How oscilloscopes work (2024)

by Chris Woodford. Last updated: April 29, 2022.

Blip... blip... blip... blip... woooooooooooooooo.... "Quicknurse, she's crashed... the paddles!" No TV hospital drama wouldbe complete without the sight and sound of a heart monitor by apatient's bedside. We've all watched those brightly lit tracesleaping up and down—but have you ever stopped to wonder exactly howthey work? Heart monitors like this are based on a kind of electronicgraph-drawing machine called an oscilloscope, which works a lot likean old-fashioned television set. Let's take a closer look at thesehandy instruments and find out how they work!

Photo: Blast from the past! Imagine trying to build a compact oscilloscope before miniature electronic components were readily available. That was the challenge that scientists at the NationalBureau of Standards and the US Navy's Bureau of Aeronautics (BuAer) faced back in the 1950s beforethey could get their hands on transistors. What they came up with was this remarkably compact machine, a Subminiaturized Radar Indicator Oscilloscope, which dates from 1954–1956.Photo courtesy of National Institute of Standards and Technology Digital Collections, Gaithersburg, MD 20899.

What is an oscilloscope?

You've almost certainly drawn charts in school and seen them innewspapers. Many of them show how a certain quantity of something(like a heart rate, the price of a corporation's shares, or acountry's exchange rate) changes over time: they have the quantityplotted in the vertical direction (known as the y-axis)and the time period plotted in the horizontal direction (the x-axis).

The trouble with charts like this is that they can take ages toplot—unless, of course, you happen to be an oscilloscope!It's a handy little gadget that draws charts automatically using signals you feed into it fromprobes hooked up to an electronic circuit, a scientific instrument,or a piece of medical monitoring equipment.

Artwork: Below: A typical chart/graph. This one shows the steady growth of e-commerce in recent years. The x-axis (time) runs horizontally across the page; the y-axis (revenue) runs vertically up the page.By courtesy of US Census Bureau.

What can we use oscilloscopes for?

Photo: A US Navy electrician uses an oscilloscope to check the performance of anelectric motor onboard an aircraft carrier. Photo by Paolo Bayas courtesy of US Navy andWikimedia Commons.

We can use oscilloscopes for looking at all kinds of signals in allkinds of ways. If you ever study electronics,you'll use oscilloscopes to watch how signals change in circuits over time; youcan also them to locate faults in broken televisions, radios, and allkinds of similar equipment. The probes on a typical oscilloscope letyou feed in electric currents through coaxial cables—but that doesn't mean an oscilloscope can only measure electricity. Plug in a transducer (which converts onekind of energy into another) and you can use an oscilloscope to measure almost anything.For example, you could use a microphone(a type of transducer thatconverts sound energy into an electrical signal) to study soundsignals with an oscilloscope; you could use a thermocouple (atransducer that converts heat into electricity) to study temperaturechanges; or you could use a piezoelectric transducer(which generates electricity when you squeeze it) to study vibrations—such as aperson's heartbeat.

One of the really useful things about oscilloscopes is the way they turn invisible signals into things we can see and comprehend. So, for example, you can't hear ultrasound—sound above the range of human hearing—by definition. But you can see and study it very easily with an oscilloscope. In the same way, oscilloscopes provide a very useful way for people with hearing impairments to see and study soundsthey may be unable to appreciate by any other means.

Photo: Making sound visible. You can use oscilloscopes as sound "visualizers". This is, in fact, the trace from a computer visualizer into which I've fed a snatch of Beethoven, but there's nothing to stop you doing similar things with a physical oscilloscope connected to a hi-fi (or an app on your phone).

How an oscilloscope works

A traditional oscilloscope works in almost exactly the same way asa traditional (cathode-ray tube) television; indeed, you'll sometimes seeoscilloscopes referred to as cathode-ray oscilloscopes or CROs. In a TV,electron beams are made to scan back and forth across a screen coatedon the back with special chemicals called phosphors. Each time thebeam hits the screen, it makes the phosphors light up. In less timethan it takes to blink an eye, the electron beams sweep across theentire screen and build up the picture you can see. Then they do itall over again. And again. And again.So you see a moving pictureinstead of a still one. (Take a quick look at our television article for a diagram showingyou how all this works in practice.) In an oscilloscope, the electronbeams work the same way but instead of building up a picture they draw a graph.When you watch a line being drawn on an oscilloscope screen, what you'reactually looking at is an electron beam wobbling up and down!

Here's something to note: the electrical signals feeding into the x and y connections effectivelybecome the x and y values on your on-screen chart. Since there's a one-to-onecorrespondence between these two things, a traditional oscilloscopeis an analog device. (Another way of lookingat it is to say the trace on the screen is an analogy of the thing you'restudying or measuring.)

Photo: An oscilloscope draws a trace (graph) of some quantity (plotted on the y-axis) that varies with time (plotted on the x-axis). One common pattern you'll see is this smoothly undulating, snake-like up-and-down trace, which is called a sine wave or sinusoidal wave (green upper line). Another quite common pattern is a sawtooth wave (the blue stepped trace shown underneath the sine wave).This is a demo screenshot from Oscium, a plug-in oscilloscope that replicates some of the features of a traditional oscilloscope on your smartphone or tablet.

Electronic graphs

How does an oscilloscope actually draw a trace?Imagine you are an oscilloscope!Imagine holding a pencil in your hand at the zero point on a pieceof graph paper. Now suppose your hand is strapped to two electric motors, one of which can move it by precise amounts in a vertical (y)direction (that is, up and down the page), while the other one canmove it in the horizontal (x) direction (across the page from side toside). The motors are connected to electronic circuitry thatcan sample signals of different kinds.

For starters, let's suppose weconnect the x-circuit to an electronic quartzclock. Each time theclock ticks, it sends a signal to the x motor that moves your handslightly to the right. So, over a period of a few seconds, your handmoves gradually to the right drawing a horizontal line as it goes.Now suppose we connect the y-circuit to some sort of electronicinstrument that detects a person's heartbeat. If the x and y circuitsare connected at the same time, your hand will move across the page,as before, but jump up vertically each time the heart beats, drawingthe classic heartbeat trace you see in TV hospital dramas. Replace the pencil and the graph paper with an electronbeam and a TV screen and you can see exactly how an oscilloscopedraws its traces. Each time a signal comes in through the y circuit,the electron beam jumps up. All the while, a time signal is makingthe trace move from left to right along the horizontal (x) axis.

Artwork: How an oscilloscope draws a sine wave. 1) Inside the cathode-ray tube (CRT), the electron gun (yellow) fires a beam of electrons (green dots) toward the phosphor screen. 2) With no signal connected to the scope, a timing circuit powers electromagnet coils (blue) that make the electron beam sweep slowly across the screen from left to right (effectively powering the x-axis of the graph). 3) When you feed an undulating signal (orange) into the oscilloscope's probes, a different circuit powers a perpendicular pair of coils (red) that make the beam sweep up and down. 4) Acting together, the coils make the electron beams sweep out an up-and-down, wiggling trace (a sine wave).

How do you use an oscilloscope?

It's simple! You connect the signal you want to study to they-circuit and use the x-circuit (sometimes called the time base) tostudy how the signal varies over time. Alternatively, you can connecta second signal to the x-circuit and then study how the y and xsignals vary together. With the oscilloscope switched on and pluggedinto a signal, you'll see a trace forming against the background ofthe on-screen "graph paper" (which is known as a graticule,marked off in squares called divisions).

If the trace is too small to see properly, you need to adjust thecalibration of the x and y axes—just like using a different sizedscale when you're plotting a chart on paper. If you turn the Time/Division control (often marked Time/Div or Secs/Div), you alter each x-axis division of the screenso the incoming signal takes more time to move across. For example,if a heartbeat is making a pulse every second and the screen is setto one second per division, you'll get a pulse appearing on eachdivision (line) of the screen. If you turn the Time/Divisioncontrol so it's set to 0.5 second per division, the pulses will spread out to takeup twice as much horizontal room (because one second of time is nowrepresented by two divisions of the screen). You can adjust the y-axiscontrol (often labeled Volts/Division or Volts/Div) in the same way.Generally, the idea is to make the trace spread out and fill theentire screen so you can use the graticule to make accurate measurements.

Types of oscilloscopes

CRO and LCD

As we've already seen, oscilloscopes were originally based oncathode-ray tubes (CRTs), which are relatively bulky, heavy,power-hungry, unreliable, and expensive. (You can see a great picture of the cathode-ray tube from an oldoscilloscope on Wikimedia Commons.) Just as CRT televisions havenow largely been replaced by more convenient LCDtechnology, so many CRT oscilloscopes have been replaced by flat-panel LCD screens.Instead of using moving electron beams to draw traces, LCDoscilloscopes use digital electronics to draw a traceinstead—effectively mimicking what's happening with the oldertechnology. LCD oscilloscopes tend to be much cheaper and morecompact: you can even fit them in your pocket!

Photo: A typical, full-size digital oscilloscope. Photo by Brian Read courtesy of US Navy and Wikimedia Commons.

Unlike traditional oscilloscopes, which use entirely analogtechnology (displaying varying signals on the screen that correspond preciselyto the signals you feed into them), LCD oscilloscopes are generally digital: they use analog-to-digital converters to turn incoming(analog) signals into numeric (digital) form and then plot those numbers on the screen instead.

Photo: Digital oscilloscopes are much smaller and more portable thanold-fashioned analog ones. This one's a handheld Fluke Scopemeter being used to check communication signals in the electronic equipment rack behind.Photo by Andrew Lee courtesy of US Air Force.

Plugin (USB) oscilloscopes

Since your computer, tablet, or smartphone already has a CRT or LCD display, there's no actual need tobuy an oscilloscope anymore for occasional hobby use. Companies such as Cleverscope sell inexpensive, plug-in oscilloscopes(with USB connectors or equivalent leads for mobile devices) that simulate the circuitry in a traditional oscilloscope and display a trace on your PC or mobile screen. How convenient is that!(You can see a picture of a typical USB oscilloscope module on Wikimedia Commons.)

Oscilloscope apps

Search your favorite app store and you'll find quite a few hits for "oscilloscope," ranging from simpledemonstrations of signal traces up to fully functional scopes that take signals from plug-in probes. The basic apps use the microphone as their very crude signal source. With a USB adapter cable for your cellphone and a plug-in probe, your mobile becomes an instant, pocket oscilloscope! Oscium makes the best-known plug-in oscilloscope probe for the iPhone/iPod Touch and Android, and doubtless they're available from other manufacturers too.

Photo: Here are screenshots of two typical oscilloscope apps. 1) The very basic Oscillo app (available from the usual app stores) draws a simple amplitude trace of whatever sound signal is currently feeding in through the microphone of your mobile device. It's a great way to demonstrate to children how sounds of different tone and pitch make waves of different shape. Here, I'm humming a single tone into my mobile to generate an approximate sine wave—and I can vary the loudness and pitch and watch what happens to the trace as I do so. What happens if I whistle or hum, or try to make my voice sound like a trumpet or a flute? It's a great introduction to oscilloscopes and a much more interesting, interactive way of learning about waves than you'll find in traditional science books. 2) The slightly more sophisticated Sound Oscilloscope app (from Denis Bolshoiden, for Android) can draw the same amplitude trace or alternatively (as shown here) a frequency trace (FFT) of a sound signal from your microphone, so that adds an extra dimension to the activity.

Find out more

On this website

• Electronics
• Integrated circuits
• LCDs (liquid-crystal displays)
• Television (basic idea of cathode-ray tubes)

On other sites

Tutorials

• Using an oscilloscope: This easy-to-follow online tutorial is a great place to start if you've never used an oscilloscope before. [Archived via the Wayback Machine.]

History and background

• Oscilloscope history, on Wikipedia, charts the development of oscilloscopes from semi-automated graph-drawing machines through to modern digital scopes.
• VintageTEK: A part-virtual, part-real museum of classic Tektronix oscilloscopes (and associated gadgetry) with exhibits dating back over half a century.

Books

• How to Diagnose and Fix Everything Electronic by Michael Geier. McGraw-Hill, 2015. Includes an introduction to oscilloscopes, multimeters, and other test tools.
• Oscilloscopes by Ian Hickman. Newnes/Elsevier, 2001. A basic primer covering the same ground as this article in much more detail: what oscilloscopes are, what they do, the different types, and what you can use them for.
• Digital Storage Oscilloscopes by Ian Hickman. Newnes/Elsevier, 1997. A somewhat dated look at modern oscilloscopes, but still worth a look.
• How to Use Oscilloscopes and Other Test Equipment by R. A. Penfold. Bernard Babani, 1989. Covers modern digital oscilloscopes and how to use them.

Articles

• Jerobeam Fenderson's Trippy Oscilloscope Music by Neel V. Patel. IEEE Spectrum, February 19, 2015. How to use a CRT oscilloscope as an old-school visualizer!
• How to Build an iPad Oscilloscope by James Turner, IEEE Spectrum, August 31, 2011and Oscilloscope Turns iPad Into a Serious Science Tool by Brian McLaughlin, Wired, December 8, 2011. Two quick introductions to the Oscium app for iPads.
• The TI-Nspire and Lissajous Figures by William Hanna, Michael S. Davis, Kathleen Lynch-Davis and Tracy Goodson Espy.The Mathematics Teacher, Vol. 103, No. 8, April 2010. If you're struggling to teach (or learn about) Lissajousfigures with your oscilloscope, here's an alternative approach.

Patents

• US Patent: 3,795,834: Oscilloscope having external trigger display mode by Roland Andrews and Robert White, Tektronix, Inc. March 5, 1974. A detailed technical explanation of a typical analog oscilloscope from the early 1970s.
• US Patent: 3,816,815: Digital oscilloscope and method of storing and displaying waveforms by William Osbon, Nicolet Instrument Corp. June 11, 1974. The first digital oscilloscope, which "converts incoming electronic impulses into binary numbers and displays, measures and stores them."
• US Patent: 1,934,322: Cathode ray oscilloscope by William Osbon, Westinghouse Electric. November 7, 1933. An early design for a CRT oscilloscope.