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LABORATORY
Equipment
needed from the stockroom: an
analog universal meter (AVM), an analog oscilloscope with the manual,
the manual for digital oscilloscope at your bench, leads.
1. THE INSTRUMENTS
AT THE WORK BENCH.
1.1 Familiarize yourself with the instruments at your bench.
Leave the digital scope for later. The dual
dc power supply output consists of two independent units which can be
connected in different configurations (such as series or dual polarity) or used independently. Check one of these units by
connecting a DVM and AVM across its terminals.
Turn the voltage adjustment knob to get several different voltage values (for
example 1.5V, 14V, 30V. Compare readings on the power supply display with readings on the DVM and the AVM. Note readings of
the voltmeters on different ranges of the
instruments.
Note:
A DVM range is set by push buttons and an AVM range by its rotary
switch. Make sure also that both instruments are set for DC measurement.
Which voltmeter range should be selected to read best a given
voltage? Comment on the precision of voltage measurements on different ranges of
the DVM and the AVM and note if its
readings agree with the power supply display.
1.2
Configure the power supply.
Next, configure the power supply for dual polarity voltage,
such as +15V and -15V with respect to ground, which is required in many electronic circuits. Using the DVM, measure
voltages between the ground (common) and "+", ground and "-"
terminal, and also between "+" and
"- "
terminals.
2. THE ANALOG OSCILLOSCOPE
We start with a simpler traditional analog oscilloscope, which
is easier to understand and to operate. This
will introduce us to the general oscilloscope measurement fundamentals which
apply also to the digital scope.
Familiarize yourself with the analog oscilloscope. Following the
scope manual identify two inputs, input
switches (dc, ac, ground), vertical and horizontal adjustments and trigger functions. Note that one of the two input terminals
of a scope is grounded (common); the one
connected to the outer shield of the coaxial cable. Other instruments (e.g. a waveform generator) or points of a circuit you want
to test may also be grounded. Be sure that
in such a case you connect the common terminals together. Ignoring this rule may even result in destruction of a tested circuit!
Can you see why?
Perform the following tests:
2.1.Time measurements
Observe a sine wave, a triangle and a square wave from the
waveform generator at your bench. Measure a
sinewave period and compare it with the generator frequency for a low frequency
(between 50 Hz and 500 Hz), and a high frequency(>100 kHz). Expand the image of the sinewave by
selecting appropriate vertical (volts/division) and horizontal (time/division)
gains.
Estimate the precision with which you can measure the period by
considering a fraction of the horizontal
scale divisions that you can read reliably.
Try internal and external triggering (for the latter use a
"sync" or trigger output of the signal
generator). Using internal triggering adjust the trigger level and observe its
effect on the starting point of the
displayed waveform. Make notes and sketch observed waveforms
for your report.
2.2. Voltage measurements
Set the waveform generator to obtain a sinewave with a frequency
of a few hundred hertz. Connect the
oscilloscope to the generator (watch those ground leads! -polarity is important
here!). Connect also the DVM to the same terminals. From the oscilloscope image measure the signal amplitude. Note the number
of vertical divisions and volts/division
scope setting. The latter should be selected so that the image is expanded on the screen for easier and more precise reading.
The color inner knob on the range switch
must be turned all the way to the "calibrated" position".
Note a reading on the DVM for the same signal that you observe
on the scope. The meter should be set for an
appropriate range of AC voltage measurement.
Next, without changing the scope settings or the generator
frequency, repeat the measurements described
above for a triangular wave and for a square wave.
Prepare a table showing the amplitudes (from the scope) and the
DVM readings for the three waveforms.
Do the scope and the voltmeter readings agree for the three waveforms?
What does the voltmeter show? Compare appropriate values from the scope
and DVM measurements by showing their difference in percent in the table.
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Hint: refresh your memory about rms for different waveforms.
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2.3 Scope AC and
DC inputs
A switch at the scope input has three positions: GROUND, DC, and
AC. In GROUND position the scope internal
circuit is disconnected from the input terminal and connected it to
ground. This helps to find zero voltage level on the display. DC position
connects the input terminal directly to the
scope circuit while the AC setting makes this connection through
a capacitor. Thus in the AC input mode any dc voltage, which may be present in the measured signal, does not show on the display
(a capacitor represents an open circuit for
dc).
To see the effect of
different input modes perform these tests:
a) Set the input switch to GROUND and by adjusting the VERTICAL
POSITION knob move the image (a straight
horizontal line) to the middle position on the display grid.
b) Switch the scope input to DC and observe a sine wave or a
triangular wave. Check if the wave is
centered on the zero (ground) level adjusted previously. Now add some dc bias
to your signal, adjust the dc level, and observe the waveform position on the
scope display. (There
is a knob on the waveform generator for setting dc or "offset" voltage).
c) Switch to AC input
and observe if changing dc bias affects the image.
d) Switch the waveform generator to produce a square wave
(without dc bias) at high frequency (>5
kHz) and observe it in both DC and AC input modes. Repeat the same at
low frequency (about 50 Hz).
Make notes of your observations and sketch waveforms seen in
tests b) and d). Explain what you
see. Why the square wave looks "strange" only in AC mode at low frequency?
2.4 Frequency range
of instruments
Measuring instruments are designed to operate within certain
voltage and frequency range and should not
be used outside their design specifications. The oscilloscopes in our laboratory
are capable of operating at fairly high frequency (tens of megahertz). What about the DVM and the AVM? To check their useful
frequency range, do the following:
· Connect an oscilloscope,
DVM, and AVM to the waveform generator at the same time.
· Set the waveform
generator to a sine wave with the amplitude of a few volts. Start with
a frequency of a few tens of hertz and measure the voltage with all three instruments.
· Increase frequency by a
decade (a factor of ten) and measure the voltage with the three
instruments again. Repeat this procedure until you reach the maximum frequency of the waveform generator.
You should notice that readings of voltmeters drop with
increasing frequency while the scope
indicates almost the same voltage (the generator output voltage may somewhat
vary with frequency too). Adjust frequency
to find its values where the readings of DVM are only
about 5% below the voltage indicated by the scope. Repeat the same for AVM. This
way you can find the useful frequency ranges for
each voltmeter, defined here as the frequency
range where a voltmeter reading is different by no more than 5% from the
"true value" assumed to be shown
by the oscilloscope.
3. THE DIGITAL
OSCILLOSCOPE
A digital oscilloscope can be a great tool, provided that you
know how to use it. It has many functions
and menus which have to be programmed before the desired mode of operation
is obtained. This may be daunting for a beginner who has to spend time studying
the manual before making any measurements. Like with a computer, it is best to start with simple operations and move to more
complex tasks as you gain experience. Familiarity
with an analog scope should help, because front panels and basic functions of digital oscilloscopes are made to resemble those of
analog scopes.
Here are some of the
most common problems that beginners have with digital scopes:
· It is often not obvious
how to access various functions of the instrument, many of which
are programmable from the front panel buttons. Consult the manual.
· The waveform you see on
the screen does not necessarily represent the input signal. It may
be a waveform stored from a prior measurement.
· You may be easily
"fooled" by a digital scope if you let it "think" for you by
selecting the "auto" mode in which
the voltage and time scales are set by the instrument program.
On occasions a noise may look like a nice signal or a momentary transient voltage like a continuous wave.
All this can be sorted
out and clarified with experience which you are about to acquire.
Measurements:
Repeat measurements 2.1 and
2.2 from the previous section using
the digital scope at your bench. Note that
you do not need to count the scale divisions to get the signal amplitude or
the period; the instrument displays this information for you. You can also
activate the vertical and horizontal cursors
and check the values of amplitude and period using the cursor
position displays.
REPORT
Describe briefly the measurement procedure and the results,
including sketches of waveforms.
Include the table of voltage vs. frequency from part 2.4. Address the topics and answer the questions printed in bold
letters in the manual.
In particular discuss these
problems:
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Do the scope and
DVM
measurements agree (to what extend - give % values)?
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What value does a DVM
measure for a sine wave? What about a square wave and
a triangular wave?
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What is the useful
frequency range of the DVM and the AVM?
Add any observations or conclusions you wish to make; they
enhance your report.
Do not forget to number figures and tables and to give them
captions (titles).
Number all pages of the report.
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