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Experiment
The Oscilloscope
The oscilloscope is used in various experiments in this course and a good understanding of its
operation is required. Besides observing the changes that occur in an electrical signal when various control
knobs are turned, measurements of peat-to-peak voltages, frequencies, and DC voltages will be made.
Diagram
The main component of the oscilloscope is the cathode ray tube (CRT) on whose screen the externally
applied electrical signal is detected. In its barest essentials the operation of the CRT can be understood as an
application of the basic principle governing the forces between charges.
A
B
C
D
E
F
Figure 1. A simplified diagram of the CRT.
The fundamental parts of the CRT diagrammed above:
A)
filament – source of electrons
B)
positively charged anode – accelerates the electrons
C)
focusing electrodes – focuses the beam of electrons
D)
vertically oriented plates used to move the beam of electrons back and forth
E)
horizontally oriented plates that deflect the beam of electrons vertically
F)
the CRT screen the beam of electrons hits
The type of oscilloscope we will be suing is called a dual trace oscilloscope. The oscilloscope allows
two signals to be applied to two separate vertical input terminals, and the two signals can then be observed
simultaneously on the screen of the CRT. We will be using only one of the inputs.
The following information describes the primary front panel features, the turn-on procedure, and the
signal application procedure for the oscilloscope.
8
12
7
2
3
4
9
5
10
11
6
10
9
8
12
7
1
Figure 2. Front panel diagram.
Primary front panel features:
1)
POWER
on-off switch
2)
INTEN
controls brightness of display
3)
FOCUS
controls sharpness and clarity of display
4)
X
controls horizontal position of display; when pulled outward, it expands the
display by a factor of 5
5)
TIME/DIV
controls sweep time
6)
VARIABLE
varies sweep speed; in CAL’D position, sweep speed is as selected
7)
Y
controls vertical position of display
8)
INPUT Y
vertical signal input
9)
VOLTS/DIV
selects vertical deflection sensitivity measured in volts per division
10)
VARIABLE
adjusts vertical deflection sensitivity between ranges; in CAL’D position,
vertical sensitivity is as selected
11)
VERT MODE selects which y-input is displayed
General procedure for turning on the oscilloscope
1)
Set INTEN control to low intensity (counterclockwise)
2)
Set VERT MODE to channel 1 or 2 (CH-1 or CH-2)
3)
Set horizontal (X) and vertical (Y) position controls to midrange
4)
Set horizontal and vertical VARIABLE controls to calibrated (CAL’D) position
5)
Set TIME/DIV control to one millisecond/cm (1 ms) position
6)
Set SOURCE to INT
7)
Be sure all other pushbutton switches are disengaged
8)
Set POWER switch to ON and allow a 30 second warm-up period
9)
Turn the intensity (INTEN) control clockwise until a trace is visible on the CRT
CAUTION:
A trace of very high intensity will burn the phosphors inside the CRT face.
To prevent such damage, always adjust the intensity knob (INTEN) as low as possible for
comfortable viewing.
10)
Set channel 1 (or 2) DC-AC switch to AC
General procedure for connecting a signal to the oscilloscope
1)
Set channel 1 (or 2) DC-AC switch to appropriate position for signal
2)
Set channel 1 (or 2) VOLTS/DIV control to 1 volt/div
3)
Connect signal to channel 1 (or 2) INPUT Y connector
4)
Position display on CRT using the horizontal (X) and vertical (Y) position controls
5)
Adjust TRIGGER control, if necessary, for a stable display
The oscilloscope is a versatile instrument from which quantitative measurements of voltage and time
can be made. Advantage is taken of this capability in order to measure the peak-to-peak voltage and
frequency for alternating current (AC) signals. In addition, the voltage across a dry cell (DC, direct current)
can be measured.
The vertical distance from a crest to a trough for a sinusoidal signal represents the peak-to-peak value
of the voltage. (Refer to Figure 3.) Count the number of vertical divisions on the CRT and multiply this value
by the value on the VOLTS/DIV setting. For example, if the number of division is 3.7 and the VOLTS/DIV
setting is 0.2, then the peak-to-peak voltage is (3.7 div) x (0.2 volts/div) = 0.74 volts. (Remember that the
calibration knob (CAL’D) must be turned all the way in the clockwise direction until it clicks.)
The frequency of the sinusoidal signal is found by first
determining the period of the oscillation, the calculating the
reciprocal (f = 1/T). The period is the time for one complete
oscillation and is represented on the oscilloscope by the
horizontal distance from a crest to an adjacent crest. (Refer to
Figure 3.) The value of the period is found by counting the
number of horizontal divisions from one crest to the next
adjacent crest, then multiplying this number by the TIME/DIV
setting. For, example, if the number of division is 6.3 and the
TIME/DIV setting is 0.5 ms/div, then the period is (6.3 div) x
(0.5 ms/div) = 3.15 ms = 3.15 x 10 -3 sec. The corresponding
frequency is 1/3.25 x 10-3 = 317 Hz. (Remember that the
calibration (CAL’D) knob must be turned clockwise until it
clicks.)
V
Peak to peak voltage
t
Period
Figure 3.
The other instrument used in the experiment is a variable frequency generator that is capable of
producing square, triangular, and sinusoidal waves of varying frequencies and amplitudes.
Apparatus:
Oscilloscope, signal generator, leads, dry cell
Procedure:
1)
Connect the leads of the oscilloscope to the signal generator – red to red and black to black.
Turn on the oscilloscope and the signal generator.
2)
Adjust the frequency knob on the signal generator until it is generating a sinusoidal signal of
1200 Hz frequency and arbitrary amplitude.
3)
Adjust the horizontal (X) and vertical (Y) position knobs until the pattern is centered on the
screen of the CRT.
4)
Change the setting of the VOLTS/DIV knob until the signal fills vertically as much of the
screen as possible.
5)
Change the setting of the TIME/DIV knob until two or three full cycles are seen of the CRT
6)
Record the number of divisions and the VOLTS/DIV setting for the peak-to-peak voltage.
7)
Record the number of divisions and the TIME/DIV setting for the period.
8)
Repeat steps (3) through (7) for two additional frequencies (7400 Hz and 12,400 Hz)
9)
Have someone from another group set the frequency to an unknown value (have them
COVER the sig. gen. display). Calculate the mystery frequency THEN read it off the sig. gen.
10)
Disconnect the oscilloscope leads from the signal generator and adjust the oscilloscope to
read DC voltages. Adjust the vertical position of the horizontal line seen on the CRT to a
convenient position. Set the VOLTS/DIV setting to 1 volt per division. Take the leads from
the oscilloscope and connect them to the dry cell. Notice that the horizontal line jumps to a
new position indicating that the voltage has changed. Record the number of divisions and
the VOLTS/DIV setting.
Results/Analysis
Show one calculation for Peak-to-Peak voltage and uncertainty:
Note:
V  reading * scale, so
V (reading)

V
reading
Frequency
(Hz)
Signal Generator Voltage Data and Results
Scale setting (volts/div)
Number of divisions
Peak-to-peak voltage (V)
1200
7400
12400
Dry Cell Data and Results
Scale setting (volts/div)
Number of divisions
Voltage (V)
Show one calculation for frequency and uncertainty:
Note:
T  reading * scale, and f 
Frequency
(Hz)
Frequency data and Results
Scale setting
(time/div)
1200
7400
12400
mystery
f T (reading)
1
, so


T
f
T
reading
Number of divisions
Period (s)
Calculated Frequency
(Hz)
Calculate the percentage error in your calculations of the frequency.
Actual frequency
Calculated frequency
Percentage error
1200
7400
12400
mystery What is the most important way to reduce the uncertainty in your measurements of either voltage or
frequency?
Comment on how the calculated frequencies and their uncertainties compare to the true frequencies.
Conclusion