| Using An Oscilloscope |
Measuring A Simple Signal
Here's the connections for a simple
signal measurement. The oscilloscope is being used to display the voltage
output from a signal generator. You should do that also.
- Set the frequency of the
signal generator output to something like 1 or 2 kHz.
- Set the amplitude of the
signal generator output to 1 or 2 volts.
- Connect the output of
the signal generator to the oscilloscope. Be sure that the two grounds are
connected together. If you use a coax cable make sure you have it connected
correctly.
Now you need to set the oscilloscope so
that it can display the signal. If you're lucky the oscillscope will have an
autoscale button. If not:
- Be sure that the
timebase is set to something like 0.5 milliseconds/cm. A 1kHz signal has a
period of one millisecond. This setting will let you see a few cycles of a
1 kHz signal.
- Be sure that the
vertical sensitivity is set to something like 0.5 volts/cm.
- Adjust the trigger. The
oscilloscope needs a signal to tell it when to start the
- display process -
moving the dot across the screen.
Triggering the oscilloscope can be
frustrating if you don't understand the process.
- The trigger can be an
external signal, the power line, or the signal you are displaying. Usually,
the dot starts across the screen when the trigger signal goes through zero
volts. If you are using the power line, then you are triggering with a
signal that usually has no relation to the signal being displayed. When that
happens it is very frustrating trying to figure out why you see chaos.
- In multi-channel scopes,
you can trigger off Channel 2, when you're only putting a signal into
Channel 1. If there is no signal going to Channel 2, then you have no
trigger signal. You need a trigger signal, so don't do that! Set the scope
to trigger off Channel 1 if your signal is going into Channel 1.
- It is possible to get
the trigger level set incorrectly without knowing it. If your signal never
gets above 5 volts and the trigger level is at 20 volts, then you can spend
a lot of time wondering why you can't see your signal.
Some Simple
Oscilloscope Experiments
Being able to use an oscilloscope is an
important skill for anyone who takes electrical measurements. In this section
we'll present a few things you can do with oscillscopes to get better acquainted
with them.
The first thing to do is to become
acquainted with the oscilloscope you have and to learn its capabilities and
limitations. Here are some things to do.
- While displaying a
sinusoidal signal, determine the limits on time resolution for your
oscilloscope. Is the lower limit in nanoseconds/cm? Is the highest setting
in seconds/cm? When changing the time scale note how the scale changes.
Your scope almost certainly has settings like 10, 20, 50, 100 and 200
milliseconds/cm. Those settings almost always proceed in a 1-2-5 sequence.
- Adjust the amplitude of
your signal and determine the limits for vertical resolution. For very
small signals you may find that the noise you see begins to get large
compared to the signal.
- Change the signal form.
You may be able to show sine, square and triangular signals. If you can
display a square wave signal, try to determine how quickly the signal
changes. It can't change instantaneously, and there will be some transient
between voltage levels that you can see. Consider whether that transient
rise-time is due to the oscilloscope or the signal generator. Or is this a
situation where you can't tell what causes the effect?
- Using a sine signal
first, set the function generator for a 1KHz signal, then measure the actual
frequency on the oscilloscope. Your scope may have a built-in function for
that, or you may have to check the length of a period using the horizontal
scale setting. It's probably best to have your scope set for .5 or .2
milliseconds/cm so that you can see a full period for a signal with a 1.0
millisecond period.
Now, you have used several of the standard
test signals to learn your way about when using a scope. Still, it's nice to
have a real signal. Here are two alternatives.
- Get a microphone and
connect it to the oscilloscope vertical input terminals. It's easier to see
things if you sing a musical note because that gives you a more-or-less
periodic signal. Try singing eee, aw and ooo into the microphone and
observe the signals. You may be able to get a reasonably nice sine wave
signal with ooo. If you do, you have a great future as a sine wave
generator!
- Get a connector that
will fit the speaker/headphone output of the sound card on the computer
you're using - and you're using a computer if you're reading this. Play a
CD - one of your favorite songs - and observe the voltage output on the
scope. The author found a pair of very cheap headphones, about $1, and cut
off the headphones and used the cord to do this.
Let's talk about the connections you need to make
to the headphone jack.
- There will be a left
channel and a right channel output. Each channel goes to the phone for one
ear.
- One channel wire will
often be encased in a braided outer covering. The braided covering
(metallic) should be connected to ground on the oscilloscope.
- Leave the other pair of
wires from the headphone jack unconnected.
- Be careful that they do
not short together, thus shorting the output from the sound board.
- Plug the jack into the
sound board (line output if possible), and play a CD to observe the output
signal.
- If you have a two
channel scope, you can look at both channels simultaneously by connecting
the other pair of output wires, to the input for the second channel on the
scope.
- Be careful with the
grounds to be sure that ground is connected to ground.
There are a lot of other things that might
be possible for you to do.
- You may be able to save
your data in a computer file. If your oscilloscope has a computer
connection, investigate saving your data in a file. There may be special
programs on the computer to do that, and they are sometimes supplied by the
scope manufacturer.
- Save your data in a flat
file. That could be a file with a txt or dat extension. A flat file doesn't
have any formatting and just contains the characters that represent your
data. You can load a flat file into other programs for analysis. Programs
you might use are Mathcad, Excel or Matlab.
- You can use FFT
techniques to get the frequency content of the signal that you saved in a
file.
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