Displaying Electrical Signals on an Oscilloscope

By Cathleen Shamieh

An oscilloscope lets you look at an electrical signal by displaying how a voltage varies with time as a trace across a display. The vertical axis voltage indicates the amount of voltage (also called amplitude), and the horizontal axis represents time. (Remember graphing equations in math class? Well, the display on a scope is really such a graph.) Oscilloscopes always sweep left to right, so you read the timeline of the signal from left to right, just as you’d read a line of English on a page.

The signal that you observe on the oscilloscope is a waveform. Some waveforms are simple, some are complex. The four most common waveforms that you encounter in electronics are

  • DC (direct current) waveform: A flat, straight line.

  • AC (alternating current) waveform: This waveform undulates over time. The most common AC waveform is a sine wave, but you may also encounter triangle waves, sawtooth waves, and other AC waveform shapes.

  • Digital waveform: A DC signal that alternates between low (usually 0 V), which indicates logical 0, and high (usually 5 V), which indicates logical 1.

  • Pulse waveform: A signal that changes abruptly between low and high states. Most pulse waveforms are digital and usually serve as a timing mark, like the starter’s gun at a race.

An oscilloscope display has a built-in grid to help you measure time along the X (horizontal) axis and voltage along the Y (vertical) axis. Using knobs on the front panel, you select the voltage scale (for instance, 5 V/division) and sweep time (for instance, 10 ms/division) of the display. As you adjust these settings, you see the voltage display change proportionally. You can read a voltage level at a particular time by determining the position of the voltage on the grid and multiplying that by the voltage scale you’ve selected.

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A DC waveform’s vertical position (amplitude) gives you the DC voltage reading. For AC signals, the oscilloscope display enables you to determine voltage levels as well as frequency (the number of cycles per second). If you count the number of horizontal divisions that one complete cycle occupies on the screen, and multiply that by the time scale (for instance, 10 ms/division), you get the period, T, of the signal (the time it takes for one cycle to complete). The frequency, f, is the reciprocal of the period; the formula for f looks like this: f = 1/T.

When you’re testing voltage levels, you can often use multimeters and oscilloscopes interchangeably. The choice of which tool you use is yours, though for routine testing procedures you may find the multimeter a little easier. In general, you may prefer to use an oscilloscope for

  • Determining visually whether an AC or digital signal has the proper timing. For example, you often need this test when you troubleshoot radio and television equipment. The service manuals and schematics for these devices often show the expected oscilloscope waveform at various points in the circuit so that you can compare. Very handy!

  • Testing pulsating signals that change very rapidly. Signals that change faster than about five million times a second (5 MHz) are hard to detect with other test equipment, such as a multimeter or logic probe.

  • Visually testing the relationship between two signals, when using a dual-trace oscilloscope, a scope with two input channels. You may need to do this test when you work with some digital circuits, for example. Often one signal triggers the circuit to generate another signal. Being able to see both signals together helps you determine whether the circuit is working as it should.

    Here is a sample dual-trace display. The top signal represents the output of a 555 timer configured as an oscillator, and the bottom signal represents the voltage across a capacitor that is connected across the 555 timer output.

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