ECE 109 Laboratory Experiment 6
Oscilloscope/Function Generator/Multimeter Operation and Limitations
ECE 109 Laboratory Experiment 6
Oscilloscope/Function Generator/Multimeter Operation and Limitations
OBJECTIVES
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Gain experience in using an oscilloscope to measure time varying signals.
Gain experience in using a signal generator to create time varying test signals.
Gain experience in properly using an oscilloscope’s controls.
Learn the frequency limitations of instruments to non-linear signals
EQUIPMENT REQUIRED
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One banana cable
Three BNC cables
One lot of clip leads and/or jumper wires
DMM (digital multimeter)
Use the DC offset in signal generator for the DC power supply
myDAQ
Signal generator
BACKGROUND
There are two oscilloscopes available. One is on the bench top, Fig. 6-1 and the other is available
in LabVIEW software for the myDAQ, Figure 6-2. An oscilloscope is primarily a voltmeter
for observing time varying signals. It has a fairly low input impedance of one megohm (1M) so
it cannot be used when a load impedance of this size would distort the signal being measured. It
is an excellent tool for measuring transient phenomenon such as impact forces on a load cell.
Modern oscilloscopes can be analog, analog/digital, or digital. A modern digital oscilloscope can
be enhanced with a large selection of software that can analyze and interpret the digital signals.
They also have built-in computers for doing signal analysis such as Fourier transforms on the
incoming signal. This type of measurement and analysis would be very useful in measuring impact
response of a suspension system. They can also measure numerous other parameters of an
incoming signal. They have built-in communication capabilities so signals can be printed or
shared with other devices on the internet.
It is important that you do not indiscriminately turn the controls especially if you have not been
instructed in their use and function. This can prevent the oscilloscope from being able to properly
display an incoming signal. Ask the instructor for assistance if you are having problems viewing
an input signal.
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ECE 109 Laboratory Experiment 6
Oscilloscope/Function Generator/Multimeter Operation and Limitations
An ideal meter will not disturb the circuit when taking measurements. Multimeters and
oscilloscopes are far from ideal instruments.
You can determine the root-mean-square (rms) value of a sine wave displayed on an oscilloscope
by the following equation: 𝑉𝑟𝑚𝑠 =
𝑉𝑝−𝑝 √2
2
2
= 0.3535𝑣𝑝−𝑝 = 0.707𝑣𝑝 . The rms value of an input
signal is what is digitally displayed by a multimeter. However, not all multimeters have this
capability. They are limited by both waveform and frequency. It is sometimes possible to
determine the algorithm used by a multimeter to determine the rms value of a waveform.
If you are using one of the new digital oscilloscopes, you can read waveform parameters on the
lower menu which displays Vrms, Vp-p, and frequency. The voltage from a household outlet is
120 VAC. This is the rms value. The peak value is 1.414 *120= 169.7 voltages. The heating value
of 120 VAC rms is exactly equal to a 120 VDC voltage source such as a photovoltaic panel.
The oscilloscopes in the ECE laboratories are not all the same. If you are having problems setting
up the oscilloscope as the instructor for help.
PROCEDURE
Part 1
1. Connect channel 1 of the oscilloscope to the signal generator and to the digital
multimeter (set to voltage). See Figure 6-1. Make sure that the ground on the
oscilloscope and signal generator are connected together. Both are internally grounded
to the building ground system.
2. Set the signal generator to 1 KHz, 5 V pk-to-pk for each of the following waveforms: sine
wave, triangle wave, and square wave. Increase the frequency to 10 kHz, and then
1MkHz. Connect a BNC cable to both the signal generator and the oscilloscope channel
1 (two cables required). Connect the red clip leads together. Plug your banana cable into
the multimeter then connect the red clip to the red clip leads going to channel 1 and the
signal generator. The instruments are internally connected to the black lead so you
shouldn’t have to do anything with the black lead. You can connect them all together if
you want. The black lead should be at earth ground potential.
3. Plot what you see on the oscilloscope screen. in Figure 3. You can copy the signal seen
on the oscilloscope and paste it into your lab report so that you don’t have to draw it by
hand.
4. Compare the readings on the multimeter with what you see on the oscilloscope. Place
the results in Table 6-1. Add dc offset to your input signal and describe what happens on
the oscilloscope. Try to read just the offset using the multimeters and the oscilloscope.
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ECE 109 Laboratory Experiment 6
Oscilloscope/Function Generator/Multimeter Operation and Limitations
Change the oscilloscope Vertical Mode from GND, to AC, and then to DC. Describe what
happens to the waveform displayed on the oscilloscope with and without DC offset. The
multimeter should not be able to read the frequency as accurately as the oscilloscope.
5. Record your readings in Table 6-1. The oscilloscope will automatically display the signal’s
voltage value and frequency automatically. Use the soft keys to select voltage and time
measurements
Figure 6-1. Typical lab bench setup
Figure 6-2 NI Elvis Oscilloscope and Function Generator
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ECE 109 Laboratory Experiment 6
Oscilloscope/Function Generator/Multimeter Operation and Limitations
Time (sec.,msec., sec.)
Figure 6-3. Oscilloscope Display (from computer or cell phone)
Table 6-1. Measured and calculated results.
Waveform
Oscilloscope
reading Vp-p
Multimeter
reading
Sine wave
Sine wave +5dc
Triangular
Square
Waveform
Sine wave
Sine wave +5dc
Triangular
Square
Frequency
Calculated RMS
voltage
1 k Hz
1 k HZ
1 kHz
1 kHz
Oscilloscope
reading Vp-p
Multimeter
reading
Frequency
Calculated RMS
voltage
10 k Hz
10 k HZ
10 kHz
0 kHz
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ECE 109 Laboratory Experiment 6
Oscilloscope/Function Generator/Multimeter Operation and Limitations
Table 6-1. Measured and calculated results. (Continued)
Waveform
Oscilloscope
reading Vp-p
Multimeter
reading
Frequency
Sine wave
Sine wave +5dc
Triangular
Square
Calculated RMS
voltage
100 k Hz
100 k HZ
100 kHz
100 kHz
Waveform
Oscilloscope
reading Vp-p
Multimeter
reading
Frequency
Sine wave
Sine wave +5dc
Triangular
Square
Calculated RMS
voltage
1 MHz
1 MHZ
1 MHz
1 MHz
6. Now slowly increase the frequency of the function generator until the multimeter has an error
of at least 10%.
7. What is the frequency limitation of the multimeter. ______________ Hertz. Sine wave
8. What is the frequency limitation of the multimeter. ______________ Hertz. Square wave
9. What is the frequency limitation of the multimeter. ______________ Hertz. Saw tooth wave
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Note: If the amount of heat (joules) generated by a DC source (𝑖𝑑𝑐
𝑅𝑇) is equal to the heat
𝑇
generated by an ac source over the same period T ,(∫0 𝑅 ∗ 𝑖 2 𝑑𝑡) then equating the energies
1
𝑇
and solving results in 𝐼𝐷𝐶 = 𝑖𝑟𝑚𝑠 = √𝑇 ∫0 𝑖𝑡2 𝑡 𝑑𝑡 . The following are 𝑉𝑟𝑚𝑠 equations for
common waveforms:
sine wave 𝑉𝑟𝑚𝑠 =
1
𝑝𝑒𝑟𝑖𝑜𝑑 𝑇 𝑜𝑓 𝑜𝑛𝑒 𝑐𝑦𝑐𝑙𝑒
𝑉𝑝−𝑝 √2
2
2
; square wave 𝑉 𝑟𝑚𝑠 = 𝑉𝑝 ; triangle wave 𝑉𝑟𝑚𝑠 =
= 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑜𝑓 𝑤𝑎𝑣𝑒𝑓𝑜𝑟𝑚 (Hz) =
𝜔 𝑟𝑎𝑑𝑖𝑎𝑛𝑠/𝑠𝑒𝑐𝑜𝑛𝑑
2𝜋
𝑉𝑝−𝑝
2√3
(1)
(2)
10. Describe how you measure the frequency of a waveform on an oscilloscope display
using the horizontal scale and the displayed waveform?
_____________________________________________________________________________
_____________________________________________________________________________
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ECE 109 Laboratory Experiment 6
Oscilloscope/Function Generator/Multimeter Operation and Limitations
11. Why does the multimeter reading decrease as the frequency increases?
_____________________________________________________________________________
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Hint: The input circuit topology to many analog voltmeters is usually a low pass filter; i.e. it has a
capacitor to ground on the input which short the input signal to ground at high frequencies.
Write a professional comprehensive lab report using a word processor. Show your results
and include a comprehensive conclusion. There are lots of sample lab reports on the
internet.
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