Measurement of Transient Signals
Objective
There are a lot of signals in the real world that last only momentarily, such as voltage spikes from
switches in electronics or out-of-range voltages on power lines in a storm. These signals are called
transient signals. In this lab you will use the oscilloscope to observe and measure a transient signal
arising from a switch. Then you will use a digital logic chip to eliminate the transient signal from the
system, and observe the result again. Finally, you’ll use the lab data and experience to write a report.
Concepts
In this lab we will look at switch “bounce.” This is an unwanted phenomenon, and we’ll learn to
measure the resulting transient, then eliminate it. So don’t get too attached to it.
A switch transition is never perfect on the time scales of digital systems. Rather than make one clean
connection, a switch will typically make several transitions during the time it takes to open and close.
This phenomenon is known as switch bounce, and is the result of many factors, including mechanical
design, age, operating inertia, and the condition of the contact surfaces. The signal shown in Figure 1
is the voltage across a switch that has been switched just once. The bounce from the switch causes
unwanted abnormalities in circuit output. For instance, if the signal in Figure 1 was used as the input
to a digital counter, it would count three times (one for each 5 V peak) instead of just once. This is
why we use need techniques to rectify such problems. One of the best methods to eliminate switch
bounce is to use a flip-flop, like you’ve seen in digital logic.
Figure 1: Example Output of Circuit with Switch Bounce
Figure 2: Prelab Logic Circuit
Equipment and Components
This experiment will require the use of your component and tool kit, the Agilent Oscilloscope,
Agilent E3060A Power supply, SPDT (single pole, double throw) switch, two 1 k resistors, and
a 7400N Quad NAND chip.
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Prelab (25 points) – Due at the beginning of lab
1. In the time-varying signals lab, you wrote about how the digital oscilloscope acquires and
displays data. For the transients lab, knowing what the ‘trigger’ on the oscilloscope does and how it
works will allow you to make the required measurements and observations. So… type a paragraph
about what the trigger is, how it works, and how to set it on the oscilloscope. Some places to find this
information are in the User’s Guide and the articles on the Agilent website. You can also use others,
but use your own words and cite your sources. (15 points)
2. On the data sheet, name the device in Figure 2, and appropriately name the four pins A, B, C,
and D according to what the device is. A good place to start would be your digital logic book.
3. Draw the function table (truth table) for the device in Figure 2 on your data sheet.
4. Bring a couple of floppy disks to the lab.
Part 1: Switch Bounce
Our first task is to create a circuit containing a switch and observe the switch bounce.
1. Construct the circuit in Figure 3. The Agilent E3060A is used for the +5V source of the
circuit.
Figure 3: Switch circuit
2. Connect channel one (CH 1) of the Agilent scope across R1 in the circuit. Adjust the
volts/division and the horizontal time scale to get a good view of the signal.
3. Set the trigger to acquire data based on what you found out from your prelab work. The
‘Trigger’ sub-panel on the oscilloscope, shown in Figure 4, is what you will be using to
adjust the edge, mode coupling, and trigger level. For the resistor you’re measuring across, it
would be a good idea to use rising edge. Since we don’t want the waveform to be displayed
until the trigger conditions are met, you’d use Normal mode. Finally, pick an appropriate
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voltage level at which you want the scope to display the signal, and adjust it using the ‘Level’
knob.
Figure 4: Run Control and Trigger sub-panels
4. After you’ve set the trigger conditions, run a ‘Single’ acquisition by hitting the ‘Single’
button on the Run Control subpanel. This allows for a single measurement that will be taken
when the oscilloscope senses an input that crosses the trigger threshold. The test setup is
now ready to collect data, so…
5. Hit the switch in circuit. This should create a picture on the oscilloscope of the voltage
across the resistor. The voltage should have a lot of bounce to it, such as in Figure 5.
Figure 5: Two More Examples of Switch Bounce Output
6. Using the cursors on the oscilloscope, measure the time it takes to go from a steady 0 V (the
first point at which the voltage is greater than 0 V) to a steady 5 (the first point at which the
voltage stays at a constant 5 V) and record it on your data sheet. The value measured here is
the full-scale rise time – the time it takes to go from a steady minimum to a steady maximum.
Although we are defining our rise time here to be full scale, in general the rise time is defined
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as the time it takes for a signal to go from 10% of its maximum value to 90% of its maximum
value.
7. Save the scope plot to disk for use in your report.
8. Have the TA verify your progress on your data sheet.
Part 2: Eliminating Switch Bounce
The circuit in Part 1 exhibits a pretty bad rise. It contains the results of unwanted inputs (noise), has
a lot of bounce, and takes a long time to actually go from 0 to 5 V. There is a better way of creating
this switch using the same components used in Part 1 plus a 7400 NAND gate chip.
1. First, build the circuit shown in Figure 6.
Figure 6: ‘De-bounced’ Switch Circuit
The pin diagram for the 7400 is shown in Figure 7. The pin numbers in Figure 6 correspond
to those on the chip shown in Figure 7.
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Figure 7: 7400 Pin Diagram
2. Connect the oscilloscope across pin x and the bottom terminal of R1 (the positive
oscilloscope lead connects to +5 V, and the negative lead connects to the output of the 7400).
Set the trigger settings on the oscilloscope as you did in Part 1, hit the “Single” button on the
oscilloscope, and then activate the switch in the circuit.
Figure 7: Debounced Circuit Output
This time, it should be noted that there is very little if any bounce to the signal, such as in
Figure 7.
3. Use the cursors to find the rise time from a stable 0 V to a stable 5 V and write it on your data
sheet.
4. Save the scope plot to disk for use in your report.
5. Have the TA verify your progress for Part 2 on your data sheet.
Post-lab: Report
Using the data obtained in the lab, prepare a short report using the following guidelines. Be sure to
use proper format, as dictated by the lab website.
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Objective: Briefly describe what a transient signal is, and also switch
bounce. Tell what switch bounce is, what causes it, and why and how we want
to get rid of it.
Procedure: The procedure consists of a past-tense paragraph format retelling
of what you did for the lab, and what you used. It should be reasonably
detailed, but not overly so.
Discussion: Discuss switch bounce, its causes, and how/why to get rid of it,
in more detail than in the objective. Include the prelab data, in paragraph
form to back this up, and describe how the NAND works to eliminate the bounce
(this might take some looking up). Tell what you expected and why, and give
the results both numerically and in description. Compare the results of the
two circuits, and compare them to what you expected (theory). Tell why it did
or didn't match.
Conclusion: Summarize all results (numerically and what they looked like),
and tell whether they compared to theory, and why or why not. And summarize
what was learned.
Some notes:
Cite any sources you use for your report.
Generally use past tense when describing what was done or what was observed.
You can use present tense when talking about theory. Use passive verbs rather
than active (i.e. "The circuit was constructed" rather than "I constructed
the circuit"_). Avoid using "I," "me," etc., as this is an academic and
industry standard.
Put figures on the first page on which you reference them. Only put results
in the discussion and conclusion, not in the objective and procedure.
Refer to the guidelines and the grading matrix for the reports, which can be
found on the 2303 policies page:
http://www.ece.mtu.edu/labs/EElabs/EE2303/EE2303LabSpecificInformationandGuidelines.pdf
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Lab 7: Measuring Transient Circuits
Name_____________________________
Section________
Prelab (due at the beginning of lab)
1. Include your paragraph on the trigger function.
2. Name the device in Figure 2 and name the pins.
3. Draw the function table for the device in Figure 2.
Part 1: Measurement of Time Varying Signals
1. Rise time for the switching circuit:
2. TA signature for Part 1: ________
Part 2: Summing Amplifier
1. Rise time for the de-bounced circuit:
2. TA signature for Part 2: ________
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