Download FYSP105/K1 (FYSP110/K1) HOW TO USE AN OSCILLOSCOPE 1

FYSP105/K1 (FYSP110/K1) HOW TO USE AN OSCILLOSCOPE
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Introduction
In this exercise you will get basic knowledge about how to use an oscilloscope. You’ll
also measure properties of components, which you are already familiar with from the
previous
FYSP104
course
(exercises
”Basic
instruments
for
electricity”
and ”Measuring resistance” , or more thoroughly in exercise ”Basic components of
electricity”).
A cathode-ray oscilloscope is basically a device for measuring electric potential
(differences). More general it can also show e.g. time dependencies between different
measured quantities, provided that they can be transformed to electric signals. Such
properties are e.g. dc and alternating (ac) currents, frequency, phase difference, time,
sound and other waves.
Before performing the exercises, please, read some of the following literature e.g.:
•
Young & Freedman: University Physics (10th ed.), chapter 24-7: The Cathode-Ray
Tube, pages 751 – 754
•
Ian Hickman; Oscilloscopes
•
B+K Precision’s guide book to oscilloscopes
•
Manual of the oscilloscope used (Hameg HM 303-6)
The following report tells very briefly how use an oscilloscope. The questions in the
text are for learning. If needed, ask help from the demonstrator.
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The oscilloscope
Main parts of an oscilloscope are
•
cathode-ray tube with deflecting plate electrodes
•
inlet channels with amplifiers; the input signal is connected to the deflectors, which
bends the electron beam to a certain point of the screen (fluorescent material)
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sweep generator and trigger, which are needed in order to get stable images from
time dependent signals.
Fig 1. Main components of a cathode-ray tube
One of the most important parts of the oscilloscope is the electron gun which with
focusing electrodes accelerates a narrow electron beam typically to 5 keV (Calculate the
speed of the electrons. Do you need Einstein’s theory?). The electron beam travels in
between two sets of deflectors (horizontal and vertical) and finally hits the fluorescent
screen where it shows up as a bright spot. The size and brightness of the spot depends of
the FOCUS applied. The place of the beam spot depends directly on the potential
(difference) of deflector-plates, which is defined by the input signal – read Young &
Freedman (10th ed.) pages 751 – 754! Normally the voltage under study is connected to
the y deflectors and an internally produced constantly rising voltage to the x deflectors,
which allows us to inspect time dependence of the input signal.
Oscilloscope has usually two input channels, which allow you two monitor two signals
simultaneously. You may study signal(s) in an electric circuitry without disturbing it
since typically the input resistance or impedance is as large as ∼1 MΩ (in some
oscilloscopes you can set the input impedance say to 50 Ω). The analogue cathode-ray
oscilloscope has been replaced by a digital oscilloscope, where the electron-tube is
replaced by amplifier and logic circuitry. Measurement conditions and the measured
target define the type of an oscilloscope to be chosen. Typical quality criteria are:
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•
number of channels needed
•
band width defines the frequency range needed. Typically few tens of MHz.
•
sensitivity defines how week signals can be monitored.
Setting the potential range
To set the range of measured potentials the amplification of the y-scale in the
oscilloscope can be changed from the VOLT/DIV button (or VERTICAL SCALE
button). Two-ray oscilloscope has two of them. This allows you to study two signals,
which may differ by several orders of amplitude-magnitude, simultaneously. The
oscilloscope is calibrated so that the point the beam hits tells directly the amplitude of
the signal(s) fed in. The red fine tuning button in VOLT/DIV should always be clicked
to Cal position. If so, the position of the VOLT/DIV button (e.g. 20 mV/cm) tells what
is the relation between the distance on the screen and the voltage to be measured. The
screen is equipped with horizontal and vertical lines which are separated by 1 cm
distance from each other.
Setting the sweep speed
The measured signal is usually connected to the vertical deflectors. The horizontal ones
can be used to study time dependency of the signal. From TIME/DIV one can select a
constant sweep speed for the electron beam in horizontal direction. The TIME/DIV
scale (e.g. 10 ms/cm) is also calibrated by the manufacturer. Make sure that the central
fine tuning knob is in the calibration position. In some cases, fine tuning button can be
used to utilise the entire horizontal screen. Remember however, that the fine tuning
knob in wrong position is the most common source of error in the oscilloscope
measurements.
Synchronisation with trigger
The trigger defines when the oscilloscope starts to display the input signal. As the result
of the well defined trigger the picture on the screen for every sweep is made to overlap
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with that from the previous sweep. The guarantees, that the user can see clear picture of
the signal instead of complete mess filling the whole display resulting from random
trigger. The horizontal sweep of the oscilloscope can be synchronised in several ways.
In NORMAL-mode the triggering depends on the value of the input potential. A
two-ray oscilloscope has two inputs and TRIGGER SOURCE should be used to select
the input channel wanted to trigger. The trigger happens when the input has reached a
value selected with TRIGGER LEVEL. SLOPE (or RISING/FALLING) defines
weather the rising or the falling edge of the input signal is wanted to give the trigger. If
the input does not reach the selected trigger level no input signal can be seen.
In AUTO mode the sweep potential starts rising from zero and drops back to zero when
reaching its maximum value (i.e. electron beam has travelled horizontally through the
display). This sequence is repeated continuously. In AUTO-mode the horizontal sweep
is completely independent from the vertical input. AUTO-mode is suitable e.g. when
you measure a DC-potential.
Usually the AUTO-mode is in fact a combination of auto and normal modes. The
trigger happens after a small delay time. If during this delay the selected trigger
conditions (the edge and trigger level) are fulfilled the trigger works as in normal mode.
Otherwise the trigger happens in auto mode.
TRIGGER SOURCE has in addition to the normal inputs 1 and 2 also two other
positions: EXT(ernal) and LINE. In EXT-position the trigger signal must be connected
to the EXT-input.
Settings of the input channel
The input can connected to deflector plates in with options DC, AC and GND. Direct
voltages are measurable with setting DC, only, as alternating ones are acceptable in both
DC and AC.
In GND (ground) position both vertical deflectors are connected to the ground potential.
Therefore there is no electric field between the deflectors and the electron beam shows
now the position of zero voltage. With setting GND the trigger should be AUTO. Why?
The position of the zero potential on the screen can be tuned with VERTICAL
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POSITION, POSITION or POS switch to a suitable place. This switch is in some cases
marked with arrow up down signs. In double-channel oscilloscopes both channels have
their own up-down switches. Digital oscilloscopes usually have only one switch, which
can handle both channels.
In DC-mode the signal must be connected to the upper deflector and the lower one to
the core of the coaxial cable and to ground of the oscilloscope. Now the position of
electron beam shows ”the absolute” potential of the measured signal compared to the
ground potential. In AC-mode the measured signal (the active center-cable) is
connected to the upper deflector and the core to lower one, but they are left without
grounding. This allows you to measure e.g. small fluctuations on top of a say 100V
DC-voltage.
XY-mode
You may use the oscilloscope as xy-plotter, when the TRIGGER is off and the two
measured signals deflect the beam in horizontal and vertical directions correspondingly.
XY-mode is suitable e.g. when studying the phase difference of two signals.
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Components of electricity briefly
Characteristic (curve) is curve, which represents current I through a component as
function of potential difference U between the in- and outlet.
Resistor is a component, which has a certain resistance R: with a potential difference U
between the in- and outlet of a resistor R through it runs a current I, which obeys the
relation I = U/R. Characteristic of a resistor is a straight line with coefficient 1/R. Note
that R of a resistor may depend of temperature and that the temperature of the resistor
usually increases with increasing I; in this case the characteristic is not a straight line.
How would it roughly look like?
Diode is a component, which allows current to run into one direction through it, only.
To the forbidden direction runs thus no current and to the allowed one only when the
potential difference reaches a certain s.c. threshold value. Above the threshold potential
difference the diode behaves like a resistor so its characteristic is linear.
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Measurements
The experimental part consists of the following exercises. Print a pre-filled
questionnaire found on the student laboratory web pages. Do not forget to use the
manual of the oscilloscope.
Getting acquainted:
Try out the AUTO and NORMAL modes of the trigger with a sinusoidal signal, which
you get either from a transformer (yellow box with text ”transformer electricity”) or
from a signal generator. Select the AUTO mode and tune the trigger so that the
sinusoidal input is seen on the screen. Now increase or decrease the trigger level so that
no trigger happens. How can you see when the critical value is reached? What happens
when the trigger level is turned so down (up)? Without touching the trigger level,
switch the oscilloscope to normal mode. Signal should disappear from the screen.
Why?
Tasks:
0. Calibration of the oscilloscope using CAL OUTPUT – option. Measure the
amplitude and one period of the pulse from CAL OUTPUT and calculate the
frequency of the signal. Is there any systematic error a) in the horizontal sweep b) in
the vertical deflection? If so, how much?
1. DC-potential. Measure electromotive force of a flat and a cylindrical battery using
the oscilloscope and with an analogue as well as a digital voltmeter. Don't forget the
error estimate!
2. AC-potential. Use about 5 V potential (the yellow transformer). Measure Umax and
Ueff with the oscilloscope as well as with the analogue and digital voltmeters.
Deduce the period time and frequency of the DC-potential. For the frequency use
whole width of the screen! Answer with reason, whether one should use one or
several periods. From what part of the period one should take the readings? Why
so?
3. Cumulative potentials. Use AC-potential and DC-potential of a flat battery as
shown in the arrangement of following figure. Can you tell where do you need the
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560 Ω resistors? Do they have to be just 560 Ω and equal in magnitude? Check the
resistances with a multimeter. Use two digital multimeters and one analogue one.
Try out different positions of the AC/GRD/DC-switches. What happens and why?
Sketch a figure.
4. Pulse generator. Signal generator is the blue box with text ”signal generator” and a
pushbutton on the side of it. Using the button you get a signal from the generator.
Determine amplitude, frequency and period of the signal. Draw a figure.
5. Signal generator. Using an oscilloscope examine the sinusoidal signal of a signal
generator (TRIO AG-202). Try out the various features of the oscilloscope
discussed earlier to obtain a proper representation of the signal. From the screen
determine the wavelength with errors and compare it with the value given by the
signal generator. Repeat this for at least 10 different frequencies on the range 10 50000 Hz. Compare the frequencies estimated from the screen and given by the
signal generator. Remember the possible systematic error found in exercise 0. Does
the amplitude of the signal depend on the frequency?
6. Diode and its characteristic. You’ll examine Ge- and Si-diodes. First test the
diodes with a digital multimeter according to the manual of the instrument and
define the threshold potential of them. Next examine the characteristic of the diodes
using the oscilloscope as a xy-plotter. Other components needed are the 4,7 kΩ and
220 Ω resistors as well as 5 V AC-potential (the transformer). Verify the resistances
with a multimeter and prepare an arrangement of the figure below. x-input is the
potential difference over the diode and y-input represents the current through it.
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While the oscilloscope can record only potential differences the current is measured
by recording the potential difference over the 220 Ω . Verify the zero-pints of both
inputs using the GRD-option. Determine the threshold potentials of the Si and Ge
diodes from the screen. The threshold voltage is read from the intersection of the
linear part and the x-axis.
Note that inputs 1 and 2 have a common ground. Therefore, make sure that your
circuit is exactly as shown. Since the grounds are the same, there is no need to
connect ground signal to channel other terminal (y-deflection in figure). Due to
setup used the polarity of the voltage connected to the Y-terminal is wrong, but this
can corrected for by pressing the INV(ert) button in.
Extra task:
Connect the same sinusoidal signal to inputs 1 and 2. Use the NORMAL mode and
select input 2 as the triggering source. Tune triggering and display conditions so that
both signals are seen. Remove input 2 by disconnecting the cable. What happens and
why? Now connect the trigger signal to input EXT and change the trigger mode to EXT.
There should be on the screen the picture of the signal connected to channel 1 and mere
line for channel 2.
In LINE triggering the trigger is taken from the line voltage. Line triggering is achieved
with the NORMAL node and ALT button pressed down. Take the input signal from the
transformer connected to the power line. What happens now and why? Take now the
input from a signal generator and examine it in LINE-position. What happens when you
change the frequency of the input signal? Why so?
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Results
A lengthy, self written report is not required. Return the notes made during the
measurements supplemented with the drawings and answers to the questions required in
Ch. 4. Error estimates are required only for those measurements indicated in the
questionnaire.