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How does an oscilloscope work


For a beginner an oscilloscope can seem quite a complex piece of equipment. How does an oscilloscope work is usually an early question for a beginner looking at the controls. There are quite a number of controls but taken one at a time there’s nothing too daunting to understand, especially when you go right back to the basic that an oscilloscope simply displays a graph of voltage against time.

The original technology of the oscilloscope was not a million miles away from that of a television. The first oscilloscopes were indeed known as cathode ray oscilloscopes. The technology in fact was probably a bit simpler than that of a television.

Originally a beam of electrons hit a phosphor screen and produced a dot in the middle of it. Although now different devices use different techniques to produce the same end the principle remains the same.

By applying a voltage to the oscilloscope probe the dot on the screen moves. A positive voltage will cause the dot to move up and a negative will move it down. What’s so fantastic about this? In isolation not a lot but it’s when you incorporate the other controls that things become interesting.

I’ll start by looking at a basic single channel analogue oscilloscope. Once you understand that the digital and storage versions are easy once you get the basics.

The screen is the first thing you notice, it has a grid overlay on it so you can measure voltage and time using it. Voltage is shown on the up and down vertical known as the Y – axis and time is shown going across horizontally on the X – axis. It’s important to know the X and Y axis names as lots of the controls refer to them.

In the original scopes when electron beams produced the dot you had an intensity control and a focus. The intensity made the dot brighter when turned up and the focus control was used to make the dot sharp and easier to read. You probably won’t see these controls anymore on the digital equivalents as the newer devices just simulate what used to happen so the optimal focus and intensity will be automatically be displayed for you.

Volts/Div.

The first control to get to grips with is the Volts/Div control. This is a bit like the voltage range control on a multimeter. It controls the amount the dot on the screen moves up or down with the voltage applied.

The grid on the front of the screen is divided up into what are known as divisions so if you set this control to 1 Volt per division a 1 Volt signal will move the dot up 1 division.

Setting this control to .5 Volts per division will mean the same 1 Volt signal will now move the dot up two divisions. You use this control to be able to see the signal on the screen. If you have a tiny signal to see you can set this control to show up to 50 millivolts per division on some oscilloscopes.

Y position or Y shift.

Somewhere near to the Volt/Div control will be the Y position or Y shift. To start with this should simple be set so that with no voltage present it makes the dot appear in the centre of the screen. You will then be able to see a positive and negative voltage equally when set in conjunction with the Volt/Div control.

X position or X shift.

Just as the Y position or Y shift control moved the dot up or down, the X position or X shift rather unsurprisingly moves the dot left or right. Set this so the dot is in the middle of the screen.

Time/Div.

At the moment the oscilloscope is just producing a dot. Using the Time/Div control you can get this to become a line.

In a similar way that the Volt/Div control works the Time/Div sets how the dot moves across the screen. If you set it to .1 second it takes .1 of a second to move across one square of the grid overlay and so take a second to move across ten of them.

You can see the dot move from left to right at this speed. As you switch this control up the moving dot will get faster and faster until it just produces a line.

Connecting a positive voltage to the oscilloscope will make the line move up the screen like the dot did.

Where an oscilloscope gets interesting is that by using the Time/Div control you can use the scope to see changing voltages.

how does an oscilloscope work triangle wave

Here you can see an oscilloscope displaying a triangle wave. The frequency of the wave is 753.0Hz. Time/Div is set to 200uS/Div and the Volts/Div is set to 2V/Div so you can see that as the triangle waves spans 5 divisions on the screen its about 10V peak to peak.

If you had an AC voltage of connected to the oscilloscope, assuming that the frequency of the voltage was 50 Hz, if the scope was set to just show a dot, dot would become a horizontal line as it would be moving up and down 50 times a second so it would be too quick to see moving.

If you altered the Time/Div control starting at .1 seconds you would get to a point where the display will show a waveform. For AC voltage it will be something like a sine wave. First of all it will show a large number of peaks but each time you switch up a setting you will half the number of waves.

how does an oscilloscope work sine wave at 500uS

A sine wave displayed at 500uS.

how does an oscilloscope work sine wave at 1ms

The same sine wave but with the Time/Div at 1mS.

That is basically how an oscilloscope works. There are a few other controls that you may need to use in practice. There a several different ways to set the trigger controls when you get more into advanced uses. While you’re learning the automatic mode is fine. You can use the other modes when you are trying to look for specific things in a signal. You may also need to use the X shift and Y shift to see things better. You may also have two channels which will lead you to have an extra set of controls which will seem more complex. It’s not, you just have an extra channel, and this can come in extremely useful if you’re trying to look at two things at once.

Now you can answer the question of how does an oscilloscope work, you’re ready for the next section on how to use an oscilloscope.