Table of Contents

Introductory Tutorial: Oscilloscope Skills

To prepare you for later experiments in this course, we have designed a set of exercises to help you familiarize yourself with the capabilities of our oscilloscopes. Some of you have only briefly used 'scopes in your first year lab courses, while others may already be adept with them due to research experience. You should learn something either way, as the capabilities of scopes are truly impressive.

Hey, I know how to use a scope already! I did this in 211! Great! We'd still encourage you to go through the activities here as a refresher; as you may learn something useful you didn't know was even a feature of a scope. You're also going to hand in a characterization of four unknown waveforms for this, so if nothing else you can skip ahead to those parts.

Exercise 0: Why an oscilloscope?

Before we get too far into how to use a scope, we should mention why we use them in the first place. After all, doesn't a multimeter measure voltage too? The answer, in short, is time. Multimeters are useful for measuring static quantities that change slowly, but if the phenomena we care about takes place in seconds or less it just can't keep up. Not to mention, even if it could update quickly enough you'd never be able to record the values to interpret what you were seeing. An oscilloscope solves both these problems by being able to capture a snapshot of changes over a timescale of our choosing.

Anatomy of an oscilloscope

While it may look rather like a largish electronic calculator that would be insanely complicated to operate with about a hundred tiny buttons and a 4.5“ wide screen, Don't Panic. Controls are put into reasonable groupings, and there is quite a bit of feedback given on the screen once you know how to look for it. As you work through these exercises, we'll introduce both the controls and the location of relevant feedback on the screen.

The front panel of our oscilloscopes. Get used to seeing it, we'll be using it a lot today.

Exercise 1: Learning the controls

Getting started

The softkeys are the context-sensitive buttons just to the right of the screen. The Default Setup is usually towards the top-right corner of Tektronix instruments, but it varies from model to model.

Right now you should be seeing a yellow line in the middle of the screen with a bit of fluctuation to it. That's perfectly normal and the noise is probably some sort of 60 Hz signal that is picked up from power lines in the room.

As you do this, look at the left edge and bottom left corner of the screen. When the channel is turned on, the bottom part of the screen will display both the channel name and what the voltage scale is. In addition, a little yellow arrow indicates where the baseline of the signal is. (A blue arrow would indicate the baseline for channel 2).

The feedback text denoting the vertical offset is highlighted in pink

When you're turning the position knob, you can see that it shifts where the signal is displayed on the screen. This doesn't affect the values that you're measuring, but if you are trying to compare things and forget that you've shifted something then it can cause issues. Not only does it tell you what the offset is, it does so in both terms of a voltage and a number of divisions. This can be handy if you want to set a particular horizontal line on the screen as a reference point when you're looking at a signal.

The baseline of the trace is offscreen, denoted by the arrow in the top-left corner highlighted in pink.

Now, you'll see on the left-hand edge that instead of indicating a position, there will be a 1 with an arrow pointing down or up, depending on which way you shifted the signal.

Our signal's back on the screen! Huzzah!

When you change the zoom level on a channel, the number of divisions that it is offset remains the same. So, if you were on a 1V scale with 1 division of offset and change to a 5V scale, you now have a 5V offset.

Attenuation settings

For a whole host of reasons that we won't get in to, there is often a good reason to use a cable that attenuates (reduces) the signal reading by a factor of 10 before it gets to the oscilloscope. This makes it the default assumption that you're using such a cable when you press Default Setup. However, when we're not doing that, we'll need to change one setting to properly read out voltages.

You'll need to do this any time you press the Default Setup button, unless you don't mind all your voltages being off by a factor of 10.

Looking at a signal

That's enough playing with the position of the signal, let's actually look at something. We'll use a function generator to start off with, since 'scopes are quite good at measuring periodic signals. We'll start with our faithful companion of periodic signals, the sine wave.

At this point, you should see something like the following on your scope:

The default state of the scope and function generator when both are on. Note that the Output1 button on the function generator is lit up

There are a few things that have changed, even though you can't see the whole signal yet.

The frequency estimation acts as a nice sanity check sometimes, but may not be the best way of reading out an unknown signal. More important is the fact that the scope is now properly triggering.

Why is triggering needed?

One thing that becomes apparent after using a scope for a bit is that it can't continuously show you what your signal is doing. It takes a bit of time for it to gather together data points and to display them. You wouldn't want to just have the screen update as fast as the machine can go either, since if it was updating at a different rate than your signal you wouldn't be able to see anything very clearly. We'll explore this in a bit.

Looking around

Now its time to get a better look at out signal. Thanks to the function generator, we already know what we should expect: a 1kHz sine wave that has a 5V difference between the peaks.

The same setup with the trace scaled to be visible. Note that the scale in the bottom-left says 10V; I forgot to reset the attenuation settings before taking the picture.
Something weird happened when I zoomed out too far?!?

If you zoom out to a 250 ms per division time base, you may see something quite unexpected: a sine wave. This is due to a phenomena called aliasing. Simply put, if a signal is getting sampled at a rate lower than its frequency, you can get something that appears to be a lower frequency signal. There's a whole theory in signal processing called the Nyquist Shannon sampling theorem that goes into this, but a rule of thumb is that you need to sample a signal at at least twice its frequency to get a reliable measurement.

Getting data

You'll eventually want to actually get some numbers out of the scope, rather than just looking at the squiggly lines.

This is how all measurements on oscilloscopes were made in ye olde days. Sure, you might use a ruler or means of magnifying the image, but for a good few decades that was it. With a digital scope, we've got some additional options that can make life easier.

Cursors

Writing on the face of a scope is a good way to invoke the wrath of its owner, but we can do one better. The Cursor button will let us pick vertical or horizontal lines that we can draw across the screen and read out values with 50% less squinting.

The basic cursor menu. The location of the cursor button is outlined in pink, while the multipurpose knob is outlined in orange.

Your screen should look something like the following:

The difference between the cursor positions is shown on the middle-right of the screen.

All that being done, you'll probably have something that's around a 5 volt difference. You'll also probably notice that the cursors only move one pixel at a time, so the readout is quantized. Long story short, that quantization factor is 4% of the voltage division you have selected.

Cursors being used for a time measurement

From here, you'll notice that there are some additional features to the time cursors. Not only do you get a readout of the time since triggering for each cursor, you get the voltage at that point as well as the frequency and period associated with the interval. Neat!

Measurements

Another option we have is to use some of the scope's built in measurement features. This can be handy when there are multiple things you need to keep track of across both signals and the cursors won't cut it.

The measurement controls, with the button outlined in pink.

The right-hand side of the screen should now have options for measurement gating, the two input channels, the Math channel, and a Snapshot option.

The scope with period and frequency measurements turned on. The period measurement it highlighted in pink.

There should now be a box in the lower-left side of the screen showing you the period of your signal.

You can turn measurements off by toggling them again. These scopes can show up to 6 automatic measurements at once.

You might notice some more fluctuation going on here than you had with the cursors. That's because the scope is continuously calculating values based off of the current set of data it has gathered. The frequency in particular seems susceptible to wandering like this. Thankfully, the scope has a handy tool that helps us smooth out these fluctuations: the Averaging option.

Multiple measurements simultaneously selected for our signal.

Averaging

Averaging is just what it says on the tin: It will collect data from multiple triggering events and average out the readings. For something with a well-defined period, this is amazing for eliminating random noise. Let's check it out.

The Oscilloscope's Acquire menu. The button is in the middle of the horizontal control knobs, highlighted in pink.

Even averaging only 16 times should be enough to get a rock solid reading on the measured values here. In fact, the scope manufacturer essentially expects that you'll use averaging when making measurements. To go back to normal:

The middle choice (Peak Detection) is something that is used for hunting noise or fast events, we'll get to its uses later. You'll also notice that you can toggle the number of waveforms averaged from 4 to 128, with the downside being that you'll have to wait a while for things to settle for high averaging values

Triggering

We only mentioned triggering earlier, but its time to go back to it again since it will be the cause of many of your scope woes. The problem is that the scope gathers data too quickly to be able to display it continuously on a screen. One way of dealing with this might be to set a refresh rate (e.g. 50 times a second), but that would become a headache if you were even slightly off from the frequency you wanted to look at. Instead, what we do is specify when to gather data. We do this by selecting a voltage (positive or negative) and a slope (again, positive or negative). Modern digital scopes are able to buffer data continuously, so they can effectively show you what happened before the trigger event! On the other hand, analog scopes could only start drawing after the trigger and thus started on the far left side of the screen instead of the center. Hopefully it will start making sense as we play with the controls

Trigger controls

The trigger controls on our scope are grouped together on the right-hand side, consisting of a knob and two buttons. Well, that's not quite true. The Run/Stop and Single buttons on the top-right corner of the scope are somewhat related.

The scope toggled down to stop. The control button is in the upper-right corner and turns red when the scope is stopped. Note that the triggering text now reads Stop in red, shown in the orange highlight.

The most obvious thing should be that the waveform stops changing. You'll also see that the indicator in the top center of the screen will change to red text reading Stop. At some point in your career you may very well spend time debugging things only to discover that the problem was you stopping the scope display.

This should get the scope to gather a single trigger event and then stop, with the text Acq Complete in the top center of the screen.

This isn't a great example of how this would normally go, so let's change our setup.

  • Change the horizontal scale to 100 ms
  • Press the Single button and watch the trigger status text at the top of the screen

You should see the scope change to Armed in yellow for a moment, followed by Trig`d in green, and then Acq Complete in red again. Since you're asking for one second of data, it takes a moment for it to be gathered in this case. This is the sort of thing that is most useful for either very slow periodic events or single shot events.

Now let's play with the triggering itself a bit.

The place the scope centers your signal should shift as you move the trigger level. If you look at the center of the screen, the voltage of the sine wave should match your triggering voltage as you shift things about. There's a caveat here…

You'll see that when you first move off the waveform, the status will change to Ready before switching to Auto and your screen turning into wiggly soup. This is a feature, not a bug! The idea is that if you don't have your triggering set up to work for your signal, you might still want to see something to get an idea of what you're looking at. Hence, the screen will be snapshoting your waveform at intervals that won't line up with its period. The specific rate isn't specified in the manual, and depends on what your timescale setting it. Enough of that for now; I'm getting dizzy.

Now that we're back to a know state, let's mess with some of the other options.

Unless you've done something quite strange, your sine wave should be centered on a point where it is at 0V and decreasing. The little indicator in the corner will also flip directions.

New behavior! Now, instead of just showing you whatever, the scope will keep the last image that triggered it on the screen while it waits. You'll see that the indicator in the top-center of the screen will now read Ready instead of Auto. Normal triggering is generally more useful for intermittent events, when you don't know when the next one will happen but you don't want to look at dead time inbetween. But, if you want to see something anyways…

This lets you demand a refreshed signal now, which can help if you're not sure what's going on but don't want to tweak the trigger settings.

The other options here don't really make sense for this signal, so we'll save them for later.

Characterizing signals

If you'd like to capture an image from the scope using a computer, you can quickly jump to the section on how to Export images here. Taking pictures with your phone is fine for now too.

We're going to be asking you to characterize signals both in these exercises and throughout the quarter during your experiments. So, let's define what that means.

For a sine wave, this would be (at a minimum) noting that it is a sine wave, what the amplitude is, if there's an offset (i.e. if the baseline is centered around 0V or something else. For a periodic signal this would be the difference between the maximum and minimum), and what the period is. Basically, enough information that someone would be able to tell if the thing they were looking at was the same or not.

exampletrace.jpg
The first waveform you were looking at. It could be characterized as “A sine wave with a 2.5V amplitude (or 5V peak-to-peak amplitude), no offset, and a frequency of 1 kHz.”

Unknown signals 1 and 2

Now that you've gotten a bit of practice manipulating the scope and measuring things, it's time to move on to practicing with a signal you don't know everything about ahead of time.

  • Connect your UChicago Signal Blackbox™ to channel 1 in place of your function generator
  • Plug in the USB cable to power it on
  • The indicator light will flash through a few colors before turning green
  • Save a photo and characterize the signal
  • Now use the button to toggle to the second waveform
    • The indicator light will turn white while the new signal is loading, and then red when the second waveform is loaded. If its another color, push the button to cycle through options.
  • Save a photo an characterize the signal.

Remember to include your photos and characterizations in your lab report!

More tools

Rise time

Next, disconnect your UChicago Signal Blackbox™ and re-connect the function generator.

Let's look at something a bit different: a square wave! Press the Home button on the function generator and select the Square option from the listing of continuous waves. You can do this either via the touch screen or using the blue right arrow beneath the dial on the function generator to change menus and then turning the dial and using it as a pushbutton.

Unless you've been playing with settings, it should match your sine wave here. However, we might also want to know something about just how “square” that square wave is. Do do that, you'll need to zoom in on the time scale. A lot.

The screen should look something like the following:

A close-up view of a square wave's transition. Note the timescale here

There are any number of metrics you could use here to characterize how long the transition is, but one is to look at the time taken to go from one fraction of the initial value to a fraction of the final value. For electronic signals, 10% - 90% is standard.

You should get something on the order of a few to tens of ns here. You can also do this measurement via the built-in functions.

The value you get here is probably pretty unstable, as on this scale there's likely to be some noise on your signal.

The utility of averaging in this case is the same as before, but it may not be sufficient to completely stabilize the readings. While doing this measurement I watched the rise time change by more than 10% of the average value. However, it took long enough that I couldn't see any difference visually. That, however, ties nicely into another scope feature we can use.

References

Sometimes we want to compare a signal to itself under a differing set of circumstances. In other words, we want to create a Reference image. Thankfully, there's a handy way to do that in the form of a little white button on the scope!

Before we can view a reference, we need to first chose what trace to save. We can do so as follows:

The bottom of the screen should read “Ch1 waveform saved to Ref A” after you do this.

Now that we've saved a waveform, we can recall it using the Ref button.

The scope displaying a saved reference. The Reference button is outlined in pink.

The scope will now draw the saved trace in white, behind any other signals. This is handy to have when you're trying to optimize some difficult to characterize feature by being able to see how it changed relative to a baseline.

There are limits to the reference feature; specifically that you can't make actual measurements with it. It only captures the image, not the underlying voltage & time measurements.

There is another useful tool for comparing signals that's better for sporadic events

Persistence

Since our scope has digitized its data, it is straightforward for it to be able to show us an image of what the trace has been doing in the past. To access this feature, do the following:

The utility button, outlined in pink, and the display option, outlined in orange.

This will keep a darkened copy of every trace that's been displayed in the background. This is again only a visual indicator; you can't make measurements based off of the persistent image. But you can do things like use cursors to get a range of values, or estimate the frequency of events

An example of using time cursors and persist to make a measurement. Here I'm using the voltage division grid to identify when the persistent signal is at $\pm2$ V and placing the time cursors at those locations. In this instance, the fastest transition time is being identified. The pink lines on the screen indicate the intersection points.

To turn persistence off again, use the Utility Menu ⇒ Display option and change the Persist option to off

Unknown signals 3 and 4

Plug the UChicago Signal Blackbox™ again, and toggle to the third waveform (the indicator will be blue for this one)

  • Save a photo and characterize the signal
    • If you're having trouble triggering, try zooming in on the signal more.

Toggle to the fourth waveform (a yellow indicator)

  • Save a photo and characterize the signal
    • If the scope is in Scan mode the automatic measurements won't work. Cursors, however, will.

Exercise 2: Looking at events

Detection of singular events is not as relevant to the electronics course this quarter. If you want practice you can jump to the PHY211 Tutorial page.

Exercise 3 - Exporting data

There are two kinds of data you can export from the 'scope. A *.jpg screenshot of what is currently shown on the screen, and a *.csv file with the voltage vs time data.

The lab computers are already set up so that you can copy a screenshot or data from them by using some Tektronix software. To do so:

Alternatively, if you have a USB flash drive, you can plug it into the socket on the front of the scope to save your data. To configure what to save, do the following: