Using An Oscilloscope

What is an oscilloscope used for?
• Measuring time-varying signals - by showing details of the waveshape
• Measuring aspects of time-varying signals
• Frequency of a signal
• Peak value of a signal
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.

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.

Given 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.

What 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 the left.
• There are numerous controls to control things like:
• The time scale of the presentation
• A vertical scale
• A cable (IEEE-488) to connect the oscilloscope to a computer.  That lets you:
• Take measurements 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.

How 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.
• Your oscilloscope 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.

Showing 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).
• A sinusoidal signal.  In other words, it looks like a familiar sine wave.

What 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.

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.

The End

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.