MAE 2055: Mechetronics I
Mechanical and Aerospace Engineering
Lab Exercise #2
Name
Partner 1
Partner 2
Partner 3
Objectives – Gain familiarity working with the oscilloscope and function generator
Pre-lab – complete prior to coming to lab
1) Your first task for the pre-lab is to read the Section 4: Measurement Fundamentals notes. Pay
particular attention to the description of the front-panel controls on oscilloscopes.
Oscilloscopes
An oscilloscope, or scope, is a measurement instrument that is used to display a voltage vs. time plot of
an electrical signal. These are very useful instruments when working with electronic circuits, because
they allow you to see electrical signals. Most scopes have multiple measurement channels – typically
two or four. This allows for the simultaneous measurement and display of multiple electrical signals.
One of the scopes available for use in the lab is shown in Figure 1.
Figure 1. One of the 2-channel oscilloscopes available in the lab.
Scope channels are connected to circuit nodes to be measured through probes. Probes are coaxial
cables with BNC connectors on one end (this end connects to the scope input) and specialized
connectors on the other end that enable connection to the circuit-under-test. An example of a scope
probe is shown in Figure 2. We know that voltage is a relative quantity. That is, the voltage of a circuit
node does not have an absolute value, but is only quantifiable in relation to some reference node. For
this reason, each scope input and each scope probe comprise two separate conductors: a signal
conductor (center conductor of the BNC connector and probe cable) and a ground conductor (the outer
conductor on the BNC connector and the outer shield of the probe cable). The ground conductor of the
scope probe should connect to the ground node in the circuit-under-test, and the signal conductor will
connect to the node whose voltage is to be measured.
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Lab Exercise #2
Figure 2. A scope probe. The probe end is shown at the top of the picture. The black wire with the clip on the end is the
ground lead. The probe cable is coiled up in the photo, and the end at the bottom connects to the scope BNC connectors.
The measured signals are plotted on a grid, or graticule, on the oscilloscope’s display. The graticule
consists of eight divisions along the vertical (voltage) axis, and typically ten or twelve divisions along the
horizontal, or time, axis. The scopes you will use in the lab have twelve horizontal (time) divisions.
Two of the most important controls provided on the front panel of an oscilloscope are used to set the
scale of each of the axes on which the measured signal is plotted. You can think of these as zoom
controls. They allow you to zoom in to or zoom out of the measured signal plotted on the screen, much
like you would zoom in or out when viewing an online map. (The main difference here being that on the
oscilloscope you have independent zoom control over the horizontal and vertical axes.) These controls
set the volts per division and the time per division on vertical and horizontal axes, respectively. That is,
these controls set the number of volts represented by each vertical division, and the number of seconds
represented by each horizontal division.
V/DIV, sensitivity
The scale of the vertical (voltage) axis is determined by the V/DIV setting. This is also referred to as the
sensitivity setting, because it determines how sensitive the scope channel is to voltage fluctuations. Each
channel has its own sensitivity setting, and each can be set independently. There are eight vertical
divisions on screen, so multiplying the V/DIV setting by eight gives the full-screen voltage range of the
scope channel at a particular setting. For example, if channel 1 is set to 200 mV/DIV, and channel two is
set to 1 V/DIV, then a full screen (in the vertical or top-to-bottom direction) represents 1.6 V for channel
1 and 8 V for channel 2. So, in this example a 1.6 V peak-to-peak sinusoid measured on channel 1 would
be displayed oscillating back and forth between the top and bottom of the screen.
Figure 3 shows the oscilloscope displays of one electrical signal at two different sensitivity (V/DIV)
settings. Figure 3(a) shows the display of a sinusoidal signal with amplitude of 500mV when the
sensitivity is set to 200 mV/DIV. In Figure 3(b), the same signal is shown at 500 mV/DIV. Notice how
changing the sensitivity setting effectively zooms in and out along the vertical axis.
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Lab Exercise #2
10 μsec/DIV
200 mV/DIV
10 μsec/DIV
500 mV/DIV
(a)
(b)
Figure 3. Scope displays of the same sinusoidal signal viewed at (a) 200 mV/DIV and (b) 500 mV/DIV.
Time/DIV, sec/DIV, sweep speed
The scale of the horizontal axis is determined by the time/DIV setting. This control may also be referred
to as the sweep speed, which is a holdover from the days of analog scopes where changing this setting
determined the speed at which the electron beam in the CRT display would sweep across the screen.
There is only one sweep speed setting, and it applies to all channels – the horizontal scale for each
channel cannot be set independently. Similar to the sensitivity setting, the sweep speed setting
determines the full-screen range of the horizontal axis. That is, it determines the length of time over
which a signal is displayed on screen. For example, if the time/DIV control is set to 10 μsec/DIV, and the
scope has 12 horizontal divisions, then from the left edge of the screen to the right edge of the screen
represents 120 μsec.
Figure 4 shows the scope display that would result when probing the same sinusoidal voltage with the
scope configured to two different sweep speeds. In Figure 4(a), the scope is set to 10 μsec/DIV. In Figure
4(b), the sweep speed is 5 μsec/DIV. Notice how adjusting the sweep speed setting allows you to
effectively zoom in and out along the horizontal (time) axis.
5 μsec/DIV
200 mV/DIV
10 μsec/DIV
200 mV/DIV
(a)
(b)
Figure 4. The same sinusoidal signal viewed at two different sweep speed settings: (a) 10 μsec/DIV and (b) 5 μsec/DIV.
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Lab Exercise #2
Offset/position
Each channel has its own offset, or position, control. This control sets the amount of voltage offset
added to the displayed signal. Another way to think of the function of the offset control is that it
changes the voltage level represented by the midpoint of the vertical axis at center screen. By changing
the center-screen voltage level, the displayed signal can be moved up or down on the screen. This is
often useful when viewing multiple signals simultaneously. Figure 5 shows a 1Vpp sinusoidal signal
displayed on the oscilloscope with the offset control set to two different levels. In Figure 5(a) the offset
is set to 0V, and the 0V points of the waveform pass through the vertical center of the screen. The offset
control is set to 1V in Figure 5(b). Here center screen represents -1V and the signal’s zero crossings are
now occur at 2 DIV, or 1V, above center screen. Notice how the offset control allows you to move the
displayed signal up and down on the scope’s display.
10 μsec/DIV
500 mV/DIV
10 μsec/DIV
500 mV/DIV
(a)
(b)
Figure 5. A sinusoidal signal viewed on the scope display with the position control set to (a) 0V and (b) 1V.
Trigger
The trigger controls on the oscilloscope may take some experimentation on your part to fully
understand, and there will be opportunity for that when you come to the lab. The basic function of the
scope’s trigger settings is to determine the horizontal placement of the signal on the screen. We may be
tempted to think of the center of the screen in the horizontal direction as a time = 0 seconds point,
because that is how we are used to plotting the mathematical expressions that may represent electrical
signals. But what does t = 0 mean in relation to an electrical signal that we probe in the lab? This is
where the trigger control comes in. The trigger setting determines which point on the measured signal
will be displayed at center screen along the time axis.
There are many different trigger modes, or types of triggers, but the default, and the type of trigger
most useful to you in this course, is edge trigger. Setting up an edge trigger requires the specification of
two values: edge direction, and a trigger level. The edge direction can be set to either positive or
negative. The trigger level is specified as a voltage value. For example, if a 5V sinusoidal signal is being
probed and the scope is set to trigger on a positive-going edge with the trigger level set to 0V, then the
signal will be displayed such that it crosses 0V with positive slope at the horizontal center of the screen.
If the trigger control is then set to a negative edge trigger with trigger level still at 0V, then the same
signal will be displayed so that at the center of the screen it is crossing 0V and has negative slope. This
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Lab Exercise #2
scenario is illustrated in Figure 6. In Figure 6(a) the signal is shown as it would appear for a positive edge
trigger at 0V. In Figure 6(b) the signal is shown as it would be displayed for a negative edge trigger at 0V.
The amplitude of the signal in either case is 5V, and the frequency is 1KHz. You should be able to
determine these values based on the V/DIV and sec/DIV settings shown on the plots.
(a)
(b)
Figure 6. The same sinusoidal signal displayed on the scope for (a) a positive edge trigger and (b) a negative edge trigger.
Trigger level is set to 0V in both cases.
The trigger source must also be specified as either channel 1 or channel 2. The scope will then trigger on
the specified channel, placing it on the screen appropriately. The signal measured with the other
channel is then display in the appropriate horizontal position relative to the signal measured on the
channel designated as the trigger source. Relative time delays or phase shifts between the signals
measured on all scope channels will be preserved in the display of those signals.
Function generator
In addition to the oscilloscope, another useful piece of lab equipment is the function generator. A
function generator is a voltage source, but unlike a DC power supply it is able to generate a wide variety
of time-varying (AC) voltage waveforms. Some of the various signals typical function generators can
produce are: sinusoids, square waves, triangle waves, sawtooth waves, noise, and pulses. In addition to
the type of output signal generated, control is also provided over output signal characteristics such as
amplitude, frequency, duty cycle, DC offset, etc. Similar to the scope the function generator connects to
the outside world through a BNC connector. This is a two-conductor coaxial connector. The center
conductor carries the output signal voltage, and the outer conductor connects to ground. The function
generator output can connect to your circuit-under-test using a coaxial BNC cable with two alligator clips
at one end. The alligator clips allow the function generator ground and output signal to connect to the
appropriate nodes of your circuit.
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Lab Exercise #2
Figure 7. Function generator.
Having read these brief descriptions of a few of the more important oscilloscope controls, and the
function generator, you should now be able to answer the following questions.
2) Write the mathematical expression for a 4Vpp (that’s 4V peak-to-peak), 25 KHz sinusoid (can be
either a sine or cosine).
v(t) =
3) Sketch this waveform on the graticule below, assuming the following scope settings:
 500 mV/DIV
 10 μsec/DIV
 Positive edge trigger – trigger level = 0V
 0V offset
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4) Next, sketch the same signal on the graticule below , now assuming the following scope
configuration:
 1 V/DIV
 5 μsec/DIV
 Negative edge trigger – trigger level = 0V
 0V offset
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Lab Exercise #2
5) When you come to the lab, you’ll be building the following circuit
v1
1KΩ
Function
Generator
v2
2KΩ
You will setup the function generator to output a 3Vpp, 50kHz, sinusoidal signal. You will then use
the scope to measure the waveforms present at v1 and v2. Write mathematical expressions for each
of these signals.
v1(t) =
v2(t) =
6) What sweep speed (time/DIV) setting is required to put three full periods of the measured signals
on screen? (Assume 12 horizontal divisions.)
time/DIV =
7) What sensitivity (V/DIV) setting is required so that the signal measured at v1 occupies six full vertical
divisions?
V/DIV =
---------------------------------- End of the pre-lab ----------------------------------
Have your instructor initial here to verify completion of the pre-lab.
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Lab Exercise #2
---------------------------------- To be completed in the lab ---------------------------------8) Construct the following circuit using resistors, a function generator, and a breadboard. Connect the
function generator to the circuit using a BNC cable with alligator clips on one end.
v1
1KΩ
Function
Generator
v2
2KΩ
9) - Setup the function generator to produce a 3V pp, 50KHz sine wave.
- Using either scope probes or BNC cables with alligator clips on the end:
-
Connect scope channel 1 to node v1 on your circuit.
Connect scope channel 2 to node v2 on your circuit. (Be sure to connect all probe ground
leads to the ground node in your circuit.)
Press Autoscale
10) Adjust the sweep speed (time/DIV, horizontal scale) control to put three full periods of the
measured signals on screen.
What is the time/DIV setting?
Verify that this agrees with your answer from question 6.
11) Adjust the channel 1 sensitivity (V/DIV, vertical scale) control to fill six divisions with the measured
signal, v1. Adjust the channel 2 sensitivity setting to the same value.
What is the V/DIV setting?
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Lab Exercise #2
Verify that this agrees with your answer from question 7.
12) Use the automated measurements on the scope to measure:
a. Vpp for Channel 1
____________________
b. Vpp for Channel 2
____________________
c. Frequency of Channel 1
____________________
Verify that the measured values agree with the expressions you came up with in question 5. If they
don’t agree, then either your expressions in question 5 are incorrect, or you haven’t set up the
instruments correctly. Fix any problems resulting in discrepancies between the predicted and
measured values.
Show the automated measurements to your instructor and get his initials here.
13) The trigger level and source were set automatically during the autoscale procedure.
What channel is the scope set to trigger on?
What is the trigger level set to?
What happens when you change the trigger level, and why?
14) Change the trigger to a falling edge trigger on channel 1.
How does this affect the displayed waveform, and why?
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