Oscilloscope

General Physics Experiment Handout
Oscilloscope
Purpose
The purpose of this exercise is to learn and familiarize the operation of an oscilloscope and a
function generator. The oscilloscope was used to measure the frequency, period, and amplitude of
signals, and observe the responses of the experimental system under varying conditions.
Principles
An oscilloscope is a type of electronic test instrument that is widely used to measure and analyze
signals. Changes in the voltage of constantly varying signals (especially high-frequency signals) can be
plotted on the oscilloscope to present the potential difference over time, thus enabling relevant
experimental results to be analyzed efficiently. Classified according to functionalities, an oscilloscope
comprises the following essential parts:
A.
The Cathode Ray Tube
As shown in the figure above, the cathode ray tube (CRT) is the most essential part of an
oscilloscope. A CRT comprise of an electron gun, deflection plates, and a screen. A description of
CRT components is listed as follows:
1. The electron gun
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The surface of the cathode in an electron gun is usually coated with materials that can easily
emit a large amount of electrons, such as Barium (Ba) or Strontium (Sr). When the filament
of a cathode is heated, it emits large quantities of electrons.
The control grid (1st grid) is a hollow and cylindrical conductor with a small opening at the
front end of the cathode. The control grid is charged negatively in relation to the cathode (0
to -20 V), and a greater potential difference allows fewer electrons to pass. This potential
difference between the control grid and the cathode is used to control the intensity of an
electron beam (screen brightness).
The preaccelerating anode (2nd grid) has an electrical potential much higher than that of the
cathode. The potential difference accelerates the electrons passing through the control grid.
The focusing anode (1st anode) concentrates divergent electrons leaving the preaccelerating
anode into a linear beam. This anode functions as an electron lens.
The accelerating anode (2nd anode) again accelerates the electrons leaving the focusing
anode, thereby achieving a greater axial velocity.
2.
Deflection plates
Vertical and horizontal deflection plates are installed inside a CRT to deflect the
electrons in the vertical (on the x-axis) or horizontal (on the y-axis) direction. The
degree of reflection is directly proportional to the potential differences of the deflection
electrodes of both directions.
The inside of a CRT screen is coated with an organic material that glows, which signifies the
location illuminated by the electron beam.
B.
Vertical and horizontal amplifiers
Signal voltages are amplified and transmitted to the deflection
plates by vertical and horizontal amplifiers. Amplified signals are
thus sufficient to be deflected to the screen with a viewable range.
C.
Time-base generator
Bright spots are produced by the electron beam that impacts the
phosphor coating on the back-end of a screen. The spots can be
deflected by applying external voltages to the two sets of deflection
plates in the CRT. Furthermore, the spot sweeps horizontally across
the screen and forms a bright line (the upper-right figure) when a
sawtooth wave (the lower-right figure) is applied to the deflection
plates. When the sweep frequency exceeds 20 Hz, human eyes can
see a continuous horizontal line connected by bright spots. This is the
result of the persistence of vision, and the horizontal line is known as
the time base. In addition to the horizontally deflecting sawtooth
signal, the desired signal voltage is introduced into the vertical deflection plates to display the
waveform of the signal as a function of time on the screen. As shown in the figure below, changes
in time-base sweep time produce an appropriate waveform on the oscilloscope screen.
D.
Trigger circuit
A trigger circuit is used to examine the test signal and determine a specific phase to
synchronize the test signal with the sawtooth wave generated by the time-base generator.
Consequently, waveforms produced by rapid sweeps show a repetitive pattern, and the
oscilloscope then displays a stable waveform. As shown in Figure 2, when the sweep frequency is
greater than the test frequency, the oscilloscope displays a waveform moving to the right;
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conversely, when the sweep frequency is smaller than the test frequency, the oscilloscope displays
a waveform moving to the left.
Figure 1. A diagram of vertical and horizontal signal direction in an oscilloscope
Figure 2. When the scan voltage and signal wave are not synchronized, the presented waveform
cannot be repeated
Experimental instruments
1.
2.
3.
4.
5.
Oscilloscope
Function generator
Power supply
BNC-BNC and BNC-alligator clip signal cables
Grid paper
Notes
※
Procedures
This experiment involves operating a function generator to generate periodic signals such as sine
waves, sawtooth waves, and square waves. In addition, the amplitude (Vp-p) of each signal must be
set to correspond to the desired characteristics by using the knobs of the generator. However, because
they are generated by the function generator, signals and waveforms as functions of time cannot be
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observed and verified without using an oscilloscope. Thus, an oscilloscope can be regarded as a type
of test equipment that measures periodic signals.
A.
Instruments
Oscilloscope
1. Determine the model name of the oscilloscope.
Two models of oscilloscopes are used in the lab, 622G and V-222.
The control knob functions of both models are listed in the appendices.
2. Moving bright spots or bright lines caused by the electron beam should be displayed on the
screen after the oscilloscope is switched on.
To synchronize the sweeps of the electron beam, first set the triggering source to the internal
signal. For 622G, adjust Knob 28; for V-222, adjust Switches 31 and 32.
Set the initial horizontal sweep speed of the electron beam to 1 ms/DIV; it takes 1 ms for the
beam to sweep across a division of the screen.
Set the MODE switch to the CH 1 position. This designates the displayed signal as that
transmitted into the CH 1 BNC connector.
Depress the GND push switch near the CH 1 connector (622G), or set the switch to the GND
position (V-222).
The input signal has a potential difference of 0 V when connected to the ground. The
oscilloscope should display a straight line.
If the screen is blank, the position or brightness of the trace may require adjustment by turning
the knobs to a desired position.
3. Ensure that vertical and horizontal switches are properly positioned
After a straight line appears on the screen, the preliminary setup of the oscilloscope should be
performed to display the waveforms correctly.
A reference signal is usually provided in every oscilloscope for users to verify the
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correctness of signals. The output connector for the reference signal, a small metal
terminal with an extremely small opening in the middle, is generally located below the
screen. The reference signals for both models used in this lab are Vp-p = 5 V and 1 kHz.
A BNC-alligator clip signal cable is used to connect the red alligator clip to the reference
signal output connector, and the BNC end should be connected to the CH 1 connector.
The black alligator clip is linked to the shell of the BNC connector. When the black clip
is connected to the CH 1 connector, it connects to the ground through the case of the
oscilloscope. Therefore, using the black alligator clip is not necessary, or it can be
connected to the ground connection on the oscilloscope. The red alligator clip is
connected to the pin inside the BNC connector.
In Procedure 2, we set the CH1 input as the GND voltage, which should now be switched
to the DC or AC position to filter the input signal. The AC position of the control
functions as a filter to remove DC voltages from the input signal and produce only AC
signals that vary over time. The waveform displayed on the screen contains both DC and
AC signals when the control is switched to the DC position.
Set the triggering source signal to CH 1. This allows the characteristics of the CH 1 signal
to be used as the triggering source.
Count the number of divisions occupied by the waveform in the horizontal and vertical
directions. Verify the Vp-p and frequency of the waveform according to the number of
divisions and settings of s/DIC and V/DIV along both directions.
If the observed waveform is not correct, check whether the switches or knobs regarding
CAL are positioned properly.
If the waveform appears unstable, check the settings of the triggering source.
Function generator
1. Transmit the output signal of the function generator to the oscilloscope by using a BNC-BNC
cable. The function generator outputs signals through the BNC connector labeled OUTPUT (in
A in the figure below).
Each group can choose either CH 1 or CH 2 for the oscilloscope input.
2. Setup of the function generator output signal
Amplitude (AMPL): (B in the figure below)
Adjustment should be performed beginning from small-value voltages. Turning the knob
completely counterclockwise sets the output voltage to zero.
Ensure that the knob is not pulled out. In normal circumstances, this knob should adhere to
the front panel. The knob is only pulled out for voltage adjustment.
Waveform: (C in the figure below)
A selection of push switches is provided for the desired output waveform.
Frequency:
Press the proper switch for the desired frequency range (C in the figure below), and make
adjustment using the DUTY knob (D in the figure below).
In addition, ensure that the DUTY switch is positioned properly.
The above adjustments do not need to be performed in order.
Refer to the appendix at the end of the handout.
D
C
E
A
B
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B.
AC signal measurement
After understanding the operation of the oscilloscope and the function generator, set up the
function generator to output the following signals to the oscilloscope. Perform relevant adjustments
for the oscilloscope to show a sharp and stable trace, and record the trace (note: specify the units of
measurement for the x- and y-axes).
1.
2.
3.
4.
Sine wave (500 Hz, Vp-p = 10 V)
Sawtooth wave (1 kHz, Vp-p = 2 V)
Square wave (5 kHz, Vp-p = 0.3 V)
Square wave (500 Hz, Vp-p = 0.1 V)
(p-p: peak to peak)
※ Verify the measurement according to the relationship between the frequency and the period.
※ Record the oscilloscope settings including VOLTS/DIV, TIME/DIV, or other crucial
selections by the side of the recorded signal waveforms (the settings should be recorded in
detail for future reference when conducting experiments requiring an oscilloscope).
C.
DC voltage measurement
In this measurement, the oscilloscope functions as a high-impedance DC voltmeter.
1. Contact the two input test leads (red and black) of the oscilloscope with each other to determine
the reference voltage.
2. Connect the test leads to a 1.5-V direct current and record the trace on the oscilloscope.
※ Record the voltages measured by the oscilloscope and compare whether this voltage is similar
to that supplied by the 1.5-V current.
Discussion questions
1.
Research and understand the mechanisms of a synchronized trigger circuit. How could a
malfunctioned device used for synchronized triggering affect the trace of the oscilloscope?
2.
Does the waveform always move toward the right of the oscilloscope screen when the internal
sweep frequency is greater than the test frequency? Discuss and explain.
3.
How does INT (internal triggering) differ from EXT (external triggering)? What are their
functions?
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Appendix A
Preliminary setup and operation methods of oscilloscope Model 622G
Before the experiment, all switches and knobs might be positioned randomly and cause the oscilloscope
to display incorrect waveforms. This problem can be avoided by examining each setting and performing
the following procedures.
When the oscilloscope is off, perform the following adjustments:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
Rotate Switches 40 and 37 (vertical POSITION control) to the mid-position.
Rotate Switch 34 (horizontal POSITION control) to the mid-position.
Rotate Switch 2 (INTENSITY control) to the mid-position.
Rotate Switch 4 (FOCUS control) to the mid-position.
Rotate Switches 10 and 14 (VOLTS/DIV select switches) to the 1V/DIV position.
Depress Switches 11 and 15 (input coupling switches) to the DC position.
Rotate Switches 13 and 17 to the CAL position.
Depress Switch 28 (trigger MODE select switches) to the AUTO position.
Rotate Switch 30 (trigger LEVEL control) to the + position.
Flip Switch 26 (SOURCE select switch) to the CH 1 position.
Rotate Switch 18 (TIME/DIV select switch) to the 0.5 ms/DIV position.
Ensure that Switch 19 is released (not in the SWP.UNCAL position).
Flip Switch 39 (MODE select switch) to the CH1 position.
Depress Switch 9 (POWER) to the ON position.
Rotate Switch 2 (INTENSITY) to adjust the trace to an appropriate intensity.
If the horizontal line does not appear, try adjusting Switch 34 (horizontal POSITION) and
Switches 40 and 37 (vertical POSITION).
Rotate Switch 4 (FOCUS) to make the horizontal line sharp and clear. Switch 2 (INTENSITY)
might also need to be adjusted.
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Appendix B
Preliminary setup and operation methods of oscilloscope Model V-222.
When the oscilloscope is off, perform the following adjustments:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
Rotate Switches 19 and 20 (vertical POSITION control) to the mid-position.
Rotate Switch 29 (horizontal POSITION control) to the mid-position.
Rotate Switch 6 (INTENSITY control) to the mid-position.
Rotate Switch 3 (FOCUS control) to the mid-position.
Rotate Switches 13 and 14 (VOLTS/DIV select switches) to the 1 V/DIV position.
Depress Switches 11 and 12 (input coupling switches) to the DC position.
Rotate Switches 15 and 16 to the CAL position.
Depress Switch 35 (trigger MODE select switches) to the AUTO position.
Rotate Switch 34 (trigger LEVEL control) to the + position.
Flip Switch 32 (SOURCE select switch) to the CH 1 position.
Flip Switch 31 to the INT position.
Rotate Switch 26 (TIME/DIV select switch) to the 0.5 ms/DIV position.
Rotate Switch 17 to the CAL position.
Flip Switch 21 (MODE select switch) to the CH1 position.
Depress Switch 1 (POWER) to the ON position.
Rotate Switch 2 (INTENSITY) to adjust the trace to an appropriate intensity.
If the horizontal line does not appear, try adjusting Switch 29 (horizontal POSITION) and
Switches 19 and 20 (vertical POSITION).
Rotate Switch 4 (FOCUS) to make the horizontal line sharp and clear. Switch 2 (INTENSITY)
might also need to be adjusted.
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Appendix C
Front panel of the function generator GFG-8015G
1.
PWR (power switch):
The power switch applies power to the function generator.
2. POWER ON (power on indicator):
An LED is used to indicate whether the power is on or off.
3. RANGE (range control):
This control, together with the multiplier, determines the frequency of the output signal, which is
given by the product of the range (e.g., 1 KHz) and the value of the multiplier. Decades of
frequency are provided by the buttons, and each button is interlocked. Pressing one button releases
all others.
4. FUNCTION (function switch):
Three interlocking buttons allows users to select the desired output waveforms. Pressing one button
releases the previously-pressed button. To satisfy most applications, square, triangular, and sine
waves are provided.
5. MULTIPLIER:
From 0.2 to 2.0, the multiplier allows frequency settings between fixed ranges. When the RANGE is
set at 100 KHz, the output signal frequency ranges between 200 Hz and 200 KHz.
6. DUTY (time symmetry knob):
The DUTY knob controls the time symmetry of high and low voltages for output waveforms in each
period. When this knob is set to the CAL position, the time symmetry of output waveforms is 50/50.
By using the DUTY knob, the period of half of the waveform can be changed when the other half
remains fixed to alter the time symmetry with various RANGE and MULTIPLIER settings. This
unique feature produces ramp waveforms, variable pulse widths, variable duty cycle pulses, and
skewed sine waves.
7. DUTY INV (ramp/pulse invert):
This button inverts the time symmetry set by the DUTY control.
8. DC OFFSET:
When the knob is pulled, the DC OFFSET knob provides DC offset control that allows the DC level
of the output waveforms to be set at a desired value.
9. AMPL/-20dB (amplitude control):
This knob adjusts the amplitude (voltage) of the output signals. When the knob is pulled, an
attenuation of 20 dB is given to the output.
10. ATT (attenuation control):
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When this control is pushed, in addition to the 20 dB provided by the amplitude control, a
maximum of 40 dB attenuation is provided at the output.
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