The cathode ray oscilloscope, one of the most useful tools in modern experimental
physics, can measure potential differences which change rapidly with time -- too rapidly to
be followed by the needle of a simple voltmeter. You will be using the oscilloscope in
subsequent labs, so it is important to become familiar with it now. You are encouraged to
explore the operation of the oscilloscope by manipulating all of the controls. There is only
No Prelab The
one precaution which you must
take Oscilloscope
in order to protect the instrument:
 WHEN
THE SPOT
THE
SCREEN
STATIONARY,
KEEP
INTENSITY
The
oscilloscope
is oneON
of the
most
versatileIStools
in the laboratory.
TheTHE
oscilloscope
can be used as a D.C.
VERY LOW, in order not to damage the fluorescent screen by "burning a hole in it" at
voltmeter,
instrument,
and a higher
frequency
The oscilloscope
allows us to view
that point.A.C.
In voltmeter,
general, doa timing
not leave
the intensity
thanmeter.
necessary
for reasonable
visibility.
signals
that are cyclic or transient. The electrical signals of the heat or light intensity of a distant quasar can
be viewed with an oscilloscope. The heart of an oscilloscope is the cathode ray tube. The principle
The Cathode
Tube
component
of Ray
which
is shown in Figure 1.
Figure 1 shows schematically the essential features of the cathode ray tube in the
oscilloscope.
Fig. Figure
1 1.
Electrons leave the heated cathode by thermionic emission. They are accelerated through
a
voltage
and emerge
a narrowthe
beam
focused
through
a holeray
in tube,
the accelerating
Thefixed
beam
of electrons,
whichascomprise
cathode
ray in
the cathode
is formed by an electron
plate. When the electron beam strikes the fluorescent screen on the face of the tube, it
gun.
The gun
consists
of a filament,
which
becomes
hot when
electrical
flows through
it. Electrons
produces
a small
luminous
spot. An
external
potential
difference
cancurrent
be measured
by
applying
it
across
a
pair
of
parallel
deflecting
plates,
through
which
the
beam
passes
on
become so energetic that they can escape the surface of the filament. The electrons are then accelerated
the way to the screen. The beam is then deflected by the resultant transverse uniform
through
series
of plates
focusThere
the electrons
into aof
small
beam. plates,
When the
hits the fluorescent
electric afield
between
thethat
plates.
are two pairs
deflecting
onebeam
for vertical
and
the
other
for
horizontal
deflection.
screen, a glowing spot is formed on the screen.
Between the electron gun and the screen are two pairs of parallel plates between which the beam passes.
One pair of plates is horizontal and consequently will defect the beam vertically. The other pair is vertical
and will deflect the beam horizontally. If the tube is long in comparison to the deflection, then the amount
of deflection is proportional to the voltage applied to the plate.
The electrical signal to be studied is usually applied through an amplifier. The vertical amplifier can be a
D.C. or A.C. amplifier and is usually calibrated such that the voltage input can be read directly from the
screen. The horizontal deflection plates are frequently connected to an internal signal. The internal signal is
a saw tooth wave generator. In the saw tooth wave (Figure 2), the voltage increases linearly with time. This
means that the horizontal deflection becomes proportional to time. The horizontal amplifier is calibrated
such that time of the sweep can be read directly from the screen.
The combination of the vertical deflection being proportional to the input voltage and the horizontal
deflection being proportional to time causes the electron beam to trace out a voltage versus time graph. If
the input voltage is repetitive, then a stationary pattern will be produced.
53 ltage which, when applied to the horizontal deflection plates, sweeps the
ally across the screen at constant velocity, and returns the beam to its initial
peats the sweep, etc.
Figure 2.
Figure 3.
Fig. 2 To accommodate variation in signals, a triggering circuit is added to the horizontal time circuit. The
triggering circuit causes the time sweep to wait until a positive voltage (or negative voltage) occurs before
the horizontal sweep starts. This eliminates the drifting of the pattern across the screen.
The time base enables one to select for the horizontal sweep (1) any one of several frequency ranges from
the internal sweep generator, (2) to calibrate the horizontal sweep such that a horizontal distance on the
screen can be read directly in time units, and (3) to select an external input (x External) to the horizontal
deflection.
Vertical input (y input) enables one to control the gain of the vertical input such that (1) the vertical distance
on the screen can be calibrated to read directly in volts, and (2) and the input signal can be read as DC or
AC voltages.
The triggering level is used to lock into the vertical input signal such that a stationary trace can be seen on
the screen. The triggering can be done by (1) the input internally or (2) by the input of an external signal.
Apparatus:
Oscilloscope
Multimeter
Frequency Generator
Coil of wire
AC Power Supply
Bar Magnet
Dry cell
Procedure:
1. Turn on the oscilloscope and let it warm up for about 30 seconds. Adjust the horizontal and vertical
position such that the trace is in the center of the screen. The triggering level should be internal and the
mode set to Auto.
2. Connect the signal (or frequency) generator to the vertical input of channel 1. (The CH 1 button should
be in). The signal generated is an A. C. voltage whose frequency and magnitude can be changed.
Observe the trace produced when the frequency is changed or the time base on the oscilloscope is
changed. Observe both the sine wave and a square wave. Become familiar with the controls on the
54 oscilloscope and the frequency generator.
3. To measure voltages on the oscilloscope, turn the vertical amplifier to calibrate. (Rotate the dial in the
middle clockwise until it locks.) To measure D.C. voltages, set the input for ground (or connect the
terminals of the input together) and set the trace on the centerline on the screen. Set the switch to D.C.
and set the amplifier to 1 V. Now measure the voltages of a dry cell. (It should be a straight line 1.5 V
from the centerline). Compare with the measurement of a D.C. Voltmeter. Record the value in the data
section. Repeat for the three other voltage outputs of Power Supplies
4. To measure A.C. voltages, connect a sine wave from the frequency generator to the oscilloscope. (Be
sure to connect the ground output of the frequency generator to the ground input of the oscilloscope.)
Record the voltages from the bottom of the valley to the top of the peak voltage or the peak-to-peak
voltage (Vpp). Now measure the same voltage using the an A.C. voltmeter. Repeat the measurements for
three different voltages. The A.C. voltmeter reads the rms voltages (Vrms) and not the peak-to-peak
voltages. The relationship is Vrms = (0.707)(Vpp/2).
5. To measure the frequency of an A.C. signal, the period is first measured and then the frequency is found
by taking the inverse of the period. The period is the time for one complete cycle and can be read
directly off the screen of the oscilloscope. Set the time base to calibrate by rotating a dial clockwise until
it locks.
6. The oscilloscope is very useful in analyzing A.C. circuits. Connect a square wave generator in series to a
resistor box and capacitor box. Connect the input of the oscilloscope across the terminals of the capacitor
box as shown in Fig. 3. (Be sure the ground of the frequency generator is connected to the ground of the
oscilloscope.) Set f = 1000 Hz, R = 1000 Ω, and C = 0.1 µf. Sketch the waveform in the data section.
The time constant is the time for the voltage of the capacitor to decrease to 1/e th (or 0.37) of is original
value. This time can be read off the screen of the oscilloscope as shown in Fig. 4.
55 Fig. 3 Fig. 4 7. The oscilloscopes are used to study transient signals. Electromagnetic induction can be studied with the
oscilloscope. Hook up a coil of wire to the vertical amplifier. Set the vertical amplifier to 0.1 V/div. and the
time base to 0.5 sec/div. Observe the form of the voltage as the signal goes across the screen when the N
pole of the magnet is inserted into the coil and then draw the magnet out. Sketch the curve produced in the
data sheet. Repeat with the south pole of the magnet
56 Data:
1. D.C. Voltage Measurements
Oscilloscope
D.C. Voltmeter
Dry Cell
Power Supply
V1
≈ +5 V ≈ +12 V ≈ -­‐15 V V2
V3
2. A.C. Voltage Measurements
Peak to Peak
Voltages
Calculated rms Voltage
A. C. Voltmeter
≈ 1.0 V ≈ 2.6 V ≈ 3.3 V V1
V2
V3
3. Frequency Measurement
Period on the
Oscilloscope
Calculated
Frequency
Frequency Meter
≈ 950 Hz ≈ 2100 Hz ≈ 17,000 Hz T1
T2
T3
4. RC Circuit (frequency = 1000 Hz, C = 0.1 μF)
Resistance
Capacitance
1.
1000 Ω
2.
2000 Ω
3.
4000 Ω
Time Constant
(RC)
57 Time constant from the
Oscilloscope
Sketch of the RC circuit.
5. Sketch of the north pole of magnet inserted into the coil and then pulled out.
6. Sketch of the south pole of magnet inserted into the coil and then pulled out.
58