Mississippi State University Department of Physics and Astronomy
PH2223
Electronic Measurements
Objective
In this experiment you will become more familiar with the types of measurements made by
an oscilloscope and a digital multimeter, two of the most versatile electronic instruments.
Caution: : In some cases oscilloscopes have induced epileptic seizures. If you
know that you are prone to such seizures, please notify your lab instructor
before lab.
Materials
1.
2.
3.
4.
5.
Analog leads to Pasco interface
Banana-to-banana wires
Fluke digital multimeter
Large aluminium component box
Oscilloscope (one for room)
6.
7.
8.
9.
Oscilloscope leads (one for room)
Pasco 750 Interface
Rheostat
Signal generator (one for room)
Introduction
Dual Trace Oscilloscope
Your lab instructor has an oscilloscope, as shown in Figure 1, accessible to demonstrate the
following points. The heart of the oscilloscope is the cathode ray tube (CRT), as we saw
in a previous experiment, “Electric Deflection of Electrons”. In the more common uses of
the CRT, a beam of electrons is deflected by the electric field between two parallel plates in
such a way that the beam moves horizontally across the face of the CRT from left to right
many times each second. The points at which the beam hits the screen are made visible by
a phosphor that glows when hit by high-speed electrons. Each position on the horizontal
line thus formed corresponds to a particular potential difference between the parallel plates.
The resulting pattern on the CRT is a plot of this input voltage versus time. The kind of
dual trace oscilloscope that we will use has several convenient features including:
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Mississippi State University Department of Physics and Astronomy
(a) Oscilloscope
PH2223
(b) Voltage V vs time t
Figure 1: The (a) oscilloscope utilizes a cathode ray tube to display (b) V vs t plots.
Figure 2: Oscilloscope details
• The time base is calibrated in sec/division (TIME/DIV).
• Two signals can be displayed at the same time via CH1 and CH2, respectively.
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PH2223
• Each y scale is calibrated in volts/division (VOLTS/DIV).
• The input signal may be AC or DC.
• The start of the horizontal trace may be triggered from CH1, CH2, external input, or
line voltage.
Procedure
Setting up the computer
These functions are replicated in the oscilloscope function of DataStudio, in conjunction
with the Pasco 750 Interface.
Figure 3: The Pasco 750 Interface. The arrow on the left indicates the portion of the
Interface that will serve as the oscilloscope; in this case we’ll use Analog Channels A and
B. The arrow on the right indicates the portion of the Interface that will serve as the signal
generator (“OUTPUT”).
Signal input
Open the DataStudio program on your computer desktop. A window panel should open with
an option to click “Create Experiment” (if not, ask your lab instructor). Select “Analog
Channel A”, and Choose sensor or instrument... “Voltage Sensor”; click OK.
Signal output
On the same image of the Interface, click on “OUTPUT” (see Figure 3), and a “Signal
Generator” window panel will open. On this panel select “Sine Wave”, which should be
selected by default, and set the Frequency to 100 Hz.
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PH2223
Displaying the output
Now in the left window panel, find“Displays”, and click double-click “Scope”. Highlight
“Voltage, ChA (V)”, and click OK. Make the graph larger by dragging the bottom right
corner, but don’t make it so large that you cover up the Signal Generator window panel.
Now in the top left window panel labeled “Data”, select and drag “Voltage, ChB (V)” onto
the Scope 1 graph. These displays will show both channels (A and B) simultaneously.
Setting up the Pasco 750 Interface
1. Connect the analog voltage probes to Channel A of the Pasco 750 Interface (if they
aren’t already connected); the connector types should serve as a clue how to connect
them.
2. Connect the other end of the voltage probe in Channel A to the Output (black to
ground, red to sine-wave representation).
3. In DataStudio, click Start.
4. If you’ve done this correctly, you should see a sine wave on the graph. You’re viewing
the signal generated by the signal generator function of the Interface. Think about
and interpret what you’re seeing. The graph’s key (or legend) indicates which curve is
which.
5. Play around with the “V/div” (for Channel A). Note that the vertical divisions change
when you select “V/div”. This is because the vertical axis represents the voltage read
by the probes in Channels A and B. You can determine the voltage by counting the
number of divisions and multiplying by the V/div.
6. Time is represented on the horizontal scale. Play around with the “ms/div” (milliseconds per division) on the horizontal scale.
7. Set the horizontal scale back to 5 ms/div and the vertical scale back to 1 ms/div.
8. Now click the button on the graph that has the number “1” on it; this stops monitoring
the data (and stops the input signal from the signal generator) and displays a single
trace. In general we don’t want to stop monitoring the signal, so click “Start” again.
9. Hover your mouse pointer over the button to the left of the “1”. You’ll see a small text
box appear labeled “Trigger”. Clicking this stabilizes the traces on the graph without
stopping the input signal. Click it.
10. Draw what you see on the screen (make a visual record of what you see).
11. In order to measure the period of the signal most accurately, adjust the time per division
to an appropriate setting. Compute the frequency. How does this value compare to
the signal generator output setting?
12. In order to measure the peak-to-peak voltage of the signal most accurately, adjust
the voltage per division to an appropriate setting. Measure and record the peak-topeak voltage. The amplitude of the signal is the zero-to-peak voltage—i.e. half the
peak-to-peak voltage.
13. Again make a drawing of what you now see.
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PH2223
Figure 4: Simple circuit with signal generator and oscilloscope. In our case the signal
generator is not a separate device but is a function of the Interface and DataStudio; likewise
for the oscilloscope.
Half-wave rectifier
Using the circuit components in the large aluminum box, make a simple circuit with a 10
kΩ resistor in series with a diode and signal generator as shown in Figure 4. Use ChA to
monitor the voltage signal from the signal generator (not shown in Figure 4). Use ChB to
monitor the voltage across the resistor, as shown in Figure 4.
According to Ohm’s Law, which you may or may not have seen yet, the current I through
the resistor is given by I = V /R, where V is the voltage (or potential difference) across the
resistor. Monitor V by means of the oscilloscope. Use a 200 Hz signal and an appropriate
sweep rate. Draw and explain what you see.
Digital Multimeter
1. Ohmmeter. Set the meter to ohms (Ω). (Be sure not to overlook the prefix k for
kilo-ohms or M for mega-ohms.) Measure the resistance of the following:
(a) Wire-wound rheostat. Connect to one end and the terminal on the upper bar.
Note and record the effect of moving the sliding contact. Are your readings
consistent with the values printed on the rheostat?
(b) Oscilloscope lead input. With the oscilloscope unplugged and set for DC signals,
measure the resistance of input for ChA. Do the same for ChB. Measure from the
red lead of ChA to the red lead of ChB. Is this what you expected?
(c) Your body. Gently hold the metal part of one ohmmeter lead in each hand. Read
the meter. What happens when you squeeze more tightly? What change occurs
when you wet your fingers and squeeze tightly? What implication does this have
in regard to shock hazards? From the resistances you observe compute the current
you would have experienced if the wires you were holding had been those to a 120
volt household appliance.
2. AC Voltmeter. In previous experiments you have used the DC voltage setting of this
meter (V ); now set it for AC volts (V∼). Use both the voltmeter and the oscilloscope
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PH2223
to measure a 500 Hz sine wave. Adjust the signal generator amplitude for one or two
volts peak-to-peak. An AC voltmeter should read root-mean-square (rms) voltage. It
should be 0.707 times the zero-to-peak voltage. Is this what your readings indicate?
Investigate the frequency response of the meter via the following. Starting with low
frequencies, use the oscilloscope and the signal generator amplitude control to keep the
peak-to-peak voltage constant.
(a) Measure the voltage reading from the voltmeter.
(b) Increase the frequency.
(c) Repeat steps 2a-2b for several frequencies spanning a reasonable range (50, 100,
200, 500, 1500, 2500, ..., 10500 Hz, for example).
If the voltmeter responding ideally, it will read the same at all frequencies. Based on
your observations, what is the highest frequency at which you have confidence in this
meter?
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Last Modified: September 14, 2014