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CHAPTER 4
OSCILLOSCOPES
4.1
Introduction
The cathode ray oscilloscope generally referred to as the oscilloscope, is probably the
most versatile electrical measuring instrument available. Some of electrical parameter can
be observed with the oscilloscope are:
1. dc and ac voltage
2. indirect measurement of ac and dc current,
3. time
4. phase relationships
5. frequency
6. a wide range of waveform evaluations such as rise time
Many nonelectrical physical quantities such pressure, strain, temperature and
acceleration can be measured by using a transducer to convert the physical parameter to
an equivalent voltage.
The oscilloscope consists of the following major subsystems:
1. Cathode ray tube or CRT
2. Vertical amplifier
3. Horizontal amplifier
4. Sweep generator
5. Trigger circuit
6. Associated power supplies
The heart of the instrument is cathode ray tube.
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4.2
The cathode ray tube
The cathode ray tube used in an oscilloscope is very similar to the picture tube in the
television set. Figure 4.1 shows a cross section of a CRT.
Figure 4.1: Cathode ray tube with the major components identified
The major components of a general purpose CRT are:
1. Evacuated glass envelope
2. Electron gun assembly
3. Deflected plate assembly
4. Accelerating anodes
5. Phosphor coated screen
Screen size diameters of approximately 1 to 25 in or larger. Most laboratory quality
oscilloscopes use a CRT that has a circular screen approximately 5in in diameter. All
electrical connections except the high voltage connection are made through the base of
the CRT. The glass envelope is evacuated to a fairly high vacuum to permit the electron
beam to traverse the tube easily.
The electron gun assembly consists of the triode section and the focus section which are
shown in Figure 4.1. The purpose of the electron gun assembly is to provide a source of
electrons, converge and focus them into a well defined beam, and accelerate them to
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toward the fluorescent screen. The electrons that make up the beam are given off by
thermionic emission from the heated cathode. The cathode is surrounded by a cylindrical
cap that is at a negative potential. This cap, which has a small hole located along the
longitudinal axis of the CRT, as shown in Figure 4.2, acts as the control grid. Because the
control grid is at a negative potential, electrons are repelled away from the cylinder walls
and, therefore, stream through the hole where they move into the electric fields of the
focusing anodes.
Figure 4.2: Triode section of the CRT
The focus lens consists of the first anode, focus ring and astigmatism aperture or second
anode. The purpose of this section is to converge and collimate the beam to obtain the
minimum size and best defined spot on the phosphor screen of the CRT.
The section of the CRT just beyond the electron gun assembly is the deflection.
Waveforms can be displayed on the CRT’s phosphor screen only if there is some means
of deflecting the electron beam both horizontally and vertically. This is the function of
the deflection plates. The deflection experienced by the electrons beam in the CRT of an
oscilloscope is called electrostatic deflection, which means that the electron beam is
deflected by the force exerted on each electron by the electric field. Electrostatic
deflection offers the advantages of higher frequency operation as well as space saving
inasmuch as the deflection plates are mounted inside the CRT. During the period of
accelerating in the electrons gun assembly, the electrons have gained kinetic energy as
they gain velocity.
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The next section of CRT is the postdeflection area. After electron pass beyond the
deflection plates, they may or not may experience not experienced additional
acceleration. This depends primarily on the maximum frequency to be applied to the
CRT. In general, if the maximum frequency to be displayed on the CRT is less than
10MH, no deflected accelerating is used. If generally necessary to increase the brightness
of the trace which otherwise may be quite dim. However, using postdeflection
acceleration generally requires that the length of the CRT be extended to achieve the
required beam deflection as a result of the CRT is extended to achieve the required beam
deflection as a result of the increased velocity.
An alternative to increased CRT length is to add a dome-shaped mesh to CRT just
beyond the deflection plates as shown in Figure 4.3. However, the mesh reduces trace
brightness and increases the size of spots unless the postdeflection accelerating voltage is
increased significantly.
Figure 4.3: CRT with dome shaped mesh to increase deflection
When the electron beam strikes the phosphor coated face of the CRT, a spot of light is
produced because the phosphor absorbs kinetic energy from electrons that strikes it and
gives up the energy in the form of light. This property if emitting light when stimulated
by electrons bombardment is called fluorescent; therefore, we can say that phosphor is
fluorescent material.
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Phosphor possesses a second desirable characteristic called phosphorescent, which mean
that the phosphor continues to emit light for a period of time after the source of excitation
is removed. The length of time phosphorescent continues is a measure of the persistence
of the fluorescent material.
Table 4.1 lists several different phosphors and some of their applications.
Phosphor
Trance
Trace
colour
Persistence
Application
P1
Yellow green
Medium
General purpose CRO
P2
Blue
Medium
Observation of low & medium speed
signals
P4
White
Medium short
Television picture tube
P7
Blue
Long
Observation of low & medium speed
signals
P11
Blue
Medium short
Photographic applications
P31
Green
Medium short
Observation of low & medium speed
signals. Most frequently used in general
purpose CRO
4.3
Basic oscilloscope controls that directly affected the beam
An intensity control as shown in Figure 4.4 is always connected to the control grid. The
potential of the control grid is negative with respect to the cathode. Therefore, as the
intensity control is adjusted, the number of electrons that pass through the small hole in
the control grid is affected, which in turn affects the brightness of the spot on the screen.
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Figure 4.4: Basic circuit of controls that determine position, intensity, and focus of the
electron beam on a CRT
The focus control is connected to the focusing anode. The focusing and accelerating
anodes form an electrostatic lens to collimate the electrons into a well defined beam.
Generally, a beam that is in sharp focus at the centre of the screen will be out of focus
near the edge of the screen because the lengths of the electron paths are different when
the beam is deflected. Adjusting the astigmatism control gives a sharp over the entire
screen.
The beam can be positioned anywhere on the screen by adjusting the controls marked
horizontal position and vertical position. When the horizontal and vertical position
controls are set to their midpoint position, the deflection voltages divide equally across
both halves of the potentiometers. There is therefore no deflection of the beam; it simply
travels along the axis of the CRT and strikes the centre of the screen. Adjusting the
horizontal and vertical position control deflect the beam to any desired position on the
screen.
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4.4
Vertical Amplifier
The vertical amplifier is the principal factor in determining the sensitivity and bandwidth
of an oscilloscope. The gain of the vertical amplifier determines the smallest signal that
the oscilloscope can be satisfactory reproduce on the CRT screen. The sensitivity of an
oscilloscope is directly proportional to gain of the vertical amplifier; that is, as gain
increases sensitivity increases, which allows us to observe smaller-amplitude signals. The
vertical sensitivity is a measure of how much the electron beam will be deflected for a
specified input signal.
Bandwidth of an oscilloscope determines the range of frequencies that can be accurately
reproduced on the CRT screen. The greater the bandwidth, the wider the range of
frequencies that can be observed with the instrument
4.5
Horizontal amplifier
The horizontal amplifier basically serves two purposes:
1. When the oscilloscope is being used in the ordinary mode of operation to display
a signal applied to the vertical input, the horizontal amplifier will amplify the
sweep generator output.
2. When the oscilloscope is being used in the X-Y mode, the signal applied to the
horizontal input terminal will be amplified by the horizontal amplifier.
4.6
Sweep generator
Oscilloscopes are used to display a waveform that varies as a function of time. If the
waveform is to be accurately reproduced, the beam must have a constant velocity. Since
the beam velocity is a function of the deflecting voltage must increase linearity with time.
A voltage with this characteristic is called a ramp voltage.
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Figure 4.5: Typical sawtooth waveform applied to the horizontal deflection plates.
If the voltage decreases rapidly to zero with waveform repeatedly reproduced, as shown
in Figure 4.5, the pattern is generally called a sawtooth waveform. During the sweep
time, Ts, the beam moves from left to right across the CRT screen. The beam is deflected
to the right by the increasing amplitude of the ramp voltage and the fact that the positive
voltage attracts the negative electrons. During retrace time, Tr, the beam returns quickly
to the left side of the screen. The control grid is generally “gated off”, which blanks out
the beam during retrace and prevents an undesirable retrace pattern from appearing on the
screen.
4.7
Attenuators
The voltage at input terminal of the vertical amplifier, which causes the beam to be
deflected off CRT screen, is quite low in amplitude. So that, high amplitude signals may
be displayed, an attenuator network is placed between the vertical input terminal and the
input of the vertical amplifier. The term “attenuate” means to reduce in size. The purpose
of the attenuator is to reduce the amplitude of the vertical input signal before applying it
to the vertical amplifier. The most basic attenuator is a simple resistive voltage divider
such as the one shown in Figure 4.6. With this circuit, the input voltage Vi, will be
attenuated by a factor of 10000 with the switch S set to position A as shown, in position
B, C, and D, the attenuation factors will be 1000, 100, and 10, respectively in switch
position E there is no attenuation.
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Figure 4.6: Resistive attenuator network
The attenuation in any switch position can be determined from the ratio of the total
resistance from the position of interest to ground to the total series resistance, written as
Vo
R
=
Vi
Rt
Where
R = total resistance from the desired attenuator terminal to ground
Rt = total series resistance
The attenuation factor is the reciprocal of the voltage divider ratio.
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4.8
Determining frequency with Lissajous Patterns
The oscilloscope can be used in X-Y mode to determine the frequency of a signal. The
frequency is determined by applying the signal of unknown frequency to either the X or
the Y input terminal and a signal of known frequency to other input terminal. The pattern
observed on the screen is called Lissajous Figure. The particular Lissajous pattern
observed depends on the ratio of the two frequencies.
f v number of horizontal tan gencies
=
fh
number of vertical tan gencies
Figure 4.7: Lissajous Patterns for integral frequencies
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Figure 4.8: Lissajous Pattern for Non-Integral frequencies
4.9
Phase angle computation
Oscilloscopes can also be used in X-Y mode to determine the phase angle between two
signals of the same frequency. The pattern displayed on the CRT screen may vary from
straight line with a positive slope, if the signals are in phase, to a straight line with a
negative slope for signals 1800 out of phase, as shown in Figure 4.8. If the phase angle is
any angle between 00 and 3600 besides 1800, a circle or ellipse, as shown in Figure 4.9,
will be displayed. The phase angle is easily determined from the ellipse. The ratio of the
Y axis intercept represented as Figure 4.9, and the maximum vertical deflection, Y2 is
equal to the sine of the phase angle, that is,
Sinθ =
Where
θ = phase angle in degrees
Y1 = Y axis intercept
Y2 = maximum vertical deflection
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Y1
Y2
Figure 4.9: Lissajous patterns for selected phase angles
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Figure 4.10: Evaluation of phase relationship
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