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Experiment 24The Oscilloscope
Advanced Readi ng:(Serway & Jewett 8
th Edition) Chapter
15, section 15-2. Chapter 33, sections33-1 and 33-2. You need to
understandwhat is meant by wavelength, frequency,
period and root mean square (rms).
Equipment:1 oscilloscope1 BNC cable1 sound level meter1
phono-BNC adapter1BNC to banana plug adapter2 tuning forks of
different
frequencies1 BK oscillator1 Oscilloscope video
1 Kelvin 200 DMM
Objective:
The object of this experiment is to learn howto use the
oscilloscope by measuring the periods and amplitudes of
various waveformsas well as analyzing a sound wave.
Theory:
The oscilloscope is an electronic instrumentwidely used in
making electronic
measurements. The most noteworthy attributeof an (ideal)
oscilloscope is that it does notaffect the quantity being
measured. The maincomponent of the scope is the
cathode-raytube, or CRT. The CRT consists of a vacuumtube in which
electrons are "boiled off" acathode and accelerated, using an
electric field,towards a phosphorescent screen. When theelectrons
strike the screen, a burst of light isgiven off. The beam is
deflected along the way by vertical and horizontal plates that
useelectric fields to deflect the electrons.
The screen of the oscilloscope has a grid on itcalled a
gradecule. The gradecule is used toread information from the screen
of theoscilloscope. The dials on the oscilloscopegive the scale of
the gradecule inVOLTS/DIVISION in the vertical directionand
SECONDS/DIVISION in the horizontaldirection (see figure 24-2).
At the beginning of this lab, a video on theoperation of the
oscilloscope may be shown.This video explains some of the theory of
theoscilloscopes operation.
Procedure:Part 1: Learning the oscilloscope controlswitches
1. The oscilloscope should be connected to anoscillator by a BNC
cable. Turn on theoscillator and the oscilloscope. You should seea
sine wave on the screen.
figure 24-1
V O L T S / D I V
SEC/DIV
Vmax Peak-to-Peak
Figure 24-2
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2. In the following section, manipulate thevarious controls on
the scopes and answer thequestions.
(a) What happens when you turn theINTENSITY cont rol?
(b) What happens when you turn the FOCUScontrol?
(c) What do the VERTICAL andHORIZONTAL POSITION controls do?
(d) What happens when you change theSEC/DIV control?
(e) What happens when you change theVOLT/DIV control?
(f) What happens when the AC-GND-DC
switch is switched to the center position?
(g) What happens when you move the triggerlevel control?
(h) What happens when the slope switch is pushed in?
(i) Why are there two VOLT/DIV controls?
Part 2: Measureme nt of frequency
3. Measure the period of the sine wave signal
from the oscillator and calculate the frequencyusing the
relationship period=1/frequency.
4. Disconnect the BNC cable from theoscillator and connect it to
the sound levelmeter. Turn on the sound level meter byrotating the
dial on the front to the 70 or 80dB position. The response switch
should beon "slow," and the weighting switch on "A".
5. Strike one of the tuning forks with therubber mallet, and
hold it in front of the soundlevel meter. Adjust the trace on the
screen
until at least one whole period appears on thescreen and the
size of the wave form on thescreen is as large as it can be.
6. Measure the period of the tuning fork andcalculate the
frequency. Calculate the percen tdifference between the frequency
printed onthe side of the tuning fork and the
experimental value. Repeat for the othertuning fork.
Part 3: Measureme nt of AC voltages.
7. Reconnect the oscillator to the
oscilloscope. Adjust the frequency of thesignal from the
oscillator so that it is close to60 hertz. Measure the peak-to-peak
voltageof a sine wave from the oscillator using theoscilloscope
.
8. Calculate the RMS voltage of the sine wavefrom the scope. The
RMS voltage is the root-mean-square of the voltage and is given
by:
V rms
=
V max( )
2
2=.707V
max
where Vmax is the amplitude of the wave or
half of the peak-to-peak voltage. Measure thevoltage of the
signal with the DMM.Remember that you are now measuring thevoltage
of a sine wave, so you need to use ACvolts instead of DC volts.
Calculate the percen t difference between the two.
9. Adjust the amplitude of the signal andrepeat steps 7 and
8.
10. Each person needs to know how tooperate the oscilloscope, so
test each other bychanging the wave forms and reading
the peak-to-peak vol tages, amplitudes, andfrequencies.
Part 4: The t est
11. On the front table, your lab instructor hasan oscilloscope
and oscillator. Find the signaland record the peak-to-peak voltage
and thefrequency. Have your lab instructor verifyyour answer before
leaving lab.
Questions/Conclusions:
1. Why should you maximize thevolts/division scale on the
gradecule whendetermining the amplitude of the waveform?
2. What effect does changing thevolts/division scale from 1 volt
per division to5 volts per division have on the incoming
