Scopes and Function Generators (Day Two In Lab)


This lab is designed to introduce you to using an oscilloscope (hereafter referred to as a scope) to display, record and make measurements of time varying signals. In future labs you will be using scopes in a wide range of applications.

The following exercises are designed to give you experience in:

  • Finding signals.
  • Understanding triggering.
  • Making measurements of waveforms on the scope.
  • Using a function generator as a signal source.

After working through some basic exercises you will then use the skills you have learned to measure the time constant of an RC circuit.

As in the first part of this two week lab, these exercises and measurements are not experiments. They are training exercises that will prepare you to use scopes and function generators as tools in future lab experiments.

Apparatus

Each group has a function generator which will be used as a source of a time varying voltage.

Each student in a group has their own scope. Just as with the exercises in building and making measurements of DC circuits, the only way to learn how to use a scope is to actually work hands on with your own. You simply won't learn how to use a scope by watching someone else manipulate the controls. Sadly you also cannot learn to use a scope by reading a detailed description of all the controls and features, you really just have to dive in and start making measurements, figuring out how things work as you go along. For this reason every student must complete each exercise on their own apparatus, but you are encouraged to work together as a group, and even communicate with other groups, to figure out how to do what needs to be done.

Digital oscilloscope


The digital oscilloscope (hereafter referred to simply as a digital scope) is a measurement tool which is very commonly used in the physics laboratory. Here we provide some details on the basic functions and operation of digital scopes. You will then make use of them to observe and measure time varying voltages in some capacitor circuits.

Principle of operation

The oscilloscope is a fast voltmeter which can be used to give a graphical display of voltage versus time or voltage versus voltage.

Voltages are applied to the central pins of BNC connectors on the front panel of the scope. The shell of the BNC is held at ground potential via the ground pin of the scope’s AC line cord. The signal voltage is measured (sampled) many times each second. Each measurement of voltage and time is displayed as a lit pixel, sweeping from left to right across the display. The line produced on the scope is often called the scope’s trace.

In order to produce a stable trace, the sweep must be in sync with the test signal. This synchronization is achieved by triggering the scope. In order to obtain proper triggering, the user must make some adjustments on the scope. Some useful terms are defined in the next section.

Definitions

  • Triggering determines when the oscilloscope draws a signal on the screen. The trace will appear stationary if the scope is triggered at the same point in each cycle of the signal. That point on the signal at which triggering occurs may be specified by picking a particular value of voltage, slope, and offset as shown in Fig. 1.
  • The trigger level sets the magnitude of the voltage necessary to initiate a sweep.
  • The toggle between slope: rising or slope: falling sets the slope of the leading edge of the signal on which the scope is to trigger.
  • Adjusting the horizontal position will move the entire signal left or right on the screen.
Figure 1: A sine wave displayed on a scope that is set to trigger when the signal is $0 \text{ V}$ and falling.
What should changing these parameters look like?

As an illustration, Fig. 2. shows how changing the trigger settings can alter how the oscilloscope shows the exact same signal.  Note that the signal is always centered where the Horizontal offset and Trigger Level arrows intersect 

Figure 2: Examples of how changing triggering settings will change how the same signal is displayed.

There are many additional parameters that can be altered to change how a signal is displayed, such as:

  • Trigger source selects which input will receive the desired triggering signal.
    • Internal triggering tells the scope to use the signal applied to channel 1 or 2 as the trigger signal.
    • External triggering tells the scope that the trigger signal is being applied to the EXT input.
    • Line triggering tells the scope to use the $60\text{ Hz}$ power line as a trigger signal.
  • AC coupling places a capacitor in series with the input to remove any DC component from the signal. (DC coupling has no capacitor.)
  • The vertical scale sets the voltage scale of the vertical axis.
  • The horizontal scale sets the scale of the horizontal axis (which is usually time).
  • Menu buttons display options to be selected by the column of buttons at the right side of the display.

Tasks

Function Generator And Scope

The only way to begin to learn how to use an oscilloscope is to jump in and start finding signals and making measurements with one. You could spend a lot of time learning about the theory of operation of scopes, and details on what all the controls and options do, but in practice that is of very little help when it comes to using them for the first time.

For this part of the lab we will give you some straightforward tasks to accomplish using the scope. It is then up to you and your lab partners, with help from your TA, to figure out how to use the apparatus to find signals and make measurements. It may seem random and mysterious at first, but the more you work with scopes the better you will understand what they do and how to use them.

The first set of tasks are to setup a function generator to produce a sinewave and then configure your scope to display the function generator output and make frequency and voltage measurements.

Finding a signal

  • Begin by using BNC cables to connect the output of your function generator to one of the two input channels of your scope. You will have to use “T” adapters to connect the single function generator output to multiple scopes.
  • Turn on both the scope and the function generator. Make sure the scope is in it's default configuration pressing the restore defaults button on the scope. This ensures that it is not a weird state due to a previous group of students.
  • Configure the function generator to output a sine wave at a frequency in the 1 to 10kHz range, and with a medium amplitude.

Now find the signal on your scope and set it so that the amplitude of the sine wave nearly fills the scope screen vertically and 5 to 10 cycles of the wave are visible on the time axis. The waveform should be stationary on the scope.

At this point you can try making changes to the waveform frequency and amplitude on the function generator, or the type of waveform (sine, square and triangle) while observing the effect on the scope.

This represents one of the primary uses of scopes, they allow you to see a time varying electrical signal in real time. As you will see in future labs this is a very useful ability when it comes to setting up, debugging and understanding sophisticated experiments.

Measuring Voltages and Times

Once you have a stable signal displayed on the scope use the cursor feature to measure the peak to peak voltage of the waveform and its frequency.

You should be able to confirm the measured frequency with the display on the function generator.

AC/DC Coupling

How the input signal enters the scope is referred to as coupling. With most scopes you have the option to have your signal be either DC coupled or AC coupled to the scope. The coupling is controlled by the first menu item of each channel. DC coupling simply sends the signal straight into the scope which displays is as is. AC coupling sends the signal through a capacitor which passes AC signals while blocking DC signals. To see how this can be useful, and to preview a trick that you will make use of in future labs, setup your function generator to add as large a DC offset to the sinewave as possible. DC offset just means adding a fixed voltage to the time varying voltage of the sinewave.

  • Notice how the DC offset affects the signal displayed on your scope.
  • Now switch the scope coupling from DC to AC and observe the effect on the signal.
  • Do this for both a sine wave and a square wave output from the function generator

Where this is useful is when you are trying to see or measure a small change in a signal that has a large DC offset. The DC offset can be so large that the signal you are looking for gets pushed too far off screen to view on the scope. By AC coupling you block the constant offset voltage, allowing you to see the changes in the signal about the offset.

Triggering Exercises

Scope triggering is one of concepts that seems to confuse people the most. In principle it is pretty straightforward. The scope displays the voltage on the input in real time, triggering just means telling the scope when to start showing what it sees on the input by setting a voltage threshold. When the input signal exceeds the voltage threshold you set, the scope begins displaying the signal seen on the input. It is a straightforward, but subtle concept. The two most important factors in setting up triggering are:

  • Deciding whether to trigger on the signal on the scope input, or on a separate signal sent to the External (ext) trigger input.
  • Deciding whether to use Auto or Normal triggering mode.

Connect the TTL/CMOS output from the function generator to the EXT input of each scope. The TTL/CMOS output simply sends out fixed amplitude pulses at the frequency set for the function generator. This output can be used to provide a convenient external trigger.

Auto

Set the scope to trigger on Channel 1 in auto mode. Now adjust the trigger threshold level up and down to see what happens. You should see that the scope always triggers, but when the triggering threshold is set too high or low the waveform is not stationary on the screen.

Set the trigger threshold to obtain a stationary waveform. Now adjust the function generator amplitude and observe the effect on how the scope is triggering. You should find that if you reduce the amplitude of the signal enough the waveform is no longer stationary on the screen.

Normal

With the scope still triggering on Channel 1, set it to normal mode. Again play the game of making adjustments to the trigger level. You should see that when the trigger threshold is set within the voltage range of the sinewave from the function generator you get a stable trigger. When the threshold is above or below the voltage range of the signal the scope stops triggering.

Again set the trigger threshold to obtain a stationary waveform. Now adjust the function generator amplitude and observe the effect on how the scope is triggering. You should see that when the amplitude of the signal goes below the trigger threshold the scope stops triggering.

This is the main difference between auto and normal triggering. Auto trigger will always show something, though it may not show what you want and waveforms are sometimes not stationary. Normal triggering on the other hand requires you to properly set the trigger threshold in order to see the waveform at all, but once set the waveform will be stationary.

Neither mode is better than the other, which one to use depends on what you are using the scope for.

External

Sometimes you want to trigger on a signal whose amplitude you know will vary too much for you to be able to set a stable trigger on the input itself, but you have some other signal which you know is always in sync with the one you wish to display. In these situations external triggering is useful.

With the function generator TTL/CMOS output connected to the EXT input of the scope, change the trigger source from Ch1 to EXT. Now observe your sinewave while varying the amplitude of the signal. You should be able to make the amplitude of the sinewave go all the way to zero while maintaining a stable trigger.

RC Time Constant Measurement

Now that you have some experience using the scope to trigger on a signal and make voltage and time measurements you can construct a simple RC circuit and measure it's time constant. If you have not seen capacitance and RC circuits in class this page gives the basics.

Using the resistor and capacitor provided, build a simple RC circuit.

Send a square wave from the function generator through the RC circuit. The square wave output of from the function generator will alternately charge the capacitor positive and negative. When the square wave output is positive it will charge the capacitor positively at a rate determined by the time constant of the RC circuit. When the output switches to negative the capacitor will begin charging in the opposite direction at the same rate.

Use the scope to measure the time constant of the capacitor discharge. Compare your measured time constant with the expected value based on the resistance and capacitance values for your components.

Setup the scope to produce a stationary trigger on either the rising or falling edge of the square wave. This is the point at which the charge across the capacitor begins changing at the rate defined by the time constant of the circuit. If you zoom in on the time base of the scope you will be able to see the exponential growth or decay of the charge across the capacitor (depending on whether you triggered in the up or down swing of the square wave). Carefully expand the scope time base and voltage setting in order to fill the screen with the exponentially changing part of the signal. Use the scopes cursor measurement features to find the 1/e point of the curve and use this to measure the time constant of the exponential, which should be equal to the RC time constant of the circuit which you should calculate using the known values of the resistor and capacitor.

Note that this is likely to be a challenging measurement simply because it requires you to construct the correct circuit, properly set the square wave to drive the charge on the capacitor, connect the scope to the capacitor, and trigger and display the signal properly on the scope. However if you can successfully do this you will have learned 90% of what you need to know in order to use the oscilloscope as a tool in future experiments. And this is the point of the exercise. We are using the charging/discharging of a capacitor to produce an appropriate signal to find and measure, and which has a known value that you can compare to so that you know you did things correctly.