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.

As in the previous 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. Each exercise is specifically designed to give you experience working with the most commonly used controls and settings on a scope, and to illustrate some key considerations in their use.

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.

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 a capacitor circuit.

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

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.

• 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.

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.

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.

• Your apparatus should already be setup so that the output of the function generator is connected to the channel 1 input of each scope in series. You will be working with BNC cables and adapters such as T connectors.
• 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. You should be able to easily deduce how to do this just by looking at the controls on the function generator.

At this point make 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 illustrates the primary use of a scope, it allows you to see in real time how a voltage signal is changing. As you will see in future labs this is a useful ability when it comes to setting up, debugging and understanding sophisticated experiments. Entire experiments can be, and are conducted with a scope as the primary instrument.

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.

Configure the function generator to output a ~1kHz sine wave with an amplitude of ~10V and no DC offset (the DC OFFSET button should be in the out position).

• Set the scope to trigger on Channel 1 in auto mode.
• 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 something changes.

• Set the trigger threshold to obtain a stationary waveform just below its peak voltage.
• Now adjust the function generator amplitude so that it goes below the trigger threshold.

Write down notes about the effects you observe, you will be asked to describe them in your out of lab assignment. Sketches and photos of the scope screen may be helpful for remembering what was happening when it comes time to work on your out of lab assignment.

Switch the scope trigger to Normal mode.

• Adjust the trigger threshold outside of the voltage range of the waveform.
• Observe whether or not the scope continues to trigger under this condition.

Now change the frequency of the sine wave and observe what is going on with the scope.

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.

Write down notes about the effects you observe, you will be asked to describe them in your out of lab assignment. Sketches and photos of the scope screen may be helpful for remembering what was happening when it comes time to work on your out of lab assignment.

Sometimes you want to trigger on a signal which is difficult to find, but you have some other easy to find 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.
• Adjust the trigger threshold to obtain a signal.
• 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.

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.

• With the scope input set to DC coupling, set the function generator to output a sine wave with a frequency of ~10kHz and an amplitude of ~100mV (You will need to press the “-20db” button on the function generator), and with a ~1V DC offset (press the “DC offset” button on the function generator). Find the signal on the scope.
• Now switch the scope input to AC coupling and observe how both the DC offset and the shape of the waveform are affected.

Do not forget to reset the -20db and DC offset buttons when you are finished with this section.

• With the scope input set to DC coupling, set the function generator to output a square wave with a frequency of ~50Hz, an amplitude of ~2V and no DC offset.
• Find the signal on the scope. You should see that a clean square wave from the function generator.
• Now switch the scope input to AC coupling and observe how the shape of the waveform is affected.

Where this knowledge 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. But introducing a capacitor into the signal path can distort the signal, so which type of coupling you use depends on what you are trying to accomplish. You also need to be able to recognize the distorting effect of AC coupling because you may not realize the scope is setup that way when it should not be.

Write down notes about the effects you observe, you will be asked to describe them in your out of lab assignment. So take detailed notes and include sketches or screenshots of the scope display.

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. Confirm the measured frequency with the display on the function generator.

You do not need propagate uncertainties for this part of the lab. However it is important to understand what is it that limits your ability to know how well you know a measured value. For both the voltage and frequency measurements determine what would be the best estimate of the uncertainty in your measured values?

You will be expected to articulate the answer to this question in your out of lab assignment. So take detailed notes and include sketches or screenshots of the scope display. [EP] [SC]

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 do not worry, your TA will go over what you need to know. Additionally this page gives the basics.

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

Send a ~50Hz square wave from the function generator through the RC circuit. The square wave 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.

Note that this may 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.

For your individual summary discuss the following.

For the scope exercises we are not looking for specific right answers. We are looking to see if you can clearly and concisely articulate what you observed. Your answers should be scientifically correct and plausible. Stick with what you understand from your work in lab and avoid including meaningless statements for the sake of adding more to the discussion. You may want to include diagrams and screenshots of scope traces to help illustrate your answers.