Using An Oscilloscope
What Is An Oscilloscope Used For?
What Is An Oscilloscope?
How Do You Use An Oscilloscope?
Displaying a Signal from a Function Generator

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.

Simulation

In this simulation, a simulated function generator is connected to a simulated oscillscope.  Both are simplified versions of real instruments.  Note the following.

• The function generator can produce a number of signals, including sine and cosine, square, triangular and sawtooth signals.  You can choose which signal the function generator produces by clicking on the appropriate button.

Notice the following in this simulation.

• An oscilloscope displays a signal, and there is a unique time when the oscillscope trace begins to move across the screen.  There may be a unique event that triggers the start of the display - when the oscilloscope trace begins to move across the screen.  In the simulation above, we have given you a button that starts the trace moving across the screen - a trigger button.

Clearly you cannot trigger an oscilloscope by hitting a button every time you want to observe a new trace on an oscilloscope.  Another alternative might be to let the oscilloscope free-run.  In other words, let the oscilloscope start another trace as soon as a trace is finished.  Here is a simulation of that situation.
Simulation - Free Running Oscillscope

In this simulation, the signal trace begins anew as soon as it reaches the right hand side of the oscilloscope screen.

• The value at which the trace starts is equal to the last value displayed at the end of the previous trace.
• That implies that the signal is displayed continuously, and that you see ever bit of the signal.
• If the sweep speed - the speed at which the trace moves across the screen - were much higher, the display would be a jumble.
• We can't speed up the sweep enough to really show you that.  We can, however, speed it up just a bit, and here is the simulation.
• Use the buttons to change the sweep speed.
• Adjust the frequency so that you don't have an integral number of cycles in one sweep.
Note the following about what happens when the sweep speed changes.
• When the sweep speed changes, the horizontal scale - the time scale - changes.  Although this is a simulated oscilloscope and function generator, we have designed things so that it is real-time.  In real oscilloscopes, everything is real time and when you change the time scale you change the sweep speed accordingly.  On an oscilloscope, you can always adjust the sweep speed to "match" the time-scale of the signal you are displaying.
• Example:  If you have a 1.0 kilohertz signal, the period is one millisecond and you would probably want a scale than ran over 2 milliseconds or something like that.

In a real oscilloscope, the trigger signal can be generated when the signal value reaches some particular level - the trigger level.  In most cases you can set the trigger level to a voltage value of your choosing.

Now that you have had a chance to experiment with the simulations above, it's time to define a few terms - and these are items you can control on most oscillscope.
• You can control the sweep speed.  Sweep speed is usually measured in units of time per distance, like milliseconds/centimeter.  This might also be referred to as the horizontal sensitivity.
• You can control the vertical sensitivity.  That's the measure of how sensitive the display dot is to voltage applied to the input terminals.  It is usually measured in volts/centimeter.

Problems

P1   In this simulation, determine the sweep speed.  Note that the grid lines are all 1 cm apart.  (Your monitor setting might change the scale!  Assume that the grid lines are all 1 cm apart.)

P2   What is the vertical sensitivity of the simulated oscillscope?