equipment).
Used for display, measurements and analysis of waveforms
(signals) in electrical and electronic circuits at frequencies
ranging from dc to GHz.
Basic instrument for the study of all types of signals.
Very fast in displaying/plotting (X-Y plotters).
Capable of generating a graph of an input signal versus time
(voltage-time mode) or a second variable (X-Y mode).
Can be employed to measure quantities such as peak
voltage, frequency, period, phase difference, pulse width etc.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Parts of a CRO:
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1) Cathode Ray Tube (CRT) and its associated controls.
An electron gunI.
II. Deflection plates
III. Phosphorescent screen
2) Input circuitry (a time-base generator, vertical and
horizontal amplifiers).
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Simple CRO construction:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
CRO construction:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Block diagram of a basic CRO :
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Operation: Vertical Deflection Plates(VDP)
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Electron beam generated from electron gun will be
accelerated towards phosphorous screen.
Beam passes through a set of parallel plates (vertical
deflection plates) oriented horizontally.
Voltage to be displayed is amplified by a vertical amplifier
and produce an electric field which deflects the path of the
electron vertically.
The polarity of the signal determines whether the
deflections will be up or down.
Magnitude of the signal determines the vertical
displacement of the electron.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Operation: Horizontal Deflection Plates(HDP)
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After passing through vertical deflection plates, the beam
passes through a set of similar plates (horizontal
deflection plates) which are oriented vertically.
The potential difference applied to these plates produce an
electric field which deflects left or right.
Under normal configuration these HDP are connected to a
time-base circuitry.
This circuit can control how fast the electron beam
sweeps from the left to right
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Operation:
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If two deflection voltages were held constant, the electron beam would
strike a point on the phosphorescent film and a stationary point would
visible on the screen.
Most voltages of interested are time-varying and so the voltage applied
to the HDP is varied with time in such a way that the spot moves from
left to right on the screen as time passes.
The phosphorescent material has the property of emitting light for
several milliseconds after the electrons have passed, the total effect is
for the electrons to leave behind a visible trail –a time-varying signal.
The horizontal deflection voltage is also varied in such a way that
when the beam reaches the right-hand edge of the screen, it starts over
at the left-right side.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Displaying a signal on the screen:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Displaying a signal on the screen:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
CRO in X-Y Mode:
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Beside showing a time-varying signal, the oscilloscope
lets you display an input signal rather than the time base
on the horizontal axis. This is called X-Y mode as both
the X and Y axis are tracing input voltage.
The waveform resulting from XY arrangement of two
periodic signals of different periods is called Lissajous
Pattern.
From the shape of Lissajous pattern we can determine
relative phases of the signals as well as frequency ratio.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
CRO controls:
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1. Y-Gain: Amplifies Y-deflection (small voltages are
amplified by built-in amplifiers before applying Y-plate)
Y-Gain=0.5V/Div
2. Time-Base: A saw-tooth wave is applied internally
across X-plates.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
CRO control: 2.Time-Base
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Controls the speed at which the spot sweeps across the
screen horizontally from left to right.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
CRO control: 2.Time-Base
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
CRO control: 2.Time-Base
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Helps to display the actual waveform applied across Y-
plates.
Calibrated in s/cm or ms/cm or µs/cm.
Gives the time required for the spot to sweep 1cm
horizontally across the screen.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Most Common Oscilloscope Controls:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Measurement of frequency:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Measurement of phase difference:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Lissajous Figures:
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Can be displayed by applying two a.c signals
simultaneously to X-plates and Y-plates of a CRO.
As the frequency, amplitude and phase difference are
altered, different patterns are seen on the screen of the
CRO.
Obtained when operated in X-Y mode.
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.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Measurement of phase difference using Lissajous Pattern:
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Obtained by applying two sinusoidal signals to
be compared at the vertical and horizontal plates.
The phase difference then measured from
resulting Lissajous pattern.
The pattern may be either a straight line or a
circle or an ellipse depending on the amount of
phase difference.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Measurement of phase difference using Lissajous Pattern:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Measurement of phase difference using Lissajous Pattern:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Measurement of phase difference using Lissajous Pattern:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Measurement of frequency using Lissajous Pattern:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Measurement of frequency using Lissajous Pattern:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Measurement of frequency using Lissajous Pattern:
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Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Storage type oscilloscope:
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Used for presentation of very slowly swept signals.
Used in mechanical and biomedical fields.
In conventional CRO the persistence of the screen is
smaller than the rate at which the signal sweeps across the
screen (the display will disappear within µs to seconds).
The persistence of the screen can be adjusted to match
with the sweep time. So the display stays for few seconds
or even hours.
A special CRT called storage tube is used which is same
as conventional CRT but with additional number of
special electrodes.
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Storage type oscilloscope:
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1) Two special guns called flood guns
2) Collimation electrodes
3) Collector mesh
4) Storage mesh or storage target
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Example1:
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Horizontal frequency=50Hz
Vertical frequency=?
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
Example2:
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Phase difference=?
Mr.P.Krishna, EEE Department, IIITN 19-Feb-20
