Oscilloscope
This complex instrument has a learning curve that extends over a life time.
Applications are found daily to use this instrument to best advantage. What
follows on this web page is an introductory look at the use of an oscilloscope.
Each brand of instrument has its own instruction manual which will detail all
procedures. Follow text links throughout this page to visit manufacturer web
sites for more details.
What does an oscilloscope do?
An oscilloscope is easily the most useful instrument available for testing
circuits because it allows you to see the signals at different points
in the circuit. The best way of investigating an electronic system is to monitor
signals at the input and output of each system block, checking that each block
is operating as expected and is correctly linked to the next. With a little
practice, you will be able to find and correct faults quickly and accurately.
An oscilloscope is an impressive piece of kit:
The diagram shows a Hameg HM 203-6 oscilloscope, a popular
instrument in UK schools. Your oscilloscope may look different but will have
similar controls.
Faced with an instrument like this, students typically respond either by
twiddling every knob and pressing every button in sight, or by adopting a glazed
expression. Neither approach is specially helpful. Following the systematic
description below will give you a clear idea of what an oscilloscope is and what
it can do.
The function of an oscilloscope is extremely simple: it draws a
V/t
graph, a graph of voltage against time, voltage on the vertical or Y-axis, and
time on the horizontal or X-axis.
As you can see, the screen of this
oscilloscope has 8 squares or divisions on the vertical axis, and 10 squares or
divsions on the horizontal axis. Usually, these squares are 1 cm in each
direction:
Many of the controls of the oscilloscope allow you to change the vertical or
horizontal scales of the V/t graph, so that you can display a
clear picture of the signal you want to investigate. 'Dual trace' oscilloscopes
display two V/t graphs at the same time, so that simultaneous
signals from different parts of an electronic system can be compared.
Up
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Setting up
1. Someone else may have been twiddling knobs and
pressing buttons before you. Before you switch the oscilloscope on, check that
all the controls are in their 'normal' positions. For the Hameg HM 203-6,
this means that:
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all push button switches are in the OUT position
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all slide switches are in the UP position
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all rotating controls are CENTRED
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the central TIME/DIV and VOLTS/DIV and the HOLD OFF controls are in the
calibrated, or CAL position
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Check through all the controls and put them in these positions:
2. Set both VOLTS/DIV controls to 1 V/DIV and the
TIME/DIV control to 2 s/DIV, its slowest setting:
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VOLTS/DIV
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TIME/DIV
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3. Switch ON, red button,
top centre:
The green LED illuminates and, after a few moments, you should see a small
bright spot, or trace, moving fairly slowly across the screen.
4. Find the Y-POS 1
control:
What happens when you twiddle this?
The Y-POS 1 allows you to move the spot up and down the screen. For the
present, adjust the trace so that it runs horizontally across the centre of the
screen.
5. Now investigate the
INTENSITY and FOCUS controls:
When these are correctly set, the spot will be reasonably bright but not
glaring, and as sharply focused as possible. (The TR control is screwdriver
adjusted. It is only needed if the spot moves at an angle rather than
horizontally across the screen with no signal connected.)
6. The TIME/DIV control
determines the horizontal scale of the graph which appears on the oscilloscope
screen.
With 10 squares across the screen and the spot moving at 0.2 s/DIV, how
long does it take for the spot to cross the screen? The answer is 0.2 x 10 = 2 s.
Count seconds. Does the spot take 2 seconds to cross the screen?
Now rotate the TIME/DIV control clockwise:
With the spot moving at 0.1 s/DIV, it will take 1 second to cross the
screen.
Continue to rotate TIME/DIV clockwise. With each new setting, the spot moves
faster. At around 10 ms/DIV, the spot is no longer separately visible.
Instead, there is a bright line across the screen. This happens because the
screen remains bright for a short time after the spot has passed, an effect
which is known as the persistence of the screen. It is useful
to think of the spot as still there, just moving too fast to be seen.
Keep rotating TIME/DIV. At faster settings, the line becomes fainter because
the spot is moving very quickly indeed. At a setting of 10 µs/DIV how long
does it take for the spot to cross the screen?
7. The VOLTS/DIV controls determine the vertical
scale of the graph drawn on the oscilloscope screen.
Check that VOLTS/DIV 1 is set at 1 V/DIV and that the adjacent controls
are set correctly:
The Hameg HM 203-6 has a built in source of
signals which allow you to check that the oscilloscope is working properly. A
connection to the input of channel 1, CH 1, of the oscilloscope can be made
using a special connector called a BNC plug, as shown below:
The diagram shows a lead with a BNC plug at one end and crocodile clips at
the other. When the crocodile clip from the red wire is clipped to the lower
metal terminal, a 2 V square wave is connected to the input of CH 1.
Adjust VOLTS/DIV and TIME/DIV until you obtain a clear picture of the 2 V
signal, which should look like this:
Check on the effect of Y-POS 1 and X-POS:
What do these controls do?
Y-POS 1 moves the whole trace vertically up and down on
the screen, while X-POS moves the whole trace from side to side on the screen.
These control are useful because the trace can be moved so that more of the
picture appears on the screen, or to make measurements easier using the grid
which covers the screen.
You have now learned about and used the most important controls on the
oscilloscope.
You know that the function of an oscilloscope is to draw a V/t
graph. You know how to put all the controls into their 'normal' positions, so
that a trace should appear when the oscilloscope is switched on. You know how
the change the horizontal scale of the V/t graph, how to
change the vertical scale, and how to connect and display a signal.
What is needed now is practice so that all of these controls become
familiar.
Up
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Connecting a function generator
The diagram shows the appearance of a Thandar TG101 function
generator, one of many types used in UK schools:
Again, your function generator, or signal generator, may look different but
is likely to have similar controls.
The Thandar TG101 has push button controls for On/Off switching and
for selecting either sine, square, or triangular wave shapes. Most often the 600
output is used. This can be connected to the CH 1 input of the oscilloscope
using a BNC-BNC lead, as follows:
Switch on the function generator and adjust the output level to produce a
visible signal on the oscilloscope screen. Adjust TIME/DIV and VOLTS/DIV to
obtain a clear display ond investigate the effects of pressing the waveform
shape buttons.
The rotating FREQUENCY control and the RANGE switch are used together to
determine the frequency of the output signal. With the settings shown in the
diagram above, the output frequency will be 1 kHz. How would you change
these setting to obtain an output frequency of 50 Hz? This is done by
moving the RANGE switch to '100' and the FREQUENCY control to '.5':
Experiment with these controls to produce other frequencies of output
signal, such as 10 Hz, or 15 kHz. Whatever frequency and amplitude of
signal you select, you should be able to change the oscilloscope settings to
give a clear V/t graph of the signal on the oscilloscope
screen.
The remaining features of the function generator are less often used. For
example, it is possible to change the output frequency by connecting suitable
signals to the 'Sweep in' input. The DC Offset switch and the Offset control
allow you to add a DC voltage component to the output signal producing a complex
waveform as described in Chapter
4.
The output level switch is normally set to 0 dB:
This gives an output signal with a peak amplitude which can be easily
adjusted up to several volts. In the -40 dB position, the amplitude of the
output signal is reduced to a few millivolts. Such small signals are used for
testing amplifier circuits.
The TTL output produces pulses between 0 V and 5 V at the selected
frequency and is used for testing logic circuits.
Up
Even a personal computer can be used as an oscilloscope with the right
software.
Oscilloscope for Windows is a Windows application that converts your PC into
a powerful dual-trace oscilloscope. Oscilloscope uses your PC's sound card as an
Analog-to-Digital Converter (ADC) to digitize any input waveform (speech, music,
electric signal, etc.) and then presents it on the monitor in real time,
allowing the user to control the display in the same way as on a conventional
"standalone" scope, for example change gain, time base or plot
Lissajous patterns.
MORE about this
application. You will even find downloads of the programs for free personal use
at the physics department web site.
SOURCE
Virtual Oscilloscope (Shockwave Simulation)
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