Basic Waveform Analysis with an Oscilloscope

Learn about the current generation of oscilloscopes and how to use them to measure various circuit elements.

The oscilloscope is an invaluable diagnostic instrument that can be used to troubleshoot problem circuits, verify product design before delivery to consumers, and reverse-engineer products for "hacks".
We will explore the various uses of an oscilloscope using the Tektronix MDO3104 that was provided by Tektronix. Part 1 will show the versatility of the current generation of oscilloscopes.

Getting a Scope

When preparing articles, we typically provide a link to purchase the parts required from various vendors. Due to the high cost of the machine used in this article, it seemed prudent to find an option for readers to acquire a machine in some other manner. If you cannot afford to purchase a new oscilloscope, you can rent, rent-to-own, lease, finance, or purchase used machines from companies such as Microlease.

A Note Before We Begin

All examples in this article are based on the Tektronix MDO3104 oscilloscope.
While preparing this article I contacted several test instrument makers —Rigol and BK Precision also offered to make test instruments available for this article and I would like to thank all companies for their generosity. They are all world-class instruments.
This article is not intended to be a how-to guide to the Tektronix MDO3104. It is meant to show you the various capabilities and functions of most mid-range oscilloscopes through the use of the Tektronix MDO3104 as an example.
I will show the steps needed to use the machine I have—it is left to the reader to see the documentation on their specific scope for specific key-presses and menu-options. I use bold to indicate physical manipulation of the scope through knob-turn or button-press, and quotation marks to indicate a menu choice.

Oscilloscope Displays

Oscilloscopes allow us to determine relationships between certain variables in electrical circuits. Early oscilloscopes were only able to show the relationship that exists between potential difference and time. Today’s oscilloscopes continue the tradition of measuring voltage vs. time while also providing an extensive collection of sophisticated data-analysis capabilities, display features, and triggering options.
To understand what electrical relationships exist in your circuits, you have to know how to interpret what is presented to you.
This is a typical single waveform display in an oscilloscope, showing the time on the horizontal axis and potential difference on the vertical axis.



In the lower left part of the image, you will see  ① 500mV
That indicates two things:

• Channel 1 is displayed on the oscilloscope in yellow.
• For channel one each grid rectangle corresponds to 500 mV in the vertical direction. So we have "500 millivolts per division" with 8 vertical rectangle visible, and thus
  500mV1division×8divisions=4V

• visible in the vertical direction

In the bottom left, you will see another box that says AFG Sine 100.00kHz 1.0000 Vpp:

• AFG Indicates the Arbitrary Function Generator is active (I used it to create this waveform)
• Sine is the shape of the waveform
• 100.000 kHz is the frequency of the waveform: 100,000 cycles each second.
• 1.0000 Vpp is the amplitude of the transmitted waveform.

In the bottom-center there is another box with:

4.00 µs                      5.00 GS/s      ① ∫          T →▼0.000000 s     1M points      0.00V

• 4.00 µs is the value of each rectangle in the horizontal direction:  "4 microseconds per division."  The display includes 10 rectangles, so
  4.00μs1 division×10divisions=40 μs

of time is visible across the entire screen.

The oscilloscope is recording 5.00 GS/s, i.e.,

5×109

• samples per second.
• Channel 1 is used to control the triggering of the waveform.
• Triggering occurs on the rising edge of the channel 1 waveform.
• The image is centered at T →▼0.000000 s from the trigger point.
• 1 million (1 M) data points will be collected.
• Triggering occurs when a rising signal passes through 0 V.

How to Make Basic Measurements with an Oscilloscope

To illustrate just how far these oscilloscopes have come over the past few decades, I will begin by showing you how many different ways the oscilloscope can be used to make basic measurements of frequency (or period) and peak-to-peak amplitude.

Activate the Arbitrary Function Generator

Begin by connecting oscilloscope channel 1 to the Arbitrary Function Generator (AFG) BNC connector on the back of the scope.  Activate the Arbitrary Function Generator by pressing the AFG button directly above the Channel 1 probe input.  Press the first bottom menu button below "Waveform" and use rotary knob Multipurpose a to select "Ramp."

Turn on Channel 1

Press the Channel 1 button to activate it.  Rotate the Horizontal Scale knob clockwise to adjust the scale to spread out a complete wave over most of the screen.  Use the Horizontal Position knob to adjust its location on the screen if you like.

Using the Graticule to Make Measurements

The lines on an oscilloscope display are called a graticule.  There are major and minor gridlines (or dots) that are used to measure waveforms.  Major gridlines are displayed as solid or dotted lines that run the width or height of the oscilloscope screen.  The voltage and time that correspond to the divisions formed by the major gridlines are shown at the bottom of the display.  Minor gridlines are subdivisions between major gridlines.  There are usually 4 or 5 subdivisions between gridlines.  In the following example, I used the Horizontal Position rotary dial to move the waveform so that the positive peaks of the waveform line up with major vertical gridlines.



In the vertical direction there are 500 mV per division and the distance from lowest point to highest point is 4 rectangles for

500mV1division×4divisions from peak to peak=2000mVpp=2Vpp

.
In the horizontal direction there are 4.00 µs per division and there are 5 divisions before the signal begins to repeat, giving

4.00μs1division×5divisions1period=20μs1period

.

Using the Cursors to Make Measurements

Digital oscilloscopes take all guesswork out of using the graticule.  Activate the cursors by pressing the Cursors button and use rotary dials Multipurpose a and Multipurpose b  to move them to the parts of the waveform that you would like to inspect.  In the following example, I moved the cursors to the positive peaks of the wave.



You will see in the upper right corner a new box that has information about the values of potential difference and time for the points a and b.  Here we are interested in the interval of time between the two points, i.e.,

Δ20.00μs

.
To determine the Peak-to-Peak potential difference, switch to horizontal cursors by pressing and holding the Cursors button again, selecting "Cursors-Screen" and "Bars-Horizontal."  Then use Multipurpose a and Multipurpose b knobs to adjust the position of the cursors and "Cursors linked" to aid your adjustment.  You can move between horizontal and vertical measurements by pressing the Select button.



Here we are interested in the potential difference between the two points, i.e.,

Δ2.000V

.

Below is a video walkthrough of the steps you'll need to follow. Each step uses orange highlights to indicate the appropriate button or knob you'll need to use on the Tektronics model.

Using the Digital Volt Meter to Make Measurements

A useful feature found in mid-range oscilloscopes is the DVM (Digital Volt Meter).  The DVM tool does everything you might expect a basic multimeter to do.  Enable it by pressing the Measure button in the Wave Inspector box and then "DVM," and use Multipurpose a to select (for example) "Frequency."  Here I am displaying the frequency (in the center) along with frequency statistics (on the right).  I could also display AC+DC RMS voltage, DC voltage, or AC RMS voltage.



Press the bottom menu button below "Add Measurement" and use Multipurpose b to select "Period", followed by "OK".  Then repeat "Add Measurement" and use Multipurpose b to select "Amplitude", followed by "OK".  Now Period & Amplitude are shown at the bottom in a less obtrusive fashion.  You may remove them by selecting the button below "Remove Measurement" followed by "Remove All Measurements".  You may simplify the display at any time by pressing the Menu Off button.



On the MDO3104, up to four measurements can be displayed at the bottom; there are numerous measurements to choose from (see http://www.tek.com/manual-topic/measure for more information).

Measurements from the Terminal

Just when you thought I'd run out of ways to measure the same waveform, here comes the terminal.  I connected to the oscilloscope with Telnet, turned the DVM on remotely, set it to record frequency, and queried the value.  To accomplish this feat, you will have to plug the oscilloscope into a switch or router on your local network with an Ethernet cable.  The oscilloscope will obtain an IP address from the DHCP server and display it on the screen.



Then use your favorite terminal program to connect with the oscilloscope.  Here I used PuTTY to connect to the IP address on the screen using the Telnet protocol.
The default port is 4000.



You can accomplish the same on a Windows machine by opening the run dialog ("Windows Key" + "R") and typing "telnet 192.168.0.40 4000".

                    DVM:MODe FREQuency           // Sets DVM Measurement mode to frequency
DVM:MEASUrement:VALue?       // Queries the oscilloscope and returns frequency measurement
                  

Measurements from a Web Browser

Why strain your neck looking up or down at the oscilloscope screen when you can look at your computer monitor instead?  Here I have logged into the web interface to inspect my waveform.  It is as simple as navigating to the IP address of your oscilloscope on port 81.

Using Advanced Math

The math tool allows all manner of mathematical functions to be performed on waveforms—from simple arithmetic to Fast Fourier Transforms and everything in between.  Here I am using the frequency measurement function to determine the frequency of my waveform.
Press the Math button to enter the math menu, followed by "Advanced Math", "Edit Expression".  Use Multipurpose a to scroll through the functions and Select to choose "Frequency(1)" and finally "OK".



The math menu is somewhat difficult to see.  It is red in the lower left corner.  Where it displays 100 kHz, it is indicating 100 kHz per division in the vertical direction.  The horizontal red line (the result of the math module's computation) is one division above zero, which indicates a constant reading of 100 kHz.  The 2.00 µs indicates that there are 2.00 µs per division in the horizontal direction.

Additional Information

For more information to help you understand this particular oscilloscope, check out these resources:

• Tektronix Oscilloscope Basics
• 3104 Teardown
• B&K Precision Oscilloscope Guide
• MDO3000 Series Manual

What's Next?

In articles that follow, I will use the Tektronix MDO3104 oscilloscope to troubleshoot a variety of digital circuits and to decode digital signals.