Download 124-02_Oscilloscope_Lab

PHYSICS 122 Lab EXPERIMENT NO. 1
The Oscilloscope
Goals:
(1) Learn how to use an oscilloscope.
(2) Measuring a DC voltage (constant in time) and an AC voltage (varying
in time as a sine wave)
Equipment:
1 oscilloscope
1 voltage generator
1 battery
1 tap key
4 wires
Fig. 1
AC Voltage Generator
Oscilloscope
Battery
(DC Voltage)
Wires
Tap Key
Introduction:
The oscilloscope is a very widely used laboratory instrument whose most usual
application is to give a visual display of time-varying voltages. While some
oscilloscopes now use liquid-crystal displays (LCDs), most (including the ones you will
use) still use a cathode-ray tube (CRT). Cathode-ray tubes were invented more than a
century ago, but they are still the most important component of television sets, computer
monitors and oscilloscopes.
The CRT works as follows. Within an evacuated tube (see Fig. 2), a heated
filament (much like the filament of an electric light-bulb) and a series of accelerating and
focusing electrodes (this assembly is called an electron gun) serve as the source of a wellcollimated beam of electrons, which impinge on the phosphorescent front surface (i.e. the
screen) of the tube, causing a visible spot to appear. Also within the CRT are pairs of
metal plates which, when an electric field is established between one or the other pair,
can deflect the electron beam horizontally or vertically, thereby moving the visible spot
to any desired point on the screen.
Fig. 2
In an oscilloscope the horizontal deflection (or sweep) plates are used to move the
visible spot at a constant speed horizontally across the screen. Thus the horizontal axis
represents time, the speed of the sweep and with that the time scale of the axis can be
adjusted by a switch. The vertical deflector plates are connected to the oscilloscope
input. Any voltage at the input will therefore deflect the visible spot vertically. Again
the scale may be adjusted by a switch. Effectively the oscilloscope enables you to
measure voltages as a function of time.
An important feature of the oscilloscope is that you can synchronize the
horizontal sweep with the input signal. This functionality is provided through the
“trigger”. In the simplest case the synchronization is achieved by requesting that the
input signal passes a voltage threshold which again is adjusted by a potential meter. As
the input signal passes the trigger threshold the horizontal beam sweep starts. An
oscilloscope is a powerful tool and has many features that this lab is designed to
familiarize you with.
Procedure:
The following instructions apply to the Elanco 20 MHz Storage Oscilloscope, Model DS203. They are meant to bring the oscilloscope into a state that allows you to make your
observations. You will not learn the functions of all the controls but focus on essentially
3 controls, which are labeled in the Fig. 4 below. With the oscilloscope directly in front
of you, set the controls as follows:
The white buttons below the display screen on the left should all be in the "out" position.
The controls to the right of the screen should be adjusted as given below:
INTENSITY:
FOCUS:
mid-range (line pointing up)
mid-range
TRIGGER LEVEL:
COUPLING:
SOURCE:
HOLDOFF:
XY:
<POS>:
POWER:
POS ^ (Ch I):
POS ^ (Ch 2):
mid-range
AC position
CH 1 position
MIN (ccw) position, knob pushed in
button in "out" position
mid-range
button in "out" position
mid-range, knob pushed in
mid-range, knob pushed in
VAR SWEEP:
AC AND DC (Ch 1)
CH I knob:
VERT MODE:
CH 2 knob:
AC GND DC (Ch 2):
TIME/DIV knob:
STORAGE/ ANALOG
calibrated (cw) position
switch to DC position
set to 1 V(volt), VAR knob in "CAL'D" (cw) position
CH 1 position
set to 1 V(volt), VAR knob in "CAL'D" (cw) position
set to DC position
set to 0.1 (s) position
button in "out" (ANALOG) position
Disconnect any wires that may be plugged into the front of the oscilloscope, and make
sure the power cord on the back is plugged into an outlet.
You are now ready to turn on the oscilloscope. Push the red "POWER" button to the "in"
position. After a few seconds, you should see a bright spot sweeping horizontally across
the screen about once per second. If you do not see the sweep, ask your TA for
assistance.
Once you have established the sweep, adjust the "INTENSITY" and "FOCUS " controls
to give you a sharp, bright spot (but not too bright; avoid a "halo") for clear viewing. Set
TIME/DIV to 1 ms – you should see a line across the screen now – and use the "POS"
controls to center the sweep on the screen horizontally and vertically. Once adjusted, set
TIME/DIV back to 0.1 s.
Push the "XY" button to the "in" position. When you do this, the internal horizontal
sweep is disabled, and you should see a stationary spot on the screen.
The Horizontal Time Axis:
Return the "XY" button to the "out" position to re-establish the horizontal sweep. Try
different settings of the "TIME/DIV" knob, and see what effect this has on the sweep
rate.
Q1. What is the relationship between the TIME/DIV setting and the horizontal
motion of the spot? (enter your observation into the lab book)
The Vertical Voltage Axis:
Now connect the battery in series with the tap key to the CH 1 input plug as shown.
Tap Key
Fig. 3
Vertical position
Vertical voltage sweep-CH 1
Horizontal time
Fig. 4
In real life:
sweep
With the CH 1 VOLTS/DIV set to 1 V and the TIME/DIV set to 0.1 s, try tapping the
key. Try several different settings of the VOLTS/DIV knob.
Q2. What is the relationship between the VOLTS/DIV setting and the vertical
deflection of the spot? (enter your observation into your lab book)
Measurement of the Battery Voltage:
A battery voltage is constant in time and is called a “DC (direct current) voltage”.
Measure the battery voltage with a voltmeter (supplied by the instructor) and record it in
your lab book
Now press the white STORAGE/ANALOG button to the "in" (STORAGE) position. Set
the VOLTS/DIV set at 0.5 V and the TIME/DIV set at 0.5 s and record these settings into
your lab book. When a new sweep starts push the tap key down ~5 times in succession, at
about once per second. You have generated a “square wave” voltage. Once the “square
wave” has been generated on your screen, quickly push the white button “SAVE ALL”
under the screen, because you need to sketch the trace into your lab book, and you
don’t want the next oscilloscope sweep to overwrite your stored picture. Release the
“SAVE ALL” button when done with the sketch.
Enter the lower and upper voltage reading into the table in your lab book. Estimate your
reading accuracy in the lower voltage reading in volts and repeat the same for the upper
voltage reading (presumably the same) and enter the values into the same table
Q3. Determine the voltage of the battery and its error with the oscilloscope.
Q4. Explain each part of the trace, i.e. label the parts of the trace that are valid for
the switch opened and closed. Enter the value of the voltage difference and the time
of one switch-off and one switch-on duration as determined from the trace with the
VOLTS/DIV and TIME/DIV settings. No errors needed. Is the time between
successive signals reasonable, i.e. consistent with the frequency with which you
tapped the switch?
Measurement of a Sinusoidal Voltage:
The voltage put out by the voltage generator changes with time and is called an “AC
(alternating current) voltage”.
Now set the STORAGE/ANALOG button to the “out” (ANALOG) position and set up
the voltage generator with the following settings:
FREQUENCY HZ:
1
MULT: (multiplier)
10k (=10,000) pushed in
Button with the sine-wave label:
pushed in
AMPLITUDE:
~ middle position
Connect the output “MAIN OUT HI” to Ch 1 of the oscilloscope and set VOLTS/DIV
to 1 Volts, TIME/DIV to 50 s. You should see ~ 4-5 sine waves.
Record TIME/DIV, VOLTS/DIV and the expected frequency from these settings in the
lab book.
Change the trigger setting toward the + and – sides noting where you loose a clear signal.
You will see a shift in the sine wave, but that is only a byproduct of what the trigger does.
Q5. Explain what the trigger does.
Now, set the trigger so that you clearly see the sign wave. Record the trace in your note
book. Measure the time difference between sine waves and get the period T from it.
Calculate the error of the time difference as you did it for the voltage difference in Q3
above. The time difference you measure is the difference between two time
measurements for which you have to estimate the reading errors and propagate them as in
Q3 above.
Q6. What is the amplitude (no error) and period (with error) of the wave?
Q7. From the period T calculate the frequency f = 1/T with error and compare to
the dialed frequency.
Write-Up
Your report must include:
1) Answers to questions 1-7.
2) Sketch of the square wave made by the battery (with labels!) and the sine wave
made by the voltage generator (with labels!).
3) The frequency that you set the ac signal generator to.
4) The Time/Div and Volt/Div setting for every measurement made.
5) The Time/Div and Volt/Div setting for every measurement made