Exercise 10
The Digital Oscilloscope and the Function Generator
I. Purpose
The purpose of this exercise is to introduce you to the use of a digital oscilloscope and a
function generator.
II. Equipment
oscilloscope
function generator
coaxial cables
III. References
Read Appendix B at the back of this manual for a brief description of the Tektronix digital
oscilloscope and the BK Precision Function Generator which you will use in this exercise.
Those things that you will need are described. The things you do not need are left out.
IV. Introduction
Your instructor will explain how to use the oscilloscope and function generator, and for your
reference, we have included a brief description in appendix B at the end of the lab manual, to
which you should refer as the use of the instruments is being described by your instructor.
V. Setting Up the Digital Oscilloscope
(1) Switch on the power to the oscilloscope using the button on the left on the top of the
instrument.
(2) Plug one end of a coaxial cable into the scope's Channel 1 input connector. Plug the other
end of the coaxial cable into the signal plug on the small black box that is bolted to your
work-bench next to the scope.
(3) Find and Press the AUTOSET button on the oscilloscope.
(4) Your instructor will now explain how to operate the oscilloscope and show you how to
find and use the VERTICAL controls, the HORIZONTAL controls, and the TRIGGER, as
well as how to read the display.
70
VI. AC and DC COUPLING
(1) The instructor will send you a square wave signal. The frequency of this signal will be
changed and you should observe the displayed waveform first with DC coupling set on
Channel 1 and then with AC coupling.
(2) Sketch the waveforms you observe in the space below and note the time scale you used.
1 kHz
DC
AC
Sec/div =
100 Hz
DC
AC
Sec/div =
1 Hz
DC
AC
Sec/div =
0 Hz
DC
-----------------------------------------------------------------------------------------------------------------------------------------AC
Sec/div =
Show your instructor and get your checksheet signed.
Question 1: As you can see from your plots above, the square wave no longer looks like a square
wave if you use ac coupling at low frequencies. Explain why the ac coupled waveforms look the
way they do.
71
VII. Using the CURSORS
(For this part, your instructor will supply a 2.5 kHz sine wave to the signal port on the black
distribution box )
(1) Push the CURSOR button in the Control Button Panel. The CURSORS allow you to make
time and voltage measurements of the displayed signal.
(2) Set the TYPE selection to Time.
(3) Note that the vertical position knob of Channel 1 controls the horizontal position of Cursor 1,
and the vertical position knob of Channel 2 controls the horizontal position of Cursor 2.
Now align Cursor 1 with a rising section of the displayed waveform and align Cursor 2 with
the following rising part of the waveform.
(4) In your spreadsheet record the Cursor times. [ The trigger time is set to be zero for this time
coordinate system. ].
(5) In the Delta window of the screen note the values of t and f. Record the values in your
spreadsheet. [ Note that t is always positive, independently of the order in time the cursors
are aligned with the signal. ].
Show your instructor and get your checksheet signed.
(6) Set the TYPE selection to Voltage.
(7) Note that the vertical position knob of Channel 1 controls the vertical position of Cursor 1,
and the vertical position knob of Channel 2 controls the vertical position of Cursor 2. Now
align Cursor 1 with the bottom of the displayed waveform and align Cursor 2 with the top of
the waveform.
(8) In your spreadsheet record the Cursor voltages. [ The GROUND potential is set to be zero
for this voltage coordinate system. ].
(9) In the Delta window of the screen note the value of V. Record the value in your
spreadsheet. [ Note that V is always positive, independently of the order in vertical
position the cursors are aligned with the signal. ].
Show your instructor and get your checksheet signed.
VIII. Using MEASURE
(1)
Push the MEASURE button in the Control Button Panel. This function allows you to make
72
a variety of automatic measurements on the displayed signal waveform.
(2)
In the top window choose "Source". Next select Channel 1 in all the other windows. Now
go back and choose "Type" in the top window. Finally select Mean, Pk-Pk, Period and Freq
in the others.
(3)
Adjust the Horizontal Controls “position” and “SEC/DIV” so that one or more periods of
the waveform is displayed (some of the MEASURE functions don't work if you have less
than a period displayed).
(4)
In the CH 1 windows of the screen note the value of V in the Pk-Pk window, the
frequency in the Freq window, and the period in the period window. Record the values in
your spreadsheet. [ Note that V is always positive.].
(5)
Compare the values you got for the various parameters when using CURSOR and
MEASURE.
Show your instructor and get your checksheet signed.
IX. The Function Generator
A function generator is an electrical source that can supply voltages that vary in time with
different functional forms, frequencies and amplitudes. They are used in a wide variety of test
and measurement applications. Typical functional forms that can be generated are sine waves,
square waves, triangle waves, and pulses.
(1)
Disconnect the cable from the black signal box and connect it to the OUTPUT jack on the
function generator.
(2)
Switch on the power to the function generator by depressing the red button on the left end
of row of switches.
(3)
Select the square wave function by depressing the third switch from the right in the row of
switches.
(4)
Select the frequency range by depressing the 1K button in the row of switches.
(5)
Set the frequency to approximately 1250 Hz by rotating the coarse frequency control knob
at the left and bottom of the front panel of the instrument.
(6)
Set the frequency to as close as possible to 1250 Hz by rotating the fine frequency control
knob which is just to the right of the coarse frequency control knob.
(7)
Adjust the amplitude of the output waveform by rotating the amplitude control knob which
is on the right end of the middle row of controls.
73
(8)
Adjust the digital oscilloscope so that you get a good display of the output signal of the
function generator.
Show your instructor and get your checksheet signed.
X. Changing the Function, Frequency and Amplitude
(1) Change the waveform to the triangular waveform by depressing the appropriate control
button.
(2) Change the waveform to the sinusoidal waveform by depressing the appropriate control
button.
(3) Change the waveform back to the square waveform by depressing the appropriate control
button.
(4) Use the MEASURE function of the oscilloscope to obtain the Pk-Pk voltage, the frequency
and the period of the waveform. Adjust the Pk-Pk voltage to about 5 volts by rotating the
amplitude control knob of the function generator. Record the results in your spreadsheet.
Show your instructor and get your checksheet signed off.
(5) Change the frequency of the square waveform to 1.25 MHz by using the frequency controls
of the function generator. Adjust the SEC/DIV knob to get a useful display.
(6) Use the MEASURE function of the oscilloscope to obtain the Pk-Pk voltage, the frequency
and the period of the waveform. Adjust the Pk-Pk voltage to about 3 volts by rotating the
amplitude control knob of the function generator. Record the results in your spreadsheet.
Show your instructor and get your checksheet signed off.
Email a copy of your work to yourself, submit to WebCT before leaving the lab, and
turn in your check sheet.
XI. Triggering Off an Unknown AC Signal
In this part, your instructor will supply you with an ac signal and you will adjust the scope
settings to get a stable trace. In particular, you will need to adjust the SEC/DIV knob, the V/DIV
knob and use the triggering section of the oscilloscope. .
On your instructor’s signal, try obtaining a stable trace for each of the following trigger choices:
(1) Trigger on the rising slope when the signal is 3/4VPk-Pk (see sketch below for trigger point 1).
(2) Trigger on the falling slope when the signal is 3/4VPk-Pk (see sketch below for trigger point
2).
(3) Trigger on the rising slope when the signal is 1/4VPk-Pk (see sketch below for trigger point 3).
74
(4) Trigger on the top of the wave (see sketch below for trigger point 4).
4
1
2
3
- For each of the above situations, record in your spreadsheet the settings in the five TRIGGER
Menu windows, the trigger level voltage, the VOLTS/DIV and the SEC/DIV settings.
Hints on Triggering the Scope
The small arrow on the right side of the scope's screen indicates the "trigger level" (the voltage at
which the scope triggers).
The number at the bottom right hand corner of the screen gives the voltage that the trigger level
is set to.
The small arrow at the top of the screen indicates the time when the scope triggers (the "starting
time" or t=0).
XII. Homework Problems
Submit via WebCT .
1. When you used the scope to measure a square wave, you found that the square wave no longer
looked like a square wave if you used ac coupling at low frequencies. Explain why anyone would
want to use ac coupling if it causes the waveform shape to change ?
2. Assuming that the battery voltage is V=1.5 V in the circuit below, find the magnitude of the
current I which flows (a) when the switch is open, and (b) when the switch is closed.
3. Consider the circuit shown below. (a) Suppose that when the switch is open you measure a
75
current flow I of 10 mA. What is the voltage of the battery? (b) Suppose that you now close
the switch and measure a current flow I of 30 mA, what is the resistance of resistor R in
ohms?
4. In the circuit below, suppose that the battery voltage is 5V, R1=100, R2=200 and
R3=300. What is the total current which flows out of the battery?
5. In the circuit below, a student measures a voltage drop of 2 volts across the resistor
R1=680. If the battery voltage is 6.4 V: (a) how much current is flowing through the
circuit, and (b) what is the resistance of R2.
76