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.
Notice the following about
this situation.
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.)
Enter your answer in the
box below, then click the button to submit your answer. You will
get a grade on a 0 (completely wrong) to 100 (perfectly accurate answer)
scale.
Your grade is:
P2
What is the vertical sensitivity of the simulated oscillscope?
Enter your answer
in the box below, then click the button to submit your answer.
Your grade is:
Here is another kind of question.
Question
Q1
You have a signal that is somewhere in the neighborhood of 100 KHz.
That's a period of 10 msec.
The oscilloscope screen is 10 cm wide. What sweep setting would you
use if you wanted to display a few cycles of your signal across the screen?
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.
