Exp. 2 INTRODUCTION TO THE OSCILLOSCOPE
PURPOSE:
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To understand the basic operation of an oscilloscope
To become familiar with the some of the functions of the oscilloscope as a precision measuring
instrument.
To demonstrate some of its limitations of oscilloscope as a precision measuring instrument.
To know the advantages of using scope probes and the importance of having them calibrated
Verify Kirchhoff Voltage Law (KVL)
This experiment relates to the following learning objectives of the course
1. Ability to interconnect equipment and devices such as multimeter, function generator, and oscilloscope to
achieve required results.
2. Acquire practice in recording data and results and maintaining a proper engineering notebook.
3. Ability to relate practical laboratory results with lecture theory
LAB EQUIPMENT:
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TPS-4000 Dual DC Regulated Power Supply
Agilent 54621A Oscilloscope
Decade Resistance Box
Decade Capacitor
Agilent 34410A Digital Multimeter
Agilent 33120A Function Generator (FG)
STUDENT PROVIDED EQUIPMENT:
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4
2
2
3
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Agilent 10074C Scope probes (one pair)
Banana-to-banana leads
BNC to BNC
Banana-to-Alligator-clips
BNC to banana
BNC T
Small flat-head screwdriver
Overview of the Operation of an Oscilloscope1
The oscilloscope is one of the most frequently used instruments in an electrical engineering laboratory. The scope
displays one or two waveforms as a function of time on its display tube. A waveform is displayed on the
fluorescent screen of the display tube by repeatedly moving an electron beam over the screen in a pattern
resembling the desired waveform. The electron beam is deflected by a set of horizontal and vertical fields, which
are called the sweep signal and the input signal, respectively. Subsystems of an oscilloscope are shown in figure 1
below.
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Adopted and edited from laboratory notes of Jan Van der Spiegel of the Univ. of Penn, ESE Dept.
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Fig. 1: Subsystems of an oscilloscope showing the display tube and the deflection system
Digital and analog scopes
There are two types of scopes, analog and digital. Digital scopes have more features than the analog scopes.
Digital scopes can process the signal and measure its amplitude, frequency, period, and rise and fall time. Some of
them have built-in mathematical functions and can do fast Fourier transforms in addition to capturing the display
and sending it out to a printer.
Scope Probe
Scope probes are used to apply the signals that we wish to view to the deflection plates which control the
horizontal and the vertical positions of the electron beam. Scope probes are high-quality connector cables that
have been carefully designed to reject stray signals originating from radio frequency (RF) or power lines. They
are used when working with low voltage signals or high-frequency signals which are susceptible to noise pick-up.
Also, scope probes have high input resistance which reduces circuit loading. A probe usually attenuates the input
signal by a factor of 10.
Fig. 2: A typical probe
The probe usually has a small box connected to it, which contains part of the attenuator (voltage divider):
Parasitic
Capacitance
Fig. 3: A 10:1 divider network of a typical probe.
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The advantage of using this 10:1 attenuator is that it reduces circuit loading. By adding a resistance of 9MΩ, the
input resistance (as seen by the circuit under test) increases from 1 MΩ to 10 MΩ. As a result, the current
supplied by the circuit to the scope will be roughly 10 times smaller and thus reduces circuit loading.
You will notice that the probe has a capacitor in parallel with a 9 MΩ resistor. Compensation of the probe is
obtained by adjusting its capacitor C (the probe adjustment capacitor shown in figure 3). This is done in order to
ensure that high frequency signals are not distorted. This is illustrated in the figure 4 below for a square wave.
When the probe is properly adjusted (compensated) a square wave will be displayed with a flat top. However, a
poorly adjusted probe can give considerable distortion and erroneous readings of the peak-to-peak amplitude of
the input signal. You should get into the habit of compensating the probe every time you use it.
Fig. 4: The effects of probe compensation: (a) correctly adjusted probe, (b) undercompensated and (c)
overcompensated probe.
To be clear, the compensation of the scope probe is obtained by adjusting the variable capacitor inside the probe
(as shown in the figure 3). For a times-10 probe, the compensating value will be 1/9 of the input capacitance of
the oscilloscope (which is shown to be 13 pF in the figure 3).
Experiment Sections:
1)
2)
3)
4)
5)
6)
Calibrating the Scope Probes
The Scope Input Modes
Dual Trace Measurements
Advantages of using scope probes and having them calibrated (optional)
The High-Pass Filter of the AC Coupled Mode (optional)
Frequency Measurements (optional)
Section 1) Calibrating the Scope Probes
In what follows, you are to connect probes to the two channels of the scope, and compensate both probes. Thus,
for example, when you are told in part d to “center the trace on the display”, this is referring to the trace of the
channel of interest.
a) Power-up the scope.
b) Use the Intensity knob to ensure the trace is not too bright.
c) Press the Auto Scale key. Also, if necessary, press the Cursor button to turn off the cursors.
d) Use the Vertical Position knob (VERTICAL section) to center the trace on the display.
e) Connect the scope probe to either channel 1 or 2.
f) Connect the tip of the probe to the calibrated output socket, i.e., the Probe Comp. terminal, on the lower right
corner of the scope.
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g) Adjust your probes to obtain very nice looking square waves. If necessary, press Auto Scale again to see the
square wave. For the Agilent 54622A scope and associated scope probes, the adjustment screw is near the
BNC connector. Use a small flat-head screwdriver to do the adjustment. Your probes may be well calibrated
but they should be compensated every time the scope is used.
h) Record both the frequency and amplitude of the square wave. Record whether the probe you are using is a
10X probe or not. Note: When making your measurements, be sure that the oscilloscope says “Probe n:1”,
where n is the appropriate value for your probe (e.g., n = 10 for a times-10 probe); if not, adjust this parameter
manually. (As explained below, such manual adjustment should not be necessary; however, this particular
scope is faulty! Therefore, when using this oscilloscope you must always check that it is adjusted to the
proper probe setting.) Also, show your calculation of the frequency of the square wave from the period.
i)
Examine the set-up and see if you can figure out how the scope knows if it is using 10X probe. See the note
below.
* Note: You have now calibrated the scope probes to work optimally with the scope. The probes should always
be calibrated every time measurements are taken with the scope. Also, for the Agilent scope and accompanying
scope probes, though the probes are 10X probes, your reading will come out “normal” due to the design of the
scope and probes. This means that the scope is designed to detect that a times-n probe is being used, and then
automatically adjust its readout to account for the voltage divider that is shown in the figure 3. That is, the
voltage displayed by the scope is not the voltage being applied directly to its jack; rather, it is the voltage being
applied to scope probe (which is n =10 times larger)
Questions: Section 1
1) What are two possible advantages of using 10X probes when taking scope measurements?
2) From step i, how did the scope know the Philips 10X probes were connected?
Section 2) The Scope Input Modes
a) Using a BNC-to-BNC cable and a BNC T (at either end of the cable), connect the Agilent 33120A function
generator directly to Channel 1 of the oscilloscope. (Thus, here you are not using a scope probe.) If
necessary, turn off Channel 2 by pressing its numbered button until the light turns off.
b) Connect the other end of the T to the Agilent 34401A multimeter. Disable the automatic ranging of the
meter, and manually set the range to 10 V.
c) Set the output termination of the function generator to high impedance, by applying the following
keystrokes: Shift, Menu On/Off, >, >, >, ∨, ∨, and > (as necessary to select High Z), then Enter. (If
the 50-ohm output termination were chosen instead, then for high-impedance loads―i.e., loads
much higher than 50 ohms, as are typical in this lab―actual output voltage would differ from
displayed voltage by a factor of 2.) Perform this adjustment in future experiments too.
d) Set the output of the generator to a 5.0 V peak-to-peak sinusoid (a.k.a. “sine wave”) at 500 Hz, with
a 0 V DC offset. Then, press Auto Scale on the scope.
e) Observe the effect on the displayed waveform of switching between AC and DC coupling on the
oscilloscope input. (This is done by pressing the Channel 1 button, then pressing the left “soft key”
below the display.) For each type of coupling, use the multimeter to measure the AC and DC
components of the sinusoid.
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f) For DC coupling, import the scope image into your report by
following the instructions given in the box on the right.
Also, use the Cursors feature of the scope to measure the
DC offset of the displayed waveform; specifically, measure
the maximum and minimum values of the waveform, and
divide their sum by 2.
Questions: Section 2
1) Would you expect the multimeter measurement to change as you
went from AC to DC coupling on the scope? Why would or
wouldn’t it change? You can devise a simple test to verify your
answer experimentally (optional).
To make a printout of the scope display,
do the following;
1. On the computer desktop, doubleclick on “Word Toolbar 5400”.
This opens a blank Word file with
the Agilent Toolbar window
displayed in it.
2. Click on the camera icon to capture
scope display in the Word file. You
can then print the display from the
Word file.
2) In parts e and g, was DC component measured by the multimeter close to the DC offset set on the function
generator? Also, was the AC component measured by the multimeter closer to the peak-to-peak value of the
sinusoid or its RMS value?
3) Under what conditions would the waveform shift up or down on the scope display as the setting was switched
from AC to DC coupling?
Section 3) Dual Trace Measurements
a) Construct the circuit shown below with Vs = 3.0 Vpp square wave @ 1300 Hz with no DC offset.
+
V1
R1
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1 kΩ
VS
FG
+
1.5 kΩ
R2 V2
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b) Display the function generator (FG) voltage VS on channel 1 and the voltage V2 across R2 on channel 2.
Measure both of these values using the scope. Notes: When using the Cursors, make sure that the Source is
set to proper channel. Use the ∆Y feature to measure the peak-to-peak voltages directly.
c) Noting that V1=Vs - V2, use the math key on the scope and display V1 by pressing the (1-2) softkey. Use the
Cursors with Math selected to measure V1. Note that Kirchhoff’s laws apply for any instant in time; this
means that the measurements of voltage at any time around a closed path should add up to zero. Before
capturing the scope image, simultaneously display all three waveforms―Vs, V1, and V2, in that order from
top to bottom, and without any overlapping―by making the following adjustments manually:
⎯
Adjust the vertical sensitivities of both scope channels and difference display (via the Math feature) to
the same value, to obtain the appropriate vertical scaling.
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⎯
For each channel, using the Ground feature under Coupling, adjust the vertical position of Vs or V2 to
make 0 V correspond exactly to a horizontal graticule line (i.e., horizontal grid lines); then, set the
coupling to DC.
⎯
Make all three vertical sensitivities visible when capturing the scope image
d) Replace R2 with a decade capacitor set to 0.1 µF and display the supply voltage on channel 1 and the voltage
across the capacitor on channel 2. Use Auto-Scale to position the two waveforms. Measure the three voltages
(Vs, V1, V2) and capture the scope image.
Questions: Section 3
1) Did KVL apply to the circuit using the square wave (Consider only the resistive circuit, i.e., there is no
capacitor)? Show explicitly that it did or did not apply.
2) Regarding the scope image in part d, the waveform for V2 shows the capacitor alternatively charging
positively and negatively as the waveform for Vs switches positive and negative, respectively. Similarly,
explain briefly (in a few sentences) the waveform for V1.
3) What would have happened to the scope display if the two probe grounds were connected to points with
different potentials in the circuit?
Section 4) Advantages of Using Scope Probes and Having Them Calibrated (optional) 2
In this section, you will learn about the advantages of using scope probes (in displaying a high frequency signal)
and the importance of having them calibrated.
1) Measuring a high frequency square wave signal
Set the waveform generator to a square wave with a frequency of 2 MHz and 2 Vpp. Display the square
wave on the oscilloscope using a BNC to BNC cable (black cable). Notice that the square wave is not very
clean and that it has a considerable amount of ringing. Make a print out of the screen. Next use the probe
scope to display the signal. Connect the probe input to the output of the function generator. Be extra careful
not to bend the probe pin (it is easily damaged). Also, connect the ground of the function generator to the
ground connector of the probe. Adjust the vertical scale of the scope and notice the waveform. It should be
much cleaner with less ringing.
2) The effects of poorly adjusted probes
Connect the probe input to the function generator output (make sure that the ground of the function generator
is connected to the probe ground). Select a 2 MHz square wave of 10 Vpp and display it on the scope. Use the
cursors on the scope to measure the waveform characteristics: peak-to-peak, Vmax values. Record the values in
your lab notebook. Now deliberately bring the scope probes out of calibration by adjusting the screw in the
probe compensation box and observe what happens to the square wave display. Take the same measurements
and record them in your lab notebook. Recalibrate as necessary.
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Partially adopted and edited from laboratory notes of Jan Van der Spiegel of the Univ. of Penn, ESE Dept.
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Questions: Section 4
1) How do the measurements of the square wave change with the adjustment (indicate percent difference if
any)?
2) What is the reason for the difference in measurements, if any?
Section 5) The High-Pass Filter of the AC coupled Mode (optional)
a) Set the FG to supply a 6.0Vpp square wave @ 3 Hz with no DC offset and display this on the scope with DC
coupling. Record the shape of the FG output as it appears on the scope display. Ask your instructor for help
on this one if you are unable to obtain a nice scope display.
b) Switch to AC coupling and once again record the shape of the FG output as it appears on the scope display.
c) Repeat a) and b) for the same square wave at 12 Hz, 20 Hz, 40 Hz, 100 Hz and 1000 Hz. Be sure to change
the time base as you raise the frequencies.
Questions: Section 5)
1) What caused the change in the shape of the square wave on the scope display when you went from AC to DC
coupling? Did it seem to improve with increasing frequency?
2) Compare the AC & DC coupled traces at a frequency of 12 Hz and at a frequency of 1000 Hz. Capture scope
waveforms.
Section 6) Frequency Measurements (optional)
Note:
This Section attempts to improve proficiency with scope measurements. It will also illustrate the scope’s
measurement accuracy relative to the nominal settings of the function generator. Record all results in each
member’s lab notebook. Make sure each test case is at least in the ballpark when it comes to measuring the
frequency before you write it in your lab manual.
a) Select one of your partners to be the test case.
b) Have the test case look away as someone else selects a frequency on the FG. The shape of the waveform as
well as the amplitude will not matter. Make sure that the FG does not display the test frequency.
c) Have the test case figure out the frequency from the scope display (Utilize the Cursors utility)
d) Repeat steps b) through c) for 6 different frequencies for each member, (the frequencies should cover a range
from 100 Hz to 10 kHz).
Questions: Section 6
1) Tabulate your results from the above section and include percent differences from the nominal values.
Would you consider the errors random or systematic?
2) Do the results in question 1 indicate that the scope is a valid frequency-measuring device in all cases? Is there
a frequency range over which the scope is accurate?
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