LPC Physics 2
Introduction to the Oscilloscope
©
2003 Las Positas College, Physics Department Staff
Introduction to the Oscilloscope
Purpose:
In this lab you will become familiar with the oscilloscope (CRO) as a measuring and
detecting device.
Equipment:
ƒ
ƒ
ƒ
ƒ
Oscilloscope
Signal Generator
Dry Cell (battery), Battery Holder
Patch Cords, BNC Cords, Alligator Clips
Theory:
The primary component of the oscilloscope is the cathode ray tube (CRT) on whose
screen the externally applied electrical signal is detected. The minimal application of the
CRT can be understood as an application of the basic physical principle that similar
electrical charges repel each other, while unlike charges attract.
Fig. 1. A simplified diagram of the CRT
The fundamental parts of the CRT diagrammed above are:
A) A filament that is heated to incandescence by passing a current through it so that
negatively charged electrons boil from it.
B) An anode, positively charged, which exerts a large accelerating force on the electrons
so that they move to the right along the horizontal axis of the tube. The resulting
beam that passes through the central opening in the anode moves along the axis of the
CRT.
C) A focusing electrode that focuses the beam into a narrow beam by the application of a
proper positive or negative voltage.
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LPC Physics 2
Introduction to the Oscilloscope
©
2003 Las Positas College, Physics Department Staff
D) A pair of vertical plates that the internal circuitry of the oscilloscope alternately
charges and discharges with charges of opposite sign so that the electron beam is
swept horizontally to the left and right of its axial path. The rate that the beam
sweeps back and forth is called the sweep speed, (s.s). This is controlled by the
“Time/div” or knob.
E) A pair of horizontal plates by means of which the vertical motion of the beam can be
controlled. This deflection is adjusted with the “volts/cm” knob. There are two
channels on the CRO, each with its own independent vertical deflection.
F) A fluorescent screen that shows where the beam of electrons is hitting the screen.
CRT Display
1.5
1
0.5
0
Series1
0
5
10
15
20
25
-0.5
-1
-1.5
Time
The Oscilloscope and Theory
The period, T, of the signal is given by the product of the horizontal sweep speed, (s.s),
and the distance x, that is, T=(s.s)(x).
The voltage of the dry cell can be found by taking the product of the vertical deflection in
cm, y, and the amplification in volts/cm, a. You thus obtain V = ay and δV = aδy + yδa.
The accuracy of the vertical deflection on the oscilloscope is +3%, and the horizontal
sweep is +4%; for instance:
δ(s.s) = (.04)(s.s).
To find the frequency of a sinusoidal signal, you can use the relation:
f =
1
T
The corresponding uncertainty in the frequency is given by:
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Eq. 1
LPC Physics 2
Introduction to the Oscilloscope
©
2003 Las Positas College, Physics Department Staff
δf =
δT
T
Eq. 2
where δT = (s.s.)(δx) + (δs.s.)(x).
Vertical lines
mark time
divisions.
on/off
intensity
Horizontal lines
mark voltage
divisions
Like the up/down
This knob tells the ‘scope what to trigger on.
knobs, this moves the
Make sure it’s set to a channel with a signal
signal to the right or
going into it.
left
Horizontal Sweep Speed
This changes the time per division
on the horizontal axis.
Leave this knob
turned all the way to
the right
focus
These switches allow you to
ground the signals. Very
useful in calculating both DC
and AC voltages
Volts / division (cm)
Changes vertical scale of
signal
It is very important that the
central portion of these
Inputs Ch1/Ch2
knobs be turned all the
The signal(s) from the way to the left until they
function generator or
“click”
circuit go in here.
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up/down knobs
change vertical
position of signal.
Turning the knobs all
the way to the left
turns the channel
“off”.
LPC Physics 2
Introduction to the Oscilloscope
©
2003 Las Positas College, Physics Department Staff
Experiment:
1. Connect the leads of the oscilloscope to the signal generator which is generating a
sinusoidal signal of arbitrary amplitude and 1 kHz frequency. Adjust the horizontal
sweep speed on the oscilloscope until two full cycles fill the screen of the CRT.
2. When the oscilloscope is adjusted, do the following:
•
•
•
Record the peak-to-peak voltage Vp-p and the uncertainty δVp-p .
Record the distance, x, in centimeters from one crest to an adjacent crest and the
corresponding uncertainty, δx.
Record the horizontal sweep speed (s.s) of the oscilloscope. Repeat Step 1 for a
sinusoidal signal of 5200 Hz.
3. Disconnect the oscilloscope leads from the signal generator and connect them to a dry
cell. Adjust the oscilloscope to read DC voltages and determine the voltage, V, of the
dry cell and its uncertainty, δV.
NOTE: When measurements are made with any measuring device, there is an
uncertainty in the reading of the measurements and also an inherent uncertainty in the
measuring device itself. These uncertainties should be noted and included as
uncertainties in the final values.
Analysis:
1. For each sinusoidal signal, indicate the peak-to-peak voltage, Vp-p, and its uncertainty,
δVp-p, and calculate the frequency, f, of each signal and its uncertainty, δf. Do the
signal generator settings and the oscilloscope readings agree? If not, why not. Can
this problem be corrected?
2. Indicate the voltage, V, of the dry cell and its uncertainty δV. Do the values printed on
the dry cell and the oscilloscope readings agree?
Results:
Write at least one paragraph describing the following:
• what you expected to learn about the lab (i.e. what was the reason for conducting
the experiment?)
• your results, and what you learned from them
• Think of at least one other experiment might you perform to verify these results
• Think of at least one new question or problem that could be answered with the
physics you have learned in this laboratory, or be extrapolated from the ideas in
this laboratory.
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LPC Physics 2
Introduction to the Oscilloscope
©
2003 Las Positas College, Physics Department Staff
Clean-Up:
Before you can leave the classroom, you must clean up your equipment, and have your
instructor sign below. If you do not turn in this page with your instructor’s signature with
your lab report, you will receive a 5% point reduction on your lab grade. How you divide
clean-up duties between lab members is up to you.
Clean-up involves:
• Completely dismantling the experimental setup
• Removing tape from anything you put tape on
• Drying-off any wet equipment
• Putting away equipment in proper boxes (if applicable)
• Returning equipment to proper cabinets, or to the cart at the front of the room
• Throwing away pieces of string, paper, and other detritus (i.e. your water bottles)
• Shutting down the computer
• Anything else that needs to be done to return the room to its pristine, pre lab form.
I certify that the equipment used by ________________________ has been cleaned up.
(student’s name)
______________________________ , _______________.
(instructor’s name)
(date)
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