1. Topics to be Discussed
• CATHODE RAY OSCILLOSCOPE (CRO).
(CRO)
• Cathode Ray Tube (CRT).
• How a CRO Displays Waveforms.
• Time-base Voltage.
• Block Diagram of a CRO.
• APPLICATIONS OF CRO.
– Measurement of Frequency.
• Lissajous patterns
• Measurement of Phase Difference.
Difference
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2. CATHODE RAY OSCILLOSCOPE (CRO)
• The cathode-ray oscilloscope (CRO) is a common
laboratory instrument that provides accurate time and
amplitude measurements of voltage signals over a wide
range of frequencies.
• Its reliability, stability, and ease of operation make it suitable
as a general purpose laboratory instrument.
• The most important and versatile electronic instrument
(used in Testing).
• It is a very fast x - y plotter and is used to display voltage
waveforms.
• The ‘stylus’ of this ‘plotter’ is a luminous spot which moves
on the screen in response to the input voltages.
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3. • An electronics engineer can “see” what is
happening in different parts of a circuit.
• The heart of the CRO is a cathode-ray tube
shown schematically in Fig. 1.
• The rest is the electronic circuitry required to
operate the CRT.
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4. Cathode Ray Tube (CRT)
• It consists of :
1. Electron Gun: Produces a sharply
focused beam of electrons.
2. Deflection System: Deflects the
electron beam, both in horizontal and
vertical directions.
3. Fluorescent Screen: Produces a bright
spot when the electron beam strikes it.
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6.  The cathode ray is a beam of electrons which are emitted by the
heated cathode (negative electrode) and accelerated toward the
fluorescent screen.
 The assembly of the cathode, intensity grid, focus grid, and
accelerating anode (positive electrode) is called an electron gun.
 Its purpose is to generate the electron beam and control its
intensity and focus between the electron gun and the fluorescent
screen are two pair of metal plates - one oriented to provide
horizontal deflection of the beam and one pair oriented to give
vertical deflection to the beam.
 These plates are thus referred to as the horizontal and vertical
deflection plates. The combination of these two deflections allows
the beam to reach any portion of the fluorescent screen.
 Wherever the electron beam hits the screen, the phosphor is
excited and light is emitted from that point. This conversion of
electron energy into light allows us to write with points or lines of
light on an otherwise darkened screen.
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7.  In the most common use of the oscilloscope the
signal to be studied is first amplified and then applied
to the vertical (deflection) plates to deflect the beam
vertically and at the same time a voltage that
increases linearly with time is applied to the horizontal
(deflection) plates thus causing the beam to be
deflected horizontally at a uniform (constant) rate.
 The signal applied to the vertical plates is thus
displayed on the screen as a function of time. The
horizontal axis serves as a uniform time scale.
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8. CRO Operation
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9.  A simplified block diagram of a typical oscilloscope is shown
in Fig. 3. In general, the instrument is operated in the following
manner.
 The signal to be displayed is amplified by the vertical
amplifier and applied to the vertical deflection plates of the
CRT. A portion of the signal in the vertical amplifier is applied
to the sweep trigger as a triggering signal.
 The sweep trigger then generates a pulse coincident with a
selected point in the cycle of the triggering signal. This pulse
turns on the sweep generator, initiating the sawtooth wave
form.
 The sawtooth wave is amplified by the horizontal amplifier
and applied to the horizontal deflection plates. Usually,
additional provisions signal are made for applying an external
triggering signal or utilizing the 60 Hz line for triggering. Also
the sweep generator may be bypassed and an external signal
applied directly to the horizontal amplifier.
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10. How a CRO Displays Waveforms?
The bright spot on the screen moves to the left
or to the right when a dc voltage is applied to
the horizontal defection plates.
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11. A solid line trace is obtained on the
screens when we apply ac voltages to
the horizontal deflection plates.
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12. Display of sine
wave on CRO
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13. Deflection Sensitivity
• It is defined as the vertical deflection of the
spot on the screen per unit deflecting
voltage applied.
• It is designated as S and is measured in
m/V.
• Deflection factor G, defined as the
reciprocal of the deflection sensitivity S.
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14. Time-base Voltage
Ideal time- Actual time- Distorted
base voltage base voltage display 14
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15. Block Diagram of a CRO
1. The CRT,
2. The vertical deflection system,
3. The horizontal defection system,
and
4. The high-voltage power supply and
the low-voltage power supply.
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18. APPLICATIONS OF CRO
• (1) Study of Waveforms :
– Dual Trace CRO.
– Dual Beam CRO.
– Storage CRO.
• (2) Measurement of Voltages.
• (3) Measurement of Current.
• (4) Measurement of Frequency.
• (5) Measurement of Phase Difference.
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19. Measurement of Frequency
• Example: A sinusoidal voltage is
displayed on a CRO. For obtaining a
convenient size of the display as shown in
figure, the vertical amplifier/attenuator is
set at 2 V/cm, and the time-base control at
0.5 ms/cm. Determine the rms value and
the frequency of the sine-wave voltage.
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21. Vp = peak value of the display × deflection
sensitivity = 3.5 cm × 2 V/cm =
7.0 V 7.0
∴ rms value = = 4.95 V
2
T = one cycle length on x-axis
× time-base setting
= 4 cm × 0.5 ms/cm = 2.0 ms
1 1
∴ f = = = 0.5 kHz
T 2 ms
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22. Lissajous patterns
• A Lissajous pattern is produced on the
screen of the CRO when two different sine-
wave voltages are simultaneously applied to
the two sets of deflection plates.
• The type of pattern produced depends on
(i) the ratio of the frequencies, and
(ii) the relative phase of the two sine-wave
voltages.
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25. Measurement of Phase Difference
http://boson.physics.sc.edu/~hoskins/Demos/CathodeRay.html
Y1
sin θ =
Y2
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