Oscilloscope Basics: Waveforms, Graph, & Measurement Reading

Types of Waves

You can classify most waves into these types:

• Sine waves.
• Square and rectangular waves.
• Sawtooth and triangle waves.
• Step and pulse shapes.
• Periodic and non-periodic signals.
• Synchronous and asynchronous signals.
• Complex waves.

Next we'll look at each of these types of waves.

Sine Waves

The sine wave is the fundamental wave shape for several reasons. It has harmonious mathematical properties"€it is the same sine shape you may have studied in trigonometry class.

The voltage in a wall outlet varies as a sine wave. Test signals produced by the oscillator circuit of a signal generator are often sine waves.

Most AC power sources produce sine waves (AC signifies alternating current, although the voltage alternates too; DC stands for direct current, which means a steady current and voltage, such as a battery produces.) The damped sine wave is a special case you may see in a circuit that oscillates, but winds down over time.

Square and Rectangular Waves

The square wave is another common wave shape. Basically, a square wave is a voltage that turns on and off (or goes high and low) at regular intervals. It is a standard wave for testing amplifiers. Good amplifiers increase the amplitude of a square wave with minimum distortion.

Television, radio, and computer circuitry often use square waves for timing signals. The rectangular wave is like the square wave except that the high and low time intervals are not of equal length. It is particularly important when analyzing digital circuitry.

Sawtooth and Triangle Waves

Sawtooth and triangle waves result from circuits designed to control voltages linearly, such as the horizontal sweep of an analog oscope or the raster scan of a television.

The transitions between voltage levels of these waves change at a constant rate. These transitions are called ramps.

Step and Pulse Shapes

Signals such as steps and pulses that occur rarely, or non-periodically, are called single-shot or transient signals.

A step indicates a sudden change in voltage, similar to the voltage change you see if you turn on a power switch.

A pulse indicates sudden changes in voltage, similar to the voltage changes you see if you turn a power switch on and then off again. A pulse might represent one bit of information traveling through a computer circuit or it might be a glitch, or defect, in a circuit.

A collection of pulses traveling together creates a pulse train. Digital components in a computer communicate with each other using pulses. These pulses may be in the form of a serial data stream or multiple signal lines may be used to represent a value in a parallel data bus. Pulses are also common in x-ray, radar, and communications equipment.

Periodic and Non-periodic Signals

Repetitive signals are referred to as periodic signals, while signals that constantly change are known as non-periodic signals. A still picture is analogous to a periodic signal, while a movie is analogous to a non-periodic signal.

Synchronous and Asynchronous Signals

When a timing relationship exists between two signals, those signals are referred to as synchronous. Clock, data, and address signals inside a computer are examples of synchronous signals.

Asynchronous signals are signals between which no timing relationship exists. Because no time correlation exists between the act of touching a key on a computer keyboard and the clock inside the computer, these signals are considered asynchronous.

Complex Waves

Some waveforms combine the characteristics of sines, squares, steps, and pulses to produce complex wave shapes. The signal information may be embedded in the form of amplitude, phase, and/or frequency variations.

For example, although the signal in Figure 6 is an ordinary composite video signal, it is composed of many cycles of higher-frequency waveforms embedded in a lower-frequency envelope.

In this example, it is important to understand the relative levels and timing relationships of the steps. To view this signal, you need an oscilloscope that captures the low-frequency envelope and blends in the higher-frequency waves in an intensity-graded fashion so that you can see their overall combination as an image you can interpret visually.

Digital phosphor oscilloscopes (DPOs) are best suited to viewing complex waves, such as the video signals shown in Figure 6. Their displays provide the necessary frequency-of-occurrence information, or intensity grading, that is essential to understanding what the waveform is really doing.

Some oscopes can display certain types of complex waveforms in special ways. For example, telecommunications data may be displayed as an eye pattern or a constellation diagram: