Download Physics 201: Experiment #2 The Photo

Physics 201: Oscilloscope Exercise
Carl Adams
Winter 2006
Purpose
To become familiar with or be reminded of a few basic oscilloscope operations.
References
XYZs of Oscilloscopes
Barry Wallbank’s lab manual for Electronics
http://en.wikipedia.org/wiki/Oscilloscope not all that bad
Review some of your circuits notes and possibly review ideas concerning harmonic
waves.
Background
The oscilloscope is a very important experimental tool for studying time varying
electrical signals. Essentially it can give a time profile of a voltage.
The core of the oscilloscope is the cathode ray tube. (All of you saw my demo, right?)
For this reason in circuit diagrams it is referred to as a CRO (cathode ray oscilloscope).
There are plates that control the deflection of the beam vertically (typically the voltage
axis) and the horizontally (typically the time axis) just as we saw in the demo. The front
of the display has a grid so that you can see the position of the trace. There are controls
for the brightness and focusing of the trace (adjust these as necessary while doing the
lab).
The input has a high impedance (it is usually listed beside the input coupling) so it will
not draw leakage current and has a fast response time. The low side, grounded side,
black probe wire is always grounded to the external ground of the building so you can’t
hook two black ground wires to two different places. The input voltage goes through a
coupler, then to a variable amplifier, and then may have some other signal added to it
(controlled by vertical offset). The coupling might be DC (direct current, no processing),
AC (alternating current, the DC part is removed by using a low pass filter), or GND
(ground, the channel is grounded out and the measured signal should be zero). The
amplifier is rated in volts per division. This is sometimes called the sensitivity. Most
scopes have at least one more channel or input with the same circuitry.
The horizontal position is governed by a sweep circuit. Its speed is given in
“time/division” units that you control. The sweep occurs at the constant, given rate from
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Oscilloscope Exercise ______________
Physics 201
left to right. You can also adjust the horizontal offset. The maximum possible frequency
of the sweep circuit gives the speed of the scope (i.e. a 20 MHz or 100 MHz scope).
The key feature of the scope and probably its most confusing is the trigger. It gets its
name because once the trigger is activated (i.e. pulled) the sweep circuit activates at the
left hand side and the input voltage is connected to the vertical deflection plates and the
trace appears. Once the trace has reached the right hand side it stops the sweep and
resets/reloads the trigger. The trigger has a level and a slope for which it activates (and a
hold-off). If you are set to Normal the level is some fraction of the full scale vertical
deflection and slope is either positive or negative. If the level is turned above the
maximum signal (or below the minimum signal) the trigger never fires and you don’t see
anything! This is the common cause a blank scope. If you use the Auto function the
scope attempts to determine what level is best. You might get your signal back but you
lose some measure of control. The other important part of the trigger circuit is the
source. If you are putting a large signal into Channel 1 or A then you probably want to
have an Internal Source/ Channel 1. You might also have a small signal and put a large
synchronous signal in either Channel 2 or the External trigger input. One other option is
Line triggering which triggers to the 60 Hz AC electrical power input (most useful for
trying to detect 60 Hz noise or if you just want it to display even roughly so you can an
idea of the signal strength). There are also trigger coupling modes which are similar to
the coupling of the input channels and TV modes which are useful for looking at signals
with extra synchronization information included (like video signals, I have never actually
tried it!)
Other important options are the Display or Mode which can be set to display both A and
B inputs or even subtract the two. XY mode is often one of the limits on the “time base”
knob. Here the time sweep and triggering circuits are disabled and the horizontal
deflection (and sensitivity) are determined by channel 1.
The scope is connecting by using the probes. Make your connections on the bread board.
Wavegenerator operation is fairly straightforward. There are also handheld multimeters
for checking resistances and capacitances.
Procedure
1. Start with a 1 kOhm resistor (look in the cabinet for brown-black-red stripes).
Install in the bread board so that it is bridging one of the long vertical channels.
Connect the red and black alligator clips from the wave generator. Turn the
power on and switch to sine wave mode. Set the frequency near 1 kHz. Don’t
connect the scope yet.
2. Turn on the scope. Do you see anything? Try Line triggering and you should see
a trace flickering across the screen in a straight line. Adjust to the slowest
possible sweep rate and you should see the cathode ray tube dot. Does the sweep
rate seem about right? (try a one-Mississippi, two-Mississippi…) Now speed it
up by adjusting the sweep rate. Do you understand what is happening?
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Oscilloscope Exercise ______________
Physics 201
3. Connect the probe into the circuit with black to ground and the probe centre to the
red. What do you see now? This is a signal that isn’t triggered. Adjust the
frequency to near 60 Hz and the sweep rate so you can see two or three cycles.
Notice how fast the trace appears to move at the different frequencies. Why?
What if it were exactly at the line frequency? Adjust the horizontal position so
you can see the start of the trace. Note how it oscillates.
4. Change to CH1 source for the trigger. The signal will either freeze or disappear.
If it disappears adjust Level until it triggers. If you do see it adjust until it
disappears. Notice how Level adjusts the trigger point. Is the Slope control right?
Try adjusting it to see what it does? Do you see why it is necessary to have this
control. This is specifically called edge triggering.
5. Trigger the signal and measure its peak to peak voltage and period.
6. Now investigate the coupling controls for CH1. You should be able adjust the
DC offset of the generator. What happens to your observed signal as you switch
between AC and DC coupling for CH1? In DC mode turn up the signal until it no
longer triggers. Can you affect trigger by adjusting the trigger coupling controls
and the level? Make some observations.
7. Now try CH1 GND coupling. Did your trace disappear? How do you get it back?
Once you get it back readjust the VERT control until the trace coincides with the
x-axis of the grid. Now you know where 0 V is.
8. Go back to DC coupling and trigger the signal. Centre the signal by adjusting the
generator offset again. Turn the frequency up to 10 kHz and again check that
period measures with frequency. (you will need to readjust the time base).
9. Check the calibrated output on the front of the oscilloscope with your probe.
10. Look at the different waveforms that the generator can produce.
11. Turn the frequency up to its maximum with a sine wave output. Do you see any
limitations? How high a frequency and how short a time base can you reach?
12. Change to square wave. Now what do you see. Turn the frequency down a bit so
you can see the signal go to its square shape and make an estimate of the rise
time. Why is there a rise time?
13. Change back to sine wave. Construct a circuit with a 1 kOhm resistor in series
with a 1 kOhm and 100 pF capacitor in parallel. Connect your probe across the 1
k that is in parallel. (check these with handheld multimeter) Now what do you
see for rise time. (turn the frequency up to see this) What is the max voltage?
The max current?
14. Hook up the second probe across the input voltage and get both signals on the
screen at once.
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