is an oscilloscope used for?
The oscilloscope is the most powerful instrument in our arsenal of electronic
instruments. It is widely used for measurement of time-varying signals.
Any time you have a signal that varies with time - slowly or quickly -
you can use an oscilloscope to measure it - to look at it, and to find
any unexpected features in it.
signals - by showing details of the waveshape
of time-varying signals
Frequency of a signal
Peak value of a signal
The features you see in a signal when you use an oscilloscope to look at
a signal are features you cannot see otherwise. In this lesson you
will learn about oscilloscopes and you should keep this goal in mind as
you proceed through the lesson.
a time varying signal that you need information about,
Be able to use an oscilloscope to portray the signal as a function of time.
Be able to measure signal parameters with an oscilloscope.
does an oscilloscope look like?
Here's a photo of
a Hewlett-Packard (HP) 54601A
Note the following features of the oscillscope
There is a CRT (Cathode
Ray Tube) screen on which the signals will be presented. That's at
There are numerous
controls to control things like:
The time scale of
A vertical scale
A cable (IEEE-488)
to connect the oscilloscope to a computer. That lets you:
with the scope
Put the measurements
in a computer file
Analyse the data
with Mathcad, Matlab, Excel, etc.
Notice that this
oscilloscope has two input channels. The controls for the two channels
are just to the right of the screen.
do you use an oscilloscope?
Plug it in.
That's not facetious.
Turn it on.
There is a push button at the lower right edge of the screen. It
says "Line" and indicates a "0" and a "1" setting. Depress that button.
Apply a signal to
the input terminals.
may have provision for more than one signal input. Choose Channel
1 if that is the case.
Make sure that the
settings match the signal. For example:
If you have a signal
at 1000 Hz, then the period of the signal is 1 millisecond (.001 sec) and
you would not want the time scale set so that you only display a microsecond
of data, and you also probably won't see much if you display 10 seconds
worth of data.
If you have a signal
that is 10 millivolts high, you won't see much if you set the oscilloscope
to shown you a signal at 20 volts full-scale. Conversely, you won't
see much of a 20 volt signal if the scope is set for 10 millivolts full-scale.
a Simple Signal on the Scope
To get familiar with the scope, you can show a sine signal on the scope.
We're going to ask that you show a signal with the following characteristics
1 volt (2v peak-to-peak)
signal. In other words, it has a peak
of 1 volt and a negative "peak" at -1 volt.
A frequency of 1000
Hz (i.e. 1 KHz).
signal. In other words, it looks like a familiar sine wave.
will the signal look like?
The oscilloscope has an illuminated dot that moves across the screen.
With no signal, it would look like the following.
When a sinusoidal signal is applied, then
the vertical position is proportional to the voltage at any instant.
If you applied a low frequency sine signal, you would get a track like
the one below.
If you have a sinusoidal signal that repeats every half millisecond - a
frequency of 2kHz - you would get a picture like this one. It would
appear to be stationary on the oscilloscope screen, but it really isn't.
It's just that it repeats so frequently that you see it as a constant image.
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.
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
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.
Triggering the oscilloscope can be frustrating if you don't understand
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
- moving the dot across the screen.
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.
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
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.
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.
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
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.
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.
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!
Let's talk about the connections you need
to make to the headphone jack.
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.
There are a lot of other things that might be possible for you to do.
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
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
Be careful with the
grounds to be sure that ground is connected to ground.
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.
That's it for this lesson on using the oscilloscope. Don't let that
stop you from trying other measurements with your oscillscope. Experiment,
and learn the other things your oscilloscope can do.