Download Excercises using the oscilloscope

Robert Röding/Mats Forsberg 2008-03-04
Institutionen för fysik, Umeå universitet
OSCILLOSKOPET
The purpose of the laboration
The purpose of this laboration is to teach you how an oscilloscope works, and give you some
experience on handling oscilloscopes as well as function generators.
Aim
After this laboration you should be able to
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Determine frequency, amplitude and phase shift, using an oscilloscope.
Obtain different types of AC-signals with a signal generator.
Use the oscilloscope in applications.
Equipment
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Oscilloscope Hitachi V-212.
Signal generator GFG-8016.
Loudspeaker and microphone
Description
The oscilloscope is an instrument which measures the electric potential as a function of time.
Essentially we electrons are fired at a flourecent screen, and the electron beam is directed horizontally
with a time varying electric field. This field is generated with a built in oscillator.
When the field strength between the deflection plates is 0 V/m, the electrons will hit the centre of the
screen (fig.1).
Fig. 1. The electron beam hits the centre of the screen.
If the field strength between the plates varies, the electron beam is deflected (fig.2).
Fig. 2. The Electron beam is deflected in an electric field.
The field strength is increased until the electron beam hits the edge of the screen. After that the
electric field is reversed and the electrons will hit the other side of the screen.
Of the oscillator generates a sawtooth potential between the plates, the electron beam will sweep over
the screen with the same frequency as the sawtooth curve.
Fig 3. Sawtooth curve
The frequency of the sweeping can be adjusted on the oscilloscope. The horizontal speed of the
sweep is expressed in terms of second/division, and with division one means one sqare in the grid on
the screen.
Once the electron beam is sweeping over the screen, the voltage we want to measure can be
cannected to the input of the oscilloscope. The input voltage is commonly reffered to as the signal.
The signal is first damped (usually with a factor 10) , and then amplified inside the oscilloscope and
finally connected to two vertical plates. The electric field between these plates deflects the electron
beam vertically.
Assume that we have the following signal:
Fig. 4. Sine signal
At a certain time t' the signal is connected to the oscilloscope. At this time the electron beam is at the
left side of the screen.
As time passes the electron beam will sweep over the screen due to the horizontal sawtooth potential,
and be deflected in the vertical direction due to the potential of the signal.
Fig. 5. How the signal can be seen on the screen after time t'.
When the beam reaches the edge of the screen it will return to the left side. Here the beam has to wait
until the signal has the same potential and sign of derivative before making the next sweep.
Otherwise the image on the screen would appear to be drifting.
Fig. 6. Difference between a trigged and an untrigged signal.
This condition of restart is called trigging. This means that periodic functions easily can be examined
with an oscilloscope.
Two channel oscilloscope
Oscilloscopes usually have two channels, which means that two signals can be measured at the same
time. In this laboration you will use a two channel oscilloscope, Hitachi V-212. A description of this
oscilloscope can be found in the appendix. There you can also find short descriptions of how the
different knobs and controls work. If needed you can also consult the manual of the instrument.
Some common abbreviations that can be encountered in the manual:
CRT
AC
DC
GND
MAG
CW
CCW
Cathode Ray Tube = katodstrålerör
Alternating Current = växelström
Direct Current = likström
Ground = jord (potential)
Magnification = Förstoring; förstärkning däremot heter GAIN
Clockwise = medurs
Counter-clockwise = moturs
Excercises using the oscilloscope
Now you are supposed to put the oscilloscope in a starting position. Begin by turning the oscilloscope
on, using the switch labeled POWER (1). The number within paranthesis refer to the number in the
manual in the appendix.
Choose to only show channel one by turning the knob MODE (18) to CH1. Set the switch INT TRIG
(26) at CH1; the trigging will now occur on channel one. Check that the trigger MODE (29) is set at
AUTO and that the trigger SOURCE (25) is set at INT.
The electron beam will now be sweeping over the screen from left to right, and can probably be seen
as a line. If nothing can be seen, try turning the position knob for channel one (16).
The focus and intensity can be adjusted with the knobs FOCUS (3) and INTENSITY (5). Do not use
more intensity than needed; otherwise the screen may be damaged. Adjust the focus so that the image
is as sharp as possible.
Set the switch AC-GND-DC for channel 1 (10) in position GND.
Adjust the time sweep TIME/DIV (22) to 1 ms.
Decrease the sweeping speed with TIME/DIV stepwise down to .1 s.
Now you should have a point sweeping over the screen with low speed from left to right.
What happend if you adjust the intensity? …………………………………………………..
How long does it take for the point to sweep over the screen?………………………………
What does .1 s mean here? ..………………………………………...……………………….
Choose to show both channels by setting the knob MODE (18) to CHOP and TIME/DIV at 1 ms.
Then place channel 1 in the middle of the upper half of she screen, and channel two at the lower half
by using the position adjustment knobs (16) and (17).
As stated previously a two channel oscilloscope can be used to study two signals at the same time.
Since we only have one electron cannon, the electron beam has to be used by both channels. This can
be done in two ways. Either we let the beam sweep through the whole screen one signal at a time
(alternernating sweep) or otherwise we let the sweep change signal rapidly and thus display small
pieces of the signals at a time(chopped sweep).
switch between MODE (18) = CHOP and ALT and see if you can see any difference on the screen.
Then adjust TIME/DIV to .1 s and compare ALT and CHOP.
Explain what happens:....…………..………………………………………………………………...
Why can't we see the electron beam jump between the two channels in the CHOP-mode? …………
…………………………………………………………………………………………………………
If we connect a signal to the oscilloscope, the signal will deflect the beam vertically at the same time
as the time sweep deflects it horisontally from left to right. This will give us an image of the curve
form of the signal.
On most oscilloscopes you can choose yourself where the trigging level should be, and if the trigging
shall take place at the downgoing or upgoing part of the signal. The knob LEVEL (28) is used to set
the level. It can be pulled out to trig at negative slope. Normally you will get a better trigging on a
large signal than a small one, and therefore you can choose if the trigging should take place on
channel 1 or channel 2. The choice is made with the switch INT TRIG (26).
On the oscilloscope there is a calibration signal ( CAL (31)). Conect this signal to channel 1. What
happens when we change from AC to DC on switch (10)? Explain!............... ……………………….
………………………………………………………………………………………………………….
Grade th axis in the figure and draw both images (AC and DC).
What frequency does the calibration signal have? ……………………………………………………
Funktionsgenerator
A function generator can be used to generate differend kinds of AC voltages.
The function generator GW, model GFG-8016G, can output sine, square and triangle voltages with
varying frequence and amplitude. The frequency is normally set with a number of buttnos, and a
knob. A continious damping of the amplitude with max 20 dB (decibel) or max 40 dB is done by
turning the amplitude knob. In the latter case this should be pulled out. This means that the signal is
damped with a factor 10 or 100 respectively. Another important function of the function generator is
frequency measurements.
The best way of familiarizing yourself with the function generator is to plug in an oscilloscope and try
out wich function the different buttons and knobs have. Do this for sine-, square and triangle voltages
and vary their frequencies. There is a row of buttons with wich you can adjust the frequency.
Observe! When you measure a voltage, it's the potential difference between two points that is
measured. When measuring with an oscilloscope it is easy to forget this since the oscilloscope is
grounded, and most signals are related to ground in one way or another. An oscilloscope
measurement can work well even thoug you only connect one cable. Try for yourselves!
Do the following excercises:
Set the oscilloscope at
MODE = CH1, TIME/DIV = 20 µs, VOLTS/DIV = 0.2 V, CH1 in position DC.
Set the sweep at the centre of the screen. Adjust different settings as needed.
Connect the signal from the function generator (OUTPUT) to channel 1 on the oscilloscope.
Move the image to the right so that you can see the beginning of the signal. Adjust the generator
setting so that you get the following six images on the oscilloscope.
Set the function generator to a sine wave with amplitude 3 V and frequency 1 kHz. The amplitude
is measured with the oscilloscope.
Measure the period of the signal with the oscilloscope and write down it's value. Does it match
the one on the function generator?
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Measure the voltage with a voltmeter in AC-setting. What doe it show? Compare with the
oscilloscope measurement and explain.
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See if you can get the same signal as the oscilloscope calibrating signal from the function
generator. (Tip: trigg the oscilloscope with the calibrating signal.)
Application: Determine the speed of sound in air
Here we will use an oscilloscope, speaker, microphone and a ruler to measure the speed of sound in
air.
Start by fastening the speaker on the optical bench. Turn down the amplitude to a minimum on the
signal generator and choose a sine wave. Connect the signal from the generator to the speaker and
channel 1 on the oscilloscope.
Place the microphone in front of the speaker and at the same height. Connect it to channel 2 on the
oscilloscope.
Adjust the sensitivity of channel 2 to max (5mV/ruta) and push the microphone as close to the
speaker as possible. Set the oscilloscope so that both signals can be seen and trigged.
Which channel is most convenient to use as trigger signal? .........................................................
Set the function fenerator at the frequency that gives the largest output from the microphone. Push the
microphone away from the speaker and look at both signals on the oscilloscope.
Explain what happens. ...................................................................................................................
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Figure out how to determine the speed of sound in air if you move the microphone a certain distance.
Your calculated speed of sound: ...................................................................................................
Extra excercise
Adjust the function generator so that is produces a square signal, then determine the speed of sound
without moving the microphone.
Your calculated speed of sound: .....................................................................................................
Bilaga – Sid 1
Bilaga – Sid 2
Bilaga – Sid 3
Bilaga – Sid 4