Download The Oscilloscope

EXPERIMENT 6
The Oscilloscope
Produced by the Physics Staff at Collin College
Copyright © Collin College Physics Department. All Rights Reserved.
University Physics II, Exp 6: The Oscilloscope
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Purpose
You will use the Scope display in Data Studio to gain some experience in the use of this
versatile laboratory tool for measuring AC voltage, frequency, and comparing AC signals
through the use of Lissajous figures.
Equipment
•
•
Voltage Sensor
Function Generator
•
Digital Multimeter
Introduction
The oscilloscope is a versatile measurement tool that can display real-time plots of voltage vs.
time or voltage vs. voltage. A typical model of an oscilloscope is shown schematically in
Figure 6.1. Its main component is the cathode ray tube (CRT), shown in Figure 6.2. The CRT
is also commonly used to produce a visual display of electronic information in other
applications, including radar and television receivers and computer monitors.
Figure 6.1
Figure 6.2
A hundred years ago, the cathode ray tube was the last word in advanced laboratory research
instruments. In appearance, the early CRTs looked much like the tubes now used in “neon”
signs. Originally, the rays that cause the glass walls of such tubes to glow were thought to be
a new kind of light called a cathode ray which was emitted by a hot wire, called a cathode,
located inside the tube.
University Physics II, Exp 6: The Oscilloscope
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In 1897, Cambridge physicist J. J. Thompson, using a tube much like the one in Figure 6.2,
showed that cathode rays were simply streams of tiny negatively-charged particles, which he
called corpuscles. Thompson won the 1906 Nobel Prize in Physics for discovering what we
now call the electron.
The interior of a cathode ray tube is evacuated to a pressure of 0.01 Pa or less. The electron
beam is produced by an assembly called the electron gun located in the neck of the tube.
Electrons are “evaporated” from a heated filament called the cathode. They are focused into a
narrow beam by a focusing electrode and are accelerated toward the fluorescent screen by the
positive anode. The anode is maintained at a high positive potential relative to the cathode, of
the order of 20 kV. This potential difference causes an electric field in the region between the
anode and the cathode, directed toward the cathode. Electrons from the cathode are
accelerated by this field. Most of them strike the anode, but some pass through a small hole to
form a narrow beam which travels at a constant velocity beyond the anode to the fluorescent
screen. The cathode, focusing electrode, and anode comprise the components of the electron
gun.
The point where the electron beam strikes the screen glows brightly (it fluoresces), and the
glow persists for a few tenths of a second after the beam is no longer there. Inside the CRT,
along the path of the electron beam between the anode and the screen, are two pairs of
deflection plates (one pair is used to deflect the beam vertically and one pair to deflect it
horizontally), as shown in Figure 6.2. If no potential difference is applied across the deflection
plates, the beam is undeflected and makes a bright spot at the center of the screen. However,
if a voltage is applied across the horizontal plates, the electron beam experiences a horizontal
force and is deflected to the right or left, depending on the polarity of the deflection voltage.
A constant deflection voltage will move the spot on the screen a fixed horizontal distance. A
varying (AC) voltage, on the other hand, will deflect the beam horizontally back and forth
since the polarity is continually changing. If the frequency of the AC voltage signal is high
enough, the spot will trace a continuous horizontal line across the screen.
Similarly, a voltage applied to the vertical deflection plates causes the spot to move vertically.
In either case, the magnitude of the deflection of the spot from the center of the screen is
proportional to the magnitude of the voltage applied to the deflection plates.
The cathode ray oscilloscope is therefore a measurement tool that can display plots of voltage
changes much like an x-y graph. Unlike a paper graph that has a beginning and an end, an
oscilloscope’s plot of voltage (known as a trace) is redrawn continuously as long as signals
are applied to the deflection plates.
In AC applications, we usually want to display the
voltage on the screen as a function of time (i.e., a
graph of voltage vs. time). Internal or external
electronic circuits are used to apply a linearly
increasing voltage vs. time signal (called a sawtooth
waveform, illustrated in Figure 6.3) to the horizontal
plates. This causes the spot to sweep from left to
right across the screen at a speed that is proportional
to the upward slope of the sawtooth waveform, then
sweep back to the left at a much faster speed
(proportional to its almost vertical downward slope).
University Physics II, Exp 6: The Oscilloscope
Figure 6.3
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With no voltage on the vertical plates you would see a continuous horizontal line across the
screen. But when an externally generated voltage having a sinusoidal form is applied to the
vertical plates, the beam is deflected vertically as it sweeps left to right causing the beam to
trace out a graph of the applied voltage vs. time as shown in Figure 6.4. Typical oscilloscopes
can display voltage vs. time in events that last as short as 100 ms or as long as 10 sec.
The oscilloscope is a very popular instrument and many
different models exist. There are generally three distinct
sets of controls on any oscilloscope. The first set
controls the electron gun, adjusting the trace focus and
intensity. The second set controls the internally or
externally applied voltage to the horizontal plates,
adjusting the electron beam’s horizontal sweep speed.
The third set controls the externally applied voltage to
the vertical deflection plates, adjusting vertical trace
amplitude. All oscilloscopes contain these controls;
however, the controls may be given different names on
different models, and some models may have additional
controls. In this experiment you will concentrate only on
the basic controls provided on the Scope display in the
Data Studio program.
Figure 6.4
Data Studio Scope Display
Emulating an oscilloscope display in Data Studio is quite simple. The pages that follow walk
you through the use of the Scope display in the Data Studio program.
Measurements Using the Oscilloscope
Perhaps the simplest measurement you can make with an oscilloscope is AC or DC voltage.
You use the oscilloscope as a high-input-impedance voltmeter simply by calibrating the volts
per scale division on the vertical trace, and connecting the unknown voltage to vertical (y)
input. You then determine the voltage by reading the vertical deflection on the screen. Note
that the oscilloscope measures instantaneous voltage, not average or rms voltage. Thus the
maximum voltage Vmax is from
zero to either peak, or one-half the
peak-to-peak voltage. For a
sinusoidal waveform, multiplying
Vmax by 1 / 2 = 0.707 gives the
root-mean-square (Vrms) value of
the voltage.
You can determine the frequency
of an AC source by calibrating the
horizontal sweep rate and then
connecting the unknown source to
the y input. The periodic sweep of
the beam causes the input pattern
to retrace itself and the pattern
appears to stand still, as in Figure
6.5.
University Physics II, Exp 6: The Oscilloscope
Figure 6.5
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By counting the number of horizontal (x) divisions required for one complete cycle of the
waveform, you can easily determine the period. The frequency is the reciprocal of the period.
You can also use the oscilloscope to measure the relative phase between two AC voltages
having the same frequency by creating a Lissajous pattern. If you apply one sinusoidal voltage
signal to the horizontal plates and the other to the vertical plates with the oscilloscope
operating in the x-y mode, a Lissajous figure will appear on the screen.
If you adjust the horizontal and vertical amplifiers so that the amplitudes of the two signals
are the same, the equations of the two waveforms on the screen would be
and
x (t ) = A sin 2πft
Equation 6.1
y(t ) = A sin(2πft + φ )
Equation 6.2
where A is the amplitude, f is the frequency, and φ is the phase difference. Using trig identities
you can expand Equation 6.2 as
y(t ) = A(sin 2πft cos φ + cos 2πft sin φ )
Equation 6.3
From Equation 6.1, when x(t) = 0, sin 2πft = 0 and cos 2πft = 1 ; so
or
y0 = A sin φ
Equation 6.4
sin φ = y0 / A
Equation 6.5
You can read both yo and A from the screen and thus determine φ.
Procedure
Selecting the Scope display
1. Plug the voltage sensor into analog channel A on the interface. Switch on the interface
and the computer. Open Data Studio.
2. Open the Scope display. It should
look like this: →
Figure 6.6
A. Voltage Measurements at 90 Hz
1. Before turning on the Signal Generator, be sure that the amplitude is turned all the
way down. Turn on the Signal Generator, select sine wave, and set the Frequency to
90.000 Hz. Measure the output of the Signal Generator with the multimeter set to
measure AC voltage (remember that the multimeter measures Vrms), set the Amplitude
(Vmax) to 3.000 V. Record these values in Table 6.1.
2. Connect the Voltage sensor to the Signal Generator output terminals. Activate the
Scope display by clicking anywhere on it. Adjust the sweep speed (x axis variable)
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and sensitivity (y axis variable) until one complete cycle of the input sine wave
appears on the screen and the pattern is almost full scale in amplitude. Record the
resulting sweep speed and sensitivity settings.
3. Read the number of divisions on the x axis for one full sine-wave cycle and compute
the period T for one cycle from the Scope sweep speed. Then use the period to
calculate the frequency f of the signal. Finally, calculate the percent difference
between your calculated frequency and the generator frequency. Record all results.
4. Read the number of vertical divisions for the peak-to-peak height of the sine wave
pattern on the screen. Calculate and record Vmax in Table 6.1. Calculate and record the
percent difference between your calculated Vmax and the generator Vmax.
B. Voltage Measurements at 300 Hz
1. Change the output frequency of the Function Generator to 300 Hz and repeat steps
A1–4. Record your results in Table 6.2.
C. Lissajous Figures
Lissajous figures are used to compare two signals using the Scope’s x-y mode to determine
their phase with respect to each other, or how one signal’s frequency compares to that of the
other. The external Signal Generator can supply the vertical voltage signal and Data Studio’s
built in Signal Generator can provide the horizontal voltage signal.
1. Click on the On button to activate the signal generator, and set the frequency to 30 Hz.
An output voltage having a sine AC waveform with an amplitude of 5.000 V should
be selected. Click on the Scope window to make it active. Drag the Output Voltage
(V) icon in the Data listing to the x axis of the Scope display.
2. Set the external Signal
Generator to apply a 60 Hz
signal to the vertical axis.
Activate the Scope display by
clicking anywhere on it. A
trace similar to this → should
appear on the display.
Figure 6.7
Note: it may be necessary to fine tune the frequency setting of the Signal Generator to obtain
a stationary image. When the image appears as expected, press STOP to freeze the display.
Print the pattern from Data Studio, record the input parameters on the print, and turn it in with
your lab report.
3. Repeat step C2 using frequencies of 30 Hz, 90 Hz, and 180 Hz for the y input signal
(external Signal Generator). Print the patterns you, label each print to indicate the
input parameters, and turn them in with your lab report.
University Physics II, Exp 6: The Oscilloscope
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