Download The Oscilloscope - Physics Introductory Labs at Stony Brook University

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Lab 2 ­ The Oscilloscope [Stony Brook Physics Laboratory Manuals]
Stony Brook Physics Laboratory Manuals
Lab 2 - The Oscilloscope
The aim of this experiment is to familiarize you with the use of a cathode-ray oscilloscope. This instrument will be employed in some
of the later experiments, as well as in many electronics and research laboratories, so it is important to clearly understand its principles
and its operation.
The oscilloscope is somewhat similar to a traditional TV, where you have an electron beam in an evacuated tube hit a phosphorcoated glass screen to make light where it hits. The electron beam is deflected up/down and left/right by voltage applied to some
plates.
Besides the CRT, inside your oscilloscope are many electronic circuits that work together and can be controlled by you with various
knobs, pushbutton switches, lever switches, and other controls. There are also “connectors” that allow you to put electrical signals
into the oscilloscope and, in some cases, take electrical signals out of them. In front of the glass-phosphor screen on the CRT is a
rectilinear grid of lines that looks like graph paper having major divisions and minor divisions (“tick marks”) along each axis. You use
this rectilinear grid to make “readings” (measurements) from the screen of the oscilloscope, which may be voltage differences
(“voltages”) or time differences or quantities related to/derived from these readings.
Equipment
Oscilloscope
Dry cell battery
2 function generators
test leads for making connections
The oscilloscope you will use
Figure 2 shows a closeup picture of an oscilloscope. It may be the same model as the one you will use or it may be a slightly different
model that looks nearly the same. It doesn't make any difference which oscilloscope you actually use during the lab.
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External voltages can be applied to the y-axis or vertical amplifiers (gray “outer knobs” with red “inner knobs” control their gain)
that create a potential difference across the vertical deflection plates (they are horizontal in space) that control the vertical motion of
the electron beam and, thereby, the vertical position of the spot on the phosphor-coated screen. The voltage applied to the x-axis or
horizontal amplifiers create a potential difference across another set of deflection plates (they are vertical in space) that control the
horizontal motion of the electron beam. This “horizontal voltage” can be generated internally with a “saw-tooth shape”, moving the
spot with constant speed from left to right, and then back almost instantaneously. The horizontal speed (i.e., the frequency of the
saw-tooth) can be controlled by the “time-base” knobs (large gray knob on the right and small gray knob above and to the left of it).
Because the time-base voltage has a “sweep” time, the displayed signal appears as a graph displaying voltage difference (vertical)
versus time (horizontal). You read off the voltage increments or time increments from the “screen” with your eye.
The vertical signal can be amplified (calibrated amplification factors) and displaced vertically (“offset”) over the screen. The intensity
and focus of the spot on the display screen can also be varied. The vertical input can be put to “ground” potential GND or 0 V), or
coupled to the input signal connector directly (DC; direct current) or via a capacitor (AC; alternating current) that blocks any
constant voltage (dc) in the signal and passes and only the time-varying (ac) part of the signal to pass. Therefore, for example, one
can study a 100 mV AC ripple on top of a 5 V DC signal).
A second vertical input is provided, which allows the “simultaneous” display of two signals. This is not a truly simultaneous display
because the electron beam making the (moving) spot on the screen switches very fast between the two input signals for the display
(CHOP), or displays them one after the other alternatively on the screen (ALT). The two signals can also be added together (ADD).
If the left-right movement of the electron beam spot and the chopping of the electron beam spot between the two signal “channels”
are both so fast that your eye, as well as the “persistence time” of the phosphors on the inside (vacuum-side) of the screen, which
emit light when hit by the electrons in the beam, make what is happening appear continuous, what you perceive is a continuous
graph on the screen. The same thing happens in a TV. The image is “rastered” so quickly that appears to be changing continuously –
not in discrete jumps – to your eye.
In some cases it is of interest to display the voltage of one signal versus that of a second signal using the X-Y mode of the scope. In
that case the second vertical input connects to the horizontal amplifier (the internal saw-tooth generator is disconnected).
The TRIGGER is an important asset of the oscilloscope: it tells the instrument to start a horizontal sweep ONLY if the vertical
signal (the signal of interest) exceeds a preset threshold. The threshold voltage is set by the (TRIGGER level) and the direction can
also be chosen (+/- for direction of change, ie. going positive or going negative—trigger polarity). Thus, the horizontal sweep
becomes synchronized with a repetitive vertical input signal: such an input signal is written many times over on the screen and
thereby appears bright and stable in (horizontal) position. The trigger for the horizontal sweep can also be taken from an external
signal (TRIGGER input), or from the 60 Hz power line frequency (LINE).
Note: The two “channels” for the vertical (voltage) display both have their own both calibrated “click settings” (the outer gray
knobs) and a variable gain (inner red) knob. Usually you want the vertical gain set to a calibrated setting; that is, a fixed rather than
variable gain! Find these knobs and play with them. When done playing, return them to their calibrated positions, which requires that
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each red knob be turned clockwise until it “clicks”. The “time base” also has a “click setting” knob (the large gray one) and a variable
knob called “var sweep” (the smaller gray knob above and to the left of the large gray one). To have the time base correspond to the
“calibrated” time settings on the big gray knob, turn the small “var sweep” knob full clockwise. The “variable” knobs must be there
for a reason. Question: Why would you want to move the variable knobs off their “calibrated” positions? The red variable knob(s)
for voltage measurements? The small gray “var sweep” knob for the time base?
The available function generators
A function generator lets you produce either a square, sine, or triangle wave, with frequencies adjusted within a certain range you can
control. For some procedures described below, you will use one and then two “function generator(s)” to produce voltage vs. time
signals to observe with your oscilloscope. Therefore, you need to know something about them. Figure 3 shows a collage of four
different functions generators, any one or two of which may be the one(s) you use. Your Lab TA will explain what you need to know
about them. The one on the upper-right has a digital readout for the frequency [in Hz, old name: cycles-per-second (cps)] of the
periodic waveforms it can generate. The other three require you to “read the buttons and dials” yourself to set the frequency of a
periodic waveform you want it to generate. You will be able to check with your oscilloscope what is the actual frequency of the
generated waveform.
Procedure
This first lab serves as a familiarization session for making electrical measurements. Your lab notebook will be somewhat minimal:
you will show a simple electrical schematic of each experiment you do, and record some numbers. You should feel free to play
around and try things out, with just one point to note: Do not have the intensity control set too high, and never allow a small very
bright spot to remain stationary on the cathode-ray tube face. The screen can easily be damaged if the intensity is too high.
Follow the following steps to explore the capabilities of the oscilloscope:
1. Adjust the focus, intensity, the vertical and horizontal positions and the sweep time with no external input connected. You are
looking for a straight horizontal line running through the center of the screen. Set the input mode for CH 1 to DC.
2. Measure the voltage of a battery. To keep the scope trace sweeping across the screen, trigger off the 60 Hz AC power line by
using LINE triggering mode. Remember to correctly take into account the scale you choose for the input and include an
estimate of the uncertainty of your measurement (for example, 2.5±0.2 divisions multiplied by 1 volt per division gives
2.5±0.2 volts).
3. Set the function generator to make a 1 kHz sine wave. Set the scope to trigger on it in DC coupling mode. Record the precise
frequency from the readout of the generator (make sure you are using the generator with a digital readout for this part!).
Measure the period of the sine wave (the time from peak to peak) in at (for example) 5.6±0.2 divisions, times 0.2 msec per
division, giving 1.12±0.04 msec. Do the two frequencies agree within uncertainty? (They may not if either the scope or the
function generator have a calibration error - an example of the difference between precision and accuracy). Measure the
amplitude, and the peak-to-peak value, of the voltage of the sine wave.
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4. Change triggering from + to -, by pulling out the trigger level knob, and note what happens. Change the triggering voltage,
and note what happens in a sketch. Change from sine to triangle, and repeat. Change from triangle to square, and repeat.
(Why do things not change much with the square wave?).
5. Now connect a second function generator to the second input. You might set the first input to a 500 Hz sine wave, and the
second to a 400 Hz sine wave. Trigger off input 1. Trigger off input 2. Note the appearance of one input when you trigger off
of the other. Can you explain what you see?
6. Now generate Lissajous figures [http://en.wikipedia.org/wiki/Lissajous_curve]! Set the scope to X-Y input mode. Try and get
both function generators to about the same frequency and amplitude. Try to fine-tune one function generator to get as near a
circle or straight line as you can get. Look at the patterns that result as you detune the frequencies. Can you explain what you
see?
7. Now for the final test. Your T.A. will come by and mess up all the knobs for you. Get back to viewing a stable sine wave.
As mentioned before your lab notebook for this experiment will be pretty simple, basically consisting of some simple sketches and
schematics.. Make comments, supported with numbers from your observations, about how precise you think the oscilloscope is for
measuring voltage and for measuring time. Can you say anything about how accurate it is? Similarly, make comments from your
observations, if you are able, about how accurate you think the function generator(s) are. Since you used two of them in one part of
the lab, you could compare them. Can one be set more precisely? more accurately? than the other?
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