Measure Electronic Waves: Overview of the Oscilloscope
An oscilloscope displays a simple graph of an electric or electronic signal. This graph, called a trace, shows how voltage changes over time. The horizontal axis of this graph, reading from left to right, represents time. The vertical axis, going up and down, represents voltage.
Although oscilloscope technology has changed over the years, even older oscilloscopes are useful for basic circuit testing. If you invest in an oscilloscope, you'll have a tool that will last you many, many years.
The most obvious feature of any oscilloscope is its screen. On older oscilloscopes, the screen is a cathode-ray tube (CRT) similar to an older television or computer monitor. On newer oscilloscopes, the screen is an LCD display like a flat-screen computer monitor.
Here are a few things to notice about the display:
Gridlines are printed on the display. On most oscilloscopes, these lines are 1 cm apart, with ten horizontal and eight vertical divisions.
The vertical and horizontal lines in the middle are thicker than the other lines and include hash marks, usually 2 mm apart. These hash marks help you pinpoint the exact position of the trace between the major intervals.
Various knobs and dials on the oscilloscope let you set the scale at which the graph of the waveform is plotted.
The vertical divisions represent voltage. On most oscilloscopes, you can set the voltage scale to as little as 5 mV (millivolts) and as much as 10 V or more.
The oscilloscope usually represents 0 V by the horizontal line in the middle — so lines in the top half of the display represent positive voltage, and lines in the bottom half are negative voltage. Thus, if the voltage scale is set to 1 V, the display can show voltages between +4 V and –4 V.
If you set the scale to 2 V, the display can show voltages between +8 V and –8 V.
The horizontal divisions represent time. The maximum time per division is typically 0.2 s (seconds), and the minimum time is typically 0.55 μs – that's half a microsecond. There are a million microseconds in a single second.
To draw a waveform, the oscilloscope actually draws a single dot that moves across the screen from left to right. Each passage of the dot from the left edge of the screen to the right edge is called a sweep.
The vertical position of the dot indicates the voltage, and the speed that the dot moves is determined by the time interval, which is sometimes called the sweep time. Thus, if you set the sweep time to 0.2 s, the dot sweeps the display once every two seconds.
Most waveforms in electronics repeat at much smaller intervals than two seconds, so you'll usually want to shorten the time interval. As you work with your oscilloscope, you'll usually need to adjust the sweep time until at least one full cycle of the waveform you're examining can be shown within the screen.