Equipment List:
Dual Trace Oscilloscope
HP function generator
Pasco frequency counter
2 BNC-to-BNC cable
1 BNC-probe
Purpose: To acquaint the student with the fundamentals of the
oscilloscope and gain confidence in measuring voltage signals with the
oscilloscope.
What there is to learn:
1. How to calibrate the scope.
2. Vertical and horizontal control of the beam.
3. How to find the ground position (zero volts reference)
4. How to remove DC offset from a signal (filtering).
5. Triggering and auto-triggering.
Introduction:
The oscilloscope is an instrument for measuring voltage as a function of time. The scope displays a graph, called a trace, of voltage (on the vertical axis) as a function of time (on the horizontal axis) and measurements are read from this graph using a grid of lines called "divisions" or simply centimeters. All the dials and switches on the scope exist to make measuring signals easy and accurate. Although the scope can do other things besides measuring voltage as a function of time, this is the instrument's primary purpose.
The following exercises should allow you to deepen your understanding of the scope. Follow them purposefully and do not rush through them and you will come to use the oscilloscope with confidence rather than frustration.
You should first complete the oscilloscope exercise sheets found in the appendix or given out by your instructor. This will prime you for what lies ahead.
While following these procedures bear in mind that the scope does not change an input signal but merely displays the same signal in different forms, large or small, compressed or extended, to aid your taking measurements. Changing a setting on the scope does not mean your measured value for the period or peak-to-peak voltage should change. The only exception to this occurs if the calibration dials have been re-set (see below, Calibration). The input signal changes when an adjustment on the function generator is made or the test circuit is changed. Adjusting the scope never changes the input signal.
What to put in your lab book: It seems natural that you would record all the specific dial settings for the measurement of a given trace. For example in the calibration setting, if you already had your voltage and time scalings recorded, the values would be quicker to set and you would be done with your calibration procedure sooner. Also, record definitions, explanations and insights so you can gain easy access to them in future crises (don't laugh).
Calibration: You should follow this procedure at the beginning
of every lab where the scope is to be used. You will notice a small and
delicate knob at the center tip of both your volts/cm dial and your
time
base dial. These smaller delicate knobs move independently from the
larger
knobs on which they sit. Note these "calibration" knobs change the
shape
(and the value of the measurement!) of the trace but again, not of the
input signal. Gently turn the calibration knobs for the voltage scaling
and the time base clockwise until they stop with a quiet click.
Start up settings:
1. Auto trigger should be on.
2. Trigger source should be on
int. (internal) not ext (external).
3. AC-GND-DC switch should be
on DC.
4. Mode switch should be on
channel
A (not dual)
5. Calibration knobs set all the
way clockwise.
Connect the BNC-probe cable from your scope's channel A to a small metal tab located somewhere on the front of your o-scope. This small tab (the output terminal) is your calibration signal's output. The terminal is connected to a square signal oscillator inside the scope, therefore the terminal provides an output signal just like the function generator except that you cannot alter its values. The terminal provides an easy to use test signal so you can find out if your scope is measuring accurately. The peak to peak voltage is specified near the terminal and is accurate to plus or minus six percent. The small slide switch on the probe should be set to 1X. If the frequency is not specified then it is the "line" frequency of 60 Hz or perhaps 1000 Hz; you should be able to tell which value for the frequency by measuring the period on the scope and take the reciprocal.
Practice measuring the test signal with the scope and see if you get the correct values for the peak to peak voltage and frequency. Is there DC offset in this signal (see DC offset below)?
Whenever you start to use an unfamiliar scope make sure the calibration knobs are turned all the way clockwise and clicked to the reference position.
Controlling the beam horizontally and vertically.
1. Getting started: Remove the BNC-probe cable and connect the
HP function generator's output with a BNC-to-BNC cable to Channel A of
the scope. The function generator should be set for a sinusoidal
output;
a small green light indicates the generator is turned on. Turn the
function
generator's frequency dial to read a very low frequency of about 2 Hz.
To achieve this, you may also need to push the correct power of ten
button.
2. Time Base setting: Adjust the scope so you can see the beam
moving up and down slowly, with no right and left motion. To do this,
turn
the time base setting to channel B or select the x-y plot feature found
nearby, which one of these options you have depends on the scope that
you
have, either way this feature will stop the beam from any left or right
motion. If the time base is set to channel B or the x-y plot feature is
selected, then the scope is waiting for you to control the horizontal
sweep
of the beam by connecting a signal to channel B. In this part you want
the beam to not move horizontally so leave channel B without an input.
It is interesting to note that the reciprocal of your time base setting
(in sec/cm) is the horizontal speed of the beam (in cm/sec).
3. Voltage scaling setting: Turn the voltage scale to a value that allows the vertical displacement of the beam to traverse at least four divisions. Understand how the amplitude dial on the function generator affects the vertical displacement of the beam on the scope because the actual signal's amplitude is changed. Changing the volts/div value on the scope does not change the amplitude of the input signal, it changes the appearance of the signal on the screen, making it easier to measure. You adjust the voltage scaling to make your function easy for you to measure not to change the value of the function.
If the function generator's output is sinusoidal and the frequency dial and order of magnitude setting for the frequency are low enough, then the motion of the beam is describing what is called simple harmonic motion. It could describe the undamped sinusoidal motion of a mass on a spring. See the effect on the motion of the beam when you push different output signals, square and triangle.
4. About the beam: Recall from your CRT lab, the
electron
beam only produces a dot but may appear to be a curve or straight line
if moving fast enough. It is not always easy to find the beam. To find
the beam use the horizontal and vertical position knobs and don't
forget
to have the intensity dial set about half way to full position. When
you
find the beam you don't want it to be too bright or the phosphors on
the
screen may become permanently damaged. If the beam has a "halo" around
it, turn the intensity down! When you are not using the scope don't
turn
it off, this shocks the circuitry, instead leave the scope on and turn
the intensity to a low level so the screen is not damaged.
Avoid turning dials aimlessly and especially quickly as if you
were spinning a roulette wheel in a casino. When you make an adjustment
to the voltage scale or the time base always consider what would happen
and why before you actually make the adjustment. Predict the result
before
you turn the dial and if you are wrong find out why and correct it. Ask
this question before changing a scale setting: "Do I want more or less
volts/div?". Answer the question then turn the dial and see if you were
right.
5. Sweeping the beam: Slowly increase the frequency of
the signal with the function generator's dial. Observe the
consequences.
Increase the frequency until the moving beam turns into a solid
straight
vertical line. Of course the beam is still moving but so fast it blends
into a solid line. The next step demonstrates the usefulness of an
oscilloscope.
Turn the time base dial from the channel B or x-y setting to a setting of 0.5 sec/div (seconds per division). Remember, the value 0.5 seconds per division means it takes the beam half a second to move one division to the right. Gradually turn the time base setting to smaller amounts of time per division. You should observe the beam going from a vertical line to a "trace" that is spread over the screen in the pattern of a sinusoid. You should understand that changing a time base setting to a smaller time per division is increasing the horizontal speed of the beam. Don't call the trace a "sine wave" because it is not a wave; call the trace "the trace" or perhaps as is common, "sinusoidal waveform".
The optimal trace contains 2 to 3 cycles on the scope face.
6. Measurements: Measure the time for one period of the oscillation you observe on the scope screen, take the reciprocal and compare with the frequency reading on the function generator; they should agree to within 5 or 10 percent. Measure the peak to peak voltage, Vp-p. In this lab you have no way of checking your Vp-p reading for accuracy other than in the Calibration section found above.
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The "ground switch": From any given trace observed on the scope face, you may be able to measure the difference between two potential values, however you cannot measure the electric potential of a single point in a circuit without knowing where the ground (V=0) value is on the scope face. By setting the scope switch to "GND" (ground reference), you will see the straight, solid, horizontal line representing the zero volts value. Using the vertical position knob you may place this reference value on a convenient grid line. Now set the ground switch to DC, you will see the ground level line disappear and the sinusoidal trace reappear. As long as you don't re-adjust the vertical position knob, you now will know where the zero volt value is.
DC offset: On your scope make sure the "DC" switch is selected
so your scope displays AC and DC signals together. Every AC signal may
or may not have a DC offset added to it. Connect a BNC-to-BNC cable
between
your generator and scope. Your HP function generator has a dial that
controls
the DC offset added to the AC signal. Find that dial on the generator
and
note that there is a blue toggle pin at the center of the dial. The
blue
toggle pin will immediately set your DC offset to zero when pushed in.
Push the toggle pin again so it is in the "out" position and your AC
signal
will have DC offset again. Confirm this effect by seeing the AC signal
on your scope move vertically as you adjust the DC offset on the
function
generator.
Next, experiment with manipulating DC offset on your scope.
Remember when you adjust your scope you aren't eliminating DC offset
from
the input signal, you are adjusting DC offset so you can see only the
AC
part of the input signal. This is accomplished by the "DC coupling"
button.
Find this switch on the scope. On De Anza scopes it is the same as the
ground switch. There is a "DC" setting and an "AC" setting. When the
switch
is set to AC, the signal has the DC offset removed (filtered). When the
switch is set to DC, the signal is displayed with DC offset. Make sure
your input signal does have a non-zero DC offset value at the function
generator.
Triggering: Recall that "triggering" is specifying the voltage at which the beam will start sweeping. Usually you should leave the trigger switch on "auto-trigger", but you should try adjusting the trigger level manually and see how it controls the beam's starting voltage. Try the procedure below.
Use the function generator as a source and a BNC-to-BNC cable to the scope. Create and display a sinusoid of frequency over 5000 Hz to reduce screen "flicker" and avoid eye strain. Now turn the "auto-trig" function off. Turn the trigger level dial until the trace disappears. Now turn on the auto-trig. Notice how the trace reappears but is now "moving" or "out of sync". You can induce that when auto-trig is on and your trace is moving out of sync, there is a good chance your trigger level setting is telling the beam to start at a voltage the signal never attains. Turn the trigger level until the trace stabilizes; now turn off auto-trig. The trace should now stay and not disappear. Continue turning the trigger level dial and see how it affects where the beam starts at the left side of the screen. If you know where the zero level is you should be able to measure the voltage of the trigger level.
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Hone your skill: Work with your lab partner and test each other to see if you can measure unknown quantities (Vp-p, frequency, DC offset, trigger level) with the scope; you will be tested on it.
See the appendix for some examples and self-quizzes on how to read
the
oscilloscope.