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Transcript
Lab 6
The Oscilloscope
OBJECTIVES
1. Become familiar with the fundamental operation of an oscilloscope.
2. Understand how to use an oscilloscope to measure the voltage and frequency for a
voltage waveform v(t) = Vm sin(2π f t).
3. Learn how to set the amplitude and frequency of a function generator using an
oscilloscope.
EQUIPMENT
Lab Kit, oscilloscope, Function Generator, and power supply.
THEORY
When an oscilloscope is used to determine an unknown dc voltage, the oscilloscope, set
in dc mode, registers a voltage shift. The dc voltage level is determine by
dc Level (V) = Deflection (cm) × Vertical Sensitivity (V/cm)
For ac voltages, the oscilloscope set in ac mode, the unknown peak-to-peak voltage can
then be determined from
ac level (V) ≡ v p-p (V) = Deflection Peak-to-Peak (cm) × Vertical Sensitivity (V/cm)
When an oscilloscope can used to set the frequency of a function generator the
horizontal sensitivity is used in the following manner: Determine the period of the desired
waveform and then calculate the number of divisions required displaying the waveform
on the horizontal axis using the provided ms/cm or s/cm on the horizontal sensitivity
control. Then adjust the function generator to provide the proper horizontal deflection fro
the desired frequency.
PROCEDURE
Part 1: Introduction
a. Turn on the oscilloscope and establish a horizontal line centered on the face of the
screen.
b.
Adjust the following controls on the oscilloscope: Focus, Intensity, Y-position, and
X-position. Observe the effect they have on the oscilloscope screen.
Part 2: DC Voltage Measurements
a. Set the DC/AC/GND switch to the GND position and adjust the Y-position control
until the 0-V reference is a line centered vertically on the screen. Turn the
DC/AC/GND switch to the dc position when finished.
b. Connect the scope to a channel and set the vertical sensitivity to 1 V/cm. Setup a
power supply to read 1.5V and use the scope probes to measure this voltage.
c. To determine the dc voltage that established the shift, multiply the vertical shift with
the vertical sensitivity. Compare the DMM reading with the oscilloscope
measurement using a percent difference. How do they compare?
d. Recalculate the voltage with sensitivities of 0.5 V/cm and 5 V/cm. Which is more
accurate? Why?
e. Disconnect the 1.5V power supply and re-establish the 0V-reference line. Adjust the
power supply to read a 3V-volt value where it goes from -1.5V to +1.5V and the
vertical sensitivity to 1 V/cm.
f. What was the effect on the magnitude and direction of the shift? Can a scope
determine whether a particular voltage is positive or negative? How?
g. Determine the magnitude (no sign) of the dc voltage and compare it with the DMM
using a percent difference. How do they compare?
7-1
Part 3: Sinusoidal Waveforms – Magnitude
Use the oscilloscope & DMM to set the magnitude of a sinusoidal voltage waveform.
Oscilloscope:
a. Connect the function generator directly to the scope probes and set the output
frequency to 500 Hz.
b. Set the volt/div to 1 V/cm, the time sweep (time/div) to 0.5 ms/cm and switch the
DC/AC/GND switch to the ac.
c. Adjust the amplitude control until the signal has a 6V peak-to-peak swing, so that the
resulting voltage waveform has an amplitude of Vm = 3.0V, and a frequency of 500
Hz. The resulting voltage waveform has the form of
=
v Vm sin ( 2π ⋅ f=
⋅ t ) 3.0V ⋅ sin ( 2π ⋅ 500t )
d. Repeat the same process with the following voltage waveforms. Make the
necessary adjustments to display them on the scope screen
• 0.2V∙ sin(2π∙500t)
• 8.0V∙ sin(2π∙500t)
Digital Multimeter
e. The sinusoidal voltage waveform (3.0V) sin(2π∙500t) has a root-mean square (rms)
voltage value determined by
V=
rms
f.
Vm 3.0V
=
= 2.12V
2
2
Connect the DMM directly across the function generator in the ac rms mode and
adjust the function generator output until Vrms = 2.12V. Then connect the function
generator directly to the scope and note what the total peak-to-peak swing is.
Repeat the same procedure with the other two voltage waveforms in part (d). How
do the rms voltages compare? Peak-to-peak voltages?
Part 4: Sinusoidal Waveforms – Frequency
Use the oscilloscope to set the frequency output of the function generator.
a. For (0.5V) sin (2π∙500t), determine the frequency and period of the voltage waveform.
b. Adjust the horizontal so the waveform appears in exactly four horizontal divisions. If
it does not, the fine-adjustment control on the function generator can be adjusted
until it is exactly 4 cm. The scope has then set the output frequency of the function
generator.
c. Measure the frequency of the following waveforms using the oscilloscope.
• 0.4V∙ sin(2π∙10,000t)
• 5.0V∙ sin(2π∙60t)
Compare the function generator frequency to the measured frequency on the
oscilloscope. How do they compare?
Part 5: Sinusoidal Waveforms on a dc Level
a. Set the function generator to an output ac voltage signal of 1.0V∙ sin(2π∙500t). Next,
setup a dc voltage value of 1.5-V on the power supply
b. Construct a series combination of a 1.5V power supply and the function generator. If
you are looking at the signal in terms of ac, you will not see a shift. However, if you
look at it as a dc signal, then you will see the 1.5V shift.
c. What was the effect of switching from the AC to DC modes? Did the shape of the
sinusoidal pattern change at all? How does the vertical shift compare to the dc level
of the power supply?
7-2