FOUNDATION EXP 3 – THE OSCILLOSCOPE
EXPERIMENT 3
THE OSCILLOSCOPE
1.0 INTRODUCTION
The aim of these tests is to introduce the basic operational procedures of the
oscilloscope so that the student will learn how the “scope” functions and how to set it
up with the optimal operating settings in the most common measurement and
diagnostic conditions.
The “cathode ray oscilloscope” is one of the most versatile tools available to an
electrical engineer for investigating circuits, but have been replaced in the lab with the
modern “digital storage oscilloscope”. Its great versatility demands a basic level of
understanding on the part of the user. Without sufficient understanding, a scope can
be virtually useless or even dangerous.
In order to help you develop a degree of familiarity with the scope and its functions,
the following procedures have been designed. They should be followed so that
familiarisation with the scope can be effectively gained.
There are two types of ‘scopes’ in the lab. One is the more conventional device as
shown in Figure 1.1, and the other is computer based. The computer based scope uses
an analogue to digital interface device, as shown in Figure 1.2, with three BNC
sockets, connected to the PC via a USB cable. Both of these will be used in this
experiment.
For each of the aspects involved in this experiment, note down all your observations
in your lab books, as well as any parameters that you are asked to measure.
Figure 1.1
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FOUNDATION EXP 3 – THE OSCILLOSCOPE
Figure 1.2
2.0 EQUIPMENT REQUIRED
1 Tektronix TDS2002 Digital Storage Scope
1 USB Instruments Scope
BNC-BNC leads (Figure 2.1)
BNC-croc clipped leads (Figure 2.2)
Frequency Generator and Counter
Breadboard
Figure 2.1
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Figure 2.2
3.0 BASIC INVESTIGATION
3.1 INITIALISATION
The type of scope installed in the lab is the Tektronix TDS2002 digital storage
oscilloscope. These scopes are small, lightweight, benchtop packages that can
be used to take ground-referenced measurements. To switch on the scope,
push the button located to the left on the top surface of the device. After a few
seconds, the main scope screen will appear, which has a black background
with dotted horizontal and vertical lines.
The scope has two channels, (so that two signals can be displayed
simultaneously) Channel 1 (CH1) and Channel 2 (CH2). Channel 1 and its
related controls have a yellow designation. This also means that when a signal
is connected to the CH1 input, a yellow trace will appear. Channel 2 is
designated with the colour blue.
First, it is necessary for the yellow (CH1) trace to be visible in the centre of
the screen in order to initialise Channel 1. This is achieved by first pressing
the yellow CH1 MENU until in the bottom left of the main screen you see
yellow text saying “CH1” followed by a voltage value. If in the same part of
the screen you see blue text, (CH2, voltage value), press the blue CH2 MENU
button until this disappears. To initialise Channel 1, press the AUTO SET
button, which is located in the top right hand corner of the scope controls. You
should now see the yellow Channel 1 horizontal trace in the centre of the
screen.
Can you initialise Channel 2 in the same way?
Next, you are to display a signal from the Function Generator on Channel 1 of
the scope screen. Connect a BNC-BNC (BNC stands for Bayonet Neill
Concelman) cable from Ch1 (Channel 1) of the scope to the OUTPUT socket
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FOUNDATION EXP 3 – THE OSCILLOSCOPE
on the Function Generator, which is the lower of the three BNC sockets on the
far lower left of the workstation. On the frequency generator, locate the
50/600 switch, and make sure it is out (50 selected). On the DISPLAY
button, make sure F/G is selected, which allows the frequency to be displayed
on the digital readout of the frequency counter (upper left quadrant). Select the
sinusoidal function on the generator and select the X100K frequency
multiplier button. Use the dial to obtain a frequency of 20kHz.
To the left of the scope control panel, there are five buttons that go from top to
bottom. On the right of the scope screen there are five parameters that
correspond to the five function buttons. If these five parameters are not
displayed, press the yellow CH1 MENU button in order to bring them up.
From top to bottom, the functions are:
i) Coupling – set this to AC by pressing the button to the right
ii) BW limit – set to off
iii) Volts/div – set to Coarse
iv) Probe – set to 1x
v) Invert – set to off
Note that the coupling button, when set to Ground, displays a horizontal line
(yellow for CH1, blue for CH2) which can be moved vertically by turning the
VERTICAL POSITION dials. This function can be used to select an
alternative zero reference if desired. Experiment with this and make a note in
your lab books regarding your observations.
3.2 BASIC CONTROLS
Figure 3.1
Figure 3.1 shows the screen of the scope, and what the axes represent. Use the
SEC/DIV (seconds per horizontal division) dial and VOLTS/DIV (volts per
vertical division) dial for Channel 1 (yellow) of the scope controls to obtain a
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FOUNDATION EXP 3 – THE OSCILLOSCOPE
trace on the scope of at least 2 periods and by adjusting the AMP button on the
frequency generator, obtain a waveform that fills at least two-thirds of the
screen. Make a note in your lab book of the VOLTS/DIV and SEC/DIV
settings which are located at the bottom of the screen, and use them to visually
measure the waveform’s frequency and peak-to-peak voltage. Take the
following measurements:
(i)
Positive peak to ground voltage
(ii)
Negative peak to ground voltage
Adjust the VERTICAL POSITION, HORIZONTAL POSITION,
VOLTS/DIV and SEC/DIV buttons, and by observing the changes made to the
waveform on the screen, explain what these controls do in your lab book.
Next press the MEASURE button, which is at the top of the scope controls.
Use the function buttons on the left of the scope controls to set the “Source” to
CH1 (top button). Press the second function button several times, and make a
note of all the parameters that this button cycles through.
Lastly, the AMP dial on the Function Generator has two ranges, one large and
one small. The dial can be pulled out to enable the shorter range, and when left
in, the longer range is enabled. Use the scope to measure the maximum peak
to peak voltage that can be produced for each of the AMP ranges.
3.3 DC OFFSET
Locate the OFFSET button on the Function Generator. Pull this button to the
“out” position, and turn the button clockwise and anticlockwise. Make a note
of what is happening on the screen of the scope as you vary the DC offset.
Next, press the yellow CH1 MENU button and select DC coupling. Observe
what happens when you adjust the “offset” button of the frequency generator.
Can you explain what is being added to the waveform on the scope screen?
Push the OFFSET button back in once you have finished with this part of the
experiment, and reselect AC coupling on the scope.
4.0 USING TWO CHANNELS
4.1 CIRCUIT AND SET UP
C=100nF
i
+
Vin
+
18
-
Vout
-
Figure 4.1
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Construct the above circuit on your breadboard. Vin is to be supplied by the
Function Generator. Use Channel 1 of the scope to measure an input
sinusoidal waveform of 20kHz with a peak to peak voltage of 8V.
4.2 PROCEDURE
On the scope controls, press the blue CH2 MENU button in order to allow two
waveforms to be displayed at the same time using the scope’s second channel,
Channel 2. Waveforms displayed on Channel 2 are blue in colour, and any
related parameters have blue text. Attach a BNC cable to the Channel 2 input
of the scope, and connect the two crocodile clipped ends to measure the output
of your circuit across the resistor. Adjust the VOLTS/DIV for Channel 2 to
obtain a suitable trace on the screen. In your lab books, draw accurately what
you see, not forgetting to note down the SEC/DIV and VOLTS/DIV for both
channels. What is the overall peak-to-peak gain of the circuit, which is a ratio
of the output and input voltages? Using the following formula, calculate this
gain in decibels.
Gain in dB = 20 log10 (Vout/Vin)
Next use the CH1 and CH2 MENU buttons to find the peak to peak values of
both waveforms, and use this to recalculate the gain. Then press the red
MATH MENU button. Press the Operation button in order to select +.
Describe what you see, and explain what is being displayed on the scope. Do
the same for when – is selected. Then press the MATH MENU to switch off
the red trace.
5.0 TRIGGER FUNCTION
5.1 EXPLANATION OF THE TRIGGER FUNCTION
The TRIGGER function controls allow the oscilloscope display to be
synchronised with the signal you want to investigate. To provide a more stable
trace on the scope screen, modern oscilloscopes have a function called the
trigger. When using triggering, the scope will pause each time the sweep
(which is the steady motion of the trace across the screen) reaches the extreme
right side of the screen. The scope then waits for a specified event before
drawing the next trace. The trigger event is usually the input waveform
reaching some user-specified threshold voltage in the specified direction
(going positive or going negative).
The effect is to resynchronise the timebase to the input signal, preventing
horizontal drift of the trace. In this way, triggering allows the display of
periodic signals such as sine waves and square waves. Trigger circuits also
allow the display of nonperiodic signals such as single pulses or pulses that
don't recur at a fixed rate.
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5.2 EXPERIMENTAL PROCEDURE
Press the TRIG MENU button located on the right of the scope controls. This
activates the trigger functions which should appear on the right side of the
screen. Leave the “Type” on “Edge”. Switch the source between CH1 and
CH2 (ignoring Ext, Ext/5 and AC Line). Can you describe what is happening?
Vary the TRIGGER LEVEL dial for both CH1 and CH2 sources. Describe
what happens and draw what you see.
Next, while still in the TRIGGER menu, switch the “Source” function to “Ext”
(for external). What can you see on the scope? Draw in your lab books what
you see. Fetch another BNC-croc clips cable. Connect the BNC end of this
cable to the EXT TRIG (trigger input) socket on the scope. Attach the
crocodile clips to the input of your circuit. Draw what is displayed on the
scope screen. Does what you see on the screen look familiar? Why?
Now attach the crocodile clips from the trigger input BNC cable to the output
of your circuit (across the resistor). Draw and explain what you see.
Can you explain the triggering function based on what you have observed?
6.0 THE PC BASED USB SCOPE
6.1 SET UP
The final part of the experiment will require the use of a PC. Switch on the PC
at your bench, and log into the ‘student’ account (password: student). Double
click on the ‘EasyScope II for DS1M12’ icon, which opens up the USB scope
window. When the USB scope window opens, two red LEDs light up on the
analogue to digital (A-to-D) converter device adjacent to two of its BNC
sockets. These LEDs indicate that the USB scope’s two channels (A and B)
have been activated. The socket for Channel A is on the right of the device,
and the middle socket is for Channel B.
6.2 PROCEDURE
Before making any connections, use the benchtop oscilloscope, the
appropriate leads and the workstation to set up a 20kHz sine wave, with a peak
to peak voltage of 8V.
Next disconnect the frequency generator from the scope, and reconnect it to
Channel A of the USB scope’s A-to-D converter. Using the mouse, select ChA
on the ‘Trigger’ panel of the EasyScope window. Adjust the T/Div and
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Volts/Div to get a suitable trace on the screen. Note in your lab books these
settings and draw what you see.
Switch the USB scope to DC coupling. On the Frequency Generator, pull out
the OFFSET control, and vary the dial. Note in your lab book what happens as
you do this. Explain what happens when you push the dial back in.
Switch the USB scope back to AC. Click on the ‘Meter A’ tab at the bottom of
the window, and note down what you see. At the bottom of this new window,
click on the ‘Configure’ tab. For Meter 1, select ‘Peak-to-peak voltage’, then
‘minimum voltage’ and finally ‘maximum voltage,’ each time noting down the
relevant figures for each parameter. Vary the “AMP” dial on the Function
Generator, and note down the subsequent changes for the three parameters
above.
Close Meter A, and click on ‘Cursor X’. Two vertical lines should appear on
the USB scope screen. These are labelled 1 and 2. Use the mouse to position
these two lines onto adjacent peaks. Note down the value of ‘Delta’ in the
bottom left of the window, and use this figure to compute the wave’s
frequency (f = 1/T). Verify if this matches what is displayed on the Frequency
Counter.
7.0 COMMENTS AND CONCLUSIONS
When writing your report in your lab book, conclude by explaining what you
have learnt about the oscilloscope and its various controls. What potential uses
do you think an oscilloscope could have? Which type of scope did you prefer
using (the PC based scope, or the conventional one), and explain the benefits
and drawbacks of both.
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