Lab 7: Introduction to the Oscilloscope
INTRODUCTION:
The
oscilloscope, or scope, is one of the most widely used electronic
instruments. It can be used to observe
and measure voltage signals, either static or time- varying, over a very wide
range of amplitudes and frequencies. The
purpose of the present experiment is to give you an introduction to the
oscilloscope and several typical applications.
The Oscilloscope
An
oscilloscope makes a plot of voltage versus time and shows it to you on the
screen. It periodically updates the
display, so if the signal changes, the display will show the change. The screen itself looks a little like a
graph. The horizontal axis is the time
and the vertical axis is the voltage, see fig. 1 below.
The
oscilloscope will allow two different signals (voltage sources) to be plotted
at the same time, one for each channel.
The oscilloscopes you will be using will also display the scale for the
plots, i.e. how much each major division represents. The scale is at the bottom of the
display. In the figure 2 below, channel
1 has a scale of 5V per major division, and channel 2’s is also 5V per major
division. If the scale is 5V, then a
signal 2 ˝ major divisions high represents 2.5´5V
= 12.5V. (Note that the triangular waveform
in the figure has a range of about 2.4 divisions from the lowest part to the
top, or 2.4 divisions peak to peak. This
means that the peak-to-peak voltage is about 12V.) Similarly a time scale of 1ms means a trace
that extends 3 major divisions horizontally is 3´1ms
= 3ms long. (In the figure above, the
scale is 500ms per major division and the length of a
cycle is about two divisions, so the period is 1,000ms
or 1ms.) Note that the voltage scales
for the two different channels may be different, but the time scale is the same
for both.
There
are three sets of controls you can vary.
(See fig. 3.) They are labeled
VERTICAL, HORIZONTAL and TRIGGER on the oscilloscope.
The VERTICAL controls are under the
vertical heading and allow you to change the voltage scale for each channel by
turning the knob labeled VOLTS/DIV. You
can also move the display for the channel up and down on the screen by turning
the knob labeled POSITION.
The
HORIZONTAL controls are under the horizontal heading and allow you to change
the time scale by turning the SEC/DIV knob.
You can also offset the display from left to right by turning the
POSITION knob.
A
final control (for this lab) is the AutoSet
button at the upper right of the scope.
When you press that the oscilloscope tries to find a suitable setting
for the voltage and time scales. It is a
handy feature of these scopes.
PART I. DC
The DC power supplies have two terminals, a positive (red) and a common (Black). In this experiment, the positive DC voltage will be measured with the oscilloscope.
Connect a probe to the scope amplifier on the left. Plug the BNC end of the probe into CH 1
(channel 1) input on the oscilloscope and connect the other end of the probe
into the + terminal of the power supply using jumper wires provided. Connect the ground strap of the probe to the
common terminal in the power supply.
Push the POWER knob to turn on
the scope. Press the AutoSet button and look at the
display. The AutoSet button is in the upper right corner of the
scope. The auto set feature will adjust
the settings on the display in an attempt to make the display appropriate to
the input.
Now
measure the voltage put out by the power supply for different knob settings on
the power supply by noting the vertical deflection on the scope and the
vertical sensitivity as you change the voltage.
Try about 4 or 5 different voltages from about 1V to 15V. Simultaneously make the same measurement on a
digital voltmeter.
(You may have to change the volts/div
setting on the scope as you change your voltage. See fig. 3 above. The VOLTS/DIV knob is just above the place
you connect the probe for that channel.)
Make a table of your data showing the voltmeter reading and the
oscilloscope reading.
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Voltage DMM |
Scope
Voltage Scale |
Divisions |
Volts
(Scope) |
Volts
(Scope – Measure) |
% diff btw
DMM & Scope |
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PART II. AC MEASUREMENTS
THE FUNCTION GENERATOR
A drawing of the function
generator is shown below. It will
generate various types of signals at different frequencies. Two typical signals are sine waves and square
waves. It also allows you to choose the
amplitude and frequency of the signals.
The frequency is determined by the push
buttons across the top (left and center ones labeled 1M, 100k, 10k, 1k, 100,
10, and 1). These select the coarse
range and the dial gives finer control of the frequency. If the dial is at 0.8 and the 1k button is
pressed, the frequency is 0.8 x 1000 = 800 Hz.
(k is shorthand for 1000.)
A drawing of the function generator is shown above. It will generate various types of signals at different frequencies. Two typical signals are sine waves and square waves. It also allows you to choose the amplitude and frequency of the signals. The frequency is determined by the push buttons across the top (left and center ones labeled 1M, 100k, 10k, 1k, 100, 10, and 1). These select the coarse range and the dial gives finer control of the frequency. If the dial is at 0.8 and the 1k button is pressed, the frequency is approximately 0.8´1000 = 800 Hz. (k is shorthand for 1000.)
The type of waveform is selected by pushing one of the three buttons in the upper right above the label function. (But not the last one on the right labeled attenuator!) They are square wave, triangle wave and sine wave. The symbol above each mimics one cycle of the waveform.
The amplitude is controlled by the dial labeled amplitude. If it is set at the min. position, the amplitude is not zero, but about 300 mV peak to peak.
A. Sine
Wave. In this section, a
time-varying signal form a signal generator will be investigated. Connect the BNC cable from the generator
output marked
1. Record your voltage scale.
2. Determine the peak to peak amplitude and calculate the peak amplitude.
3. Calculate the RMS amplitude from the peak amplitude.
4. Use the Measure button on the scope to have it find the peak and RMS. How does your RMS compare the to scope’s value?
|
Voltage scale |
Div |
Pk to Pk Voltage |
Peak Voltage. |
RMS From Pk |
RMS Scope |
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B. Square
Wave. In this section, a
time-varying signal form a signal generator will be investigated. Connect the BNC cable from the generator
output marked
1. Record your time scale and measure the period (time for one complete waveform) in horizontal divisions and in time. Calculate the corresponding frequency.
2. Repeat for frequency setting of 100kHz. (You can change the frequency just by pushing the other frequency range buttons, i.e. the 100k.) You will need to push the AutoSet button after each change of frequency, or change the time scale manually by turning the knob under HORIZONTAL that is labeled SEC/DIV.
Make a table for these like the one below. (Remember f = 1/T.) Watch your units!
|
Generator frequency (Hz) |
Scope div. |
Time per div. |
Period |
Frequency (Hz) |
Scope Measurement Freq. |
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3. Timing Measurements
The Oscilloscope is often used for
timing measurements. For instance if you
want to find out how long it takes for an electrical signal to travel from one
end of a cable to the other end of the cable, you can use the oscilloscope to
measure that. You can hook up a
generator to the cable with a BNC “Tee” with one end of the Tee going to
channel 1 on the scope and the other end to the first end of the cable. The other end can go to channel 2 on the
scope. Let the generator put out a
Square Wave of about 200kHz and measure the time difference between when the
leading edge of the square wave arrives at channel 1 and when it arrives at
channel 2.
You can do a similar measurement just
using channel 1 if you don’t connect the other end of the cable to
anything. When the wave reached the
unattached end, it with reflect back.
You can measure the time between the arrival of the first edge and the
arrival of the reflection. Now the
signal has traveled twice the length of the cable (down and back).
If you measure the length of the cable, you can calculate the speed of the electromagnetic wave going down the cable.
I want you to measure the time it takes for the wave to go down the cable and measure the length of the cable and calculate the wave speed. You should sketch what you see on the screen of the oscilloscope and label the signals showing where you took the time difference.
If the wave speed is given by
find
the value of k for your cable.