Recently I had an opportunity to talk to some budding engineers about oscilloscopes. It turns out that working in the electronics industry has caused me to take more than a few things for granted. When they asked “what’s a waveform” I wasn’t prepared with a quick answer. This led to the notion that a blog post on basic waveforms might help those who are just starting to learn about those squiggles on their scope displays. While waveforms are important in many disciplines, such as biotechnology, chemistry, seismology and many others, I’m going to stick to electronics. I invite others to add their thoughts on waveforms as well.
Oscilloscopes and Waveforms
When engineers, technicians and scientists want to see how voltage changes over time they reach for oscilloscopes. Often these changes represent information, and so we refer to them as signals. Oscilloscopes allow observation of one or more voltages that vary over time and present them as a two-dimensional graph. The vertical axis (Y) represents voltage and the horizontal (X) represents time.
It’s not as easy as it sounds. In the world of modern electronics, signals change millions of times per second. When you plot these changes on an oscilloscope, the graph will take on some shape. This shape is a waveform, and its characteristics can tell you a lot. (There are two excellent references on oscilloscopes below, but check out the graphic at Oscilloscopes: A Look Inside to learn more about a scope’s inner workings and basic waveforms.)
Waveforms can tell you many things about a signal including:
- The minimum and maximum voltages of a signal
- The frequency of an oscillating signal
- How a circuit changes a signal as it moves through the circuit
- How frequency or timing of the signal changes over time
- Whether or not a malfunctioning component is distorting the signal
- How much of the signal is noise and whether the noise is changing with time
Generic Waveforms
The term “wave” generally refers to a pattern that repeats over time. Examples include sound waves, brain waves, ocean waves and voltage waves.
Waveforms are a graphic representation of a wave. A voltage waveform is made up of time on the (X) axis and voltage on the (Y) axis.
Waveforms reveal a great deal about a signal. When there is a change in the height of the waveform, the voltage has changed and when there is a flat horizontal line you know that there has been no change for a period of time.
Straight segments show a linear change, a rise or fall of voltage at a steady rate. Sharp waves indicate sudden change in voltage. Simple enough, right?
Some generic waveforms and shape characteristics include:
- Sine Waves – A fundamental wave shape with harmonious mathematical properties. Sine waves can be the voltage in your wall outlet, test signals produced by the oscillator circuit of a signal generator and many AC power sources.
- Square and Rectangular Waves – The square wave is another common wave shape that turns on and off (high or low) at regular intervals. It is often used to test amplifiers – television, radio and computer circuitry often use these types of waves for timing signals. Rectangular waves are a lot like square waves except that the high and low time intervals are not of equal length. These types of waves are important when analyzing digital circuitry.
- Sawtooth and Triangle Waves – These jagged waveforms are made up of straight lines. They may be used to test systems designed to “scan” or “sweep” in a linear fashion, such as laser scanners, electron microscopes, and radio frequency scanners.
- Step and Pulse Shapes – Step signals indicate a sudden change you would see if you turned on a power switch, and a pulse signal is similar to the voltage changes you would see if you turned a power switch on and then off again.
Most real-world signals don’t have a tidy shape and regular, repetitive pattern. Many signals are not repetitive at all. And many signals combine characteristics from two or more of the generic waveforms above. As information technology has progressed engineers have worked to stuff more and more information into signals, so waveforms have become increasingly complex
Measurement
There are a handful of ways to quantify the characteristics of waveforms and by measuring these characteristics we can judge the quality of signals. Below are some of the most common characteristics of waveforms, all of which can be measured by looking at the X-Y graph on a scope display:
- Voltage – This is the amount of electric potential, or signal strength, between two points in a circuit. It’s common to measure the voltage from the highest peak to the lowest “valley”, called peak-to-peak voltage. There are several other voltage characteristics, including maximum voltage (see also Amplitude, below), minimum voltage, and root-mean-square voltage.
- Amplitude – Refers to the amount of voltage between two points in a circuit, measured from ground or zero volts to the maximum voltage.
- Offset Voltage – The average voltage of a waveform can be shifted above or below zero volts. This shift is called offset and it’s easy to measure by looking at the waveform on a scope.
- Frequency and Period – If a signal repeats, it has frequency. A repetitive signal also has a period, which is the amount of time it takes the signal to complete one cycle. Frequency and period signals are reciprocals of each other so that 1/period equals the frequency and 1/frequency equals the period. Electronic signals cover a wide spectrum from a few cycles per second (hertz), up to billions of cycles per second (gigahertz), or even higher.
- Rise and Fall Time – It takes some time for a signal to change from a low level to a high level, or vice versa. As you might guess, this is an important characteristic of waveforms in digital systems that encode information as high and low voltage levels. For modern digital systems, these times are really short – on the order of billionths of seconds.
Modern digital oscilloscopes have functions that make waveform measurements easier. There are front-panel buttons and/or screen-based menus from which you can select fully automated measurements including amplitude, period, rise/fall time and many more.
Wrapping it Up
So there it is, in simple terms, the basic ins-and-outs waveforms, and what you can expect to see on the display of an oscilloscope. If you are craving more, be sure to check out this How To Use an Oscilloscope video from a couple of our Tektronix experts or visit our website and download the comprehensive 60-page Oscilloscope Guide.
This is by no means the final word on waveforms. How would you describe what one is likely to see on a scope display? Chime in below!
Since publishing the above blog, Tektronix is currently offering discounts up to 89% on software application modules for qualifying oscilloscopes. Check out the details here and save away!