Scope Primer

Updated Thursday December 31, 2020

Note: This is a very brief introduction to the operation of an oscilloscope. For more details you should read a more detailed primer like the one offered by Tektronix. The section on operations is particularly relevant.

Overview

An oscilloscope is fundamentally used to make a plot of voltage vs. time. This is typically only useful for a periodic signal, and an oscilloscope has built-in circuitry, called a trigger, so that the same part of a periodic signal is shown on the screen each time. To properly operate an oscilloscope, you really only need to worry about a few things:

Note that the digital scopes we have in the lab advertise an automatic button which is supposed to properly set up the scope for typical periodic signals. I have never had much luck with this button, but you might want to try it out.

Signal Source

To measure a signal you have to provide a signal to the scope. The signal must come on a cable with a BNC connector. If you are trying to measure something else, you will need an adapter. One useful scope accessory is a scope probe, which will allow you to measure voltages anywhere you can get access to an exposed conductor. Be careful with high voltages.

Your signal cable should be plugged into one of the BNC inputs on the front of the scope. There will be either two or four inputs labelled accordingly. One you have plugged it in, you must make sure the scope is actually displaying that channel. Above the input should be a colored button with the channel name. The corresponding color should be shown on the display. If not, push the button and the screen menu should change to that channel name. Various adjustable channel settings will also appear next to the four soft buttons immediately next to the screen. You can play with these later. If you want to turn off other channels, push the appropriate colored button once to bring up the menu for that channel, then push it again and it should disappear from the screen.

Trigger

Getting a trigger can sometimes be the most frustrating part of operating an oscilloscope. On the right you will find the Trig button to bring up the trigger menu. From the soft buttons, you can now adjust the trigger settings. Modern oscilloscopes are designed to be very flexible, which can lead to great confusion. The trigger is satisfied when some condition happens in time. This condition can occur on any of the input channels, or even from other information available to the scope (like the power line frequency). First, select the channel you want to trigger on, which should be the same channel you plugged the signal cable into. Second select a trigger mode. Auto is the easiest to deal with at this point. Third, select a trigger type. An edge trigger, the most common, is satisfied when the voltage rises or falls (adjustable) past some threshold value. You can adjust the threshold with the level knob which should be next to the trigger button on the right of the front panel. If you scan this around, you should be able to fairly quickly get a trigger locked which will appear as a stable pattern on the display. Our scopes have the nice feature of giving a visual indicator of where the trigger level is. Look for the yellow arrow on the side of the display as you move the level around. There should be in indication in the upper right corner of the display when you get a good trigger. If you can not, ask for help.

Horizontal scale

Even though you have a trigger, you may not yet be able to see anything useful. Next you must adjust the gain and offset of the X and Y axes so you can clearly see the desired features on the display. The horizontal scale is also the time axis. The big knob to the right will adjust the time interval per division from something very short (ns) up to something very long (seconds). There is also a button to bring up more options on the on screen menu. Twist this knob around until the signal you see on the screen is close to one full period. There is also a smaller horizontal offset knob which will move the entire display left and right. This is useful because by default the trigger comes right in the middle of the display. If you want to see more of what comes later, you can shift the whole thing over to get a better view. On the display, color coded by channel, the scope will tell you the time per division of your current setting.

Vertical scale

The vertical scale has a similar gain knob just above the channel input. This can be used to adjust the gain in volts. A second smaller knob can be used to move the entire trace up and down vertically. This is especially useful if you want to compare two inputs, but you don't want them drawn on top of each other. From the color-coded channel menu button, there are several other channel options which are worth mentioning. One is the coupling, which has the options AC/DC/Ground. This changes how the voltage input is coupled to the internal circuitry. Ground means the input is grounded, which is only useful to figure out where ground (V=0) is on your scope. If you can't find a signal, make sure you aren't simply displaying ground. AC allows you to remove any DC offset from your signal, while DC gives the pure signal in volts as it comes into the input. Another option is the Filter which can be different things like HF reject and so on. This can be used in special circumstances to reject high (or low) frequency noise. It is best to turn this off. Also make sure the probe setting is x1 (unless you are using a probe) or else your voltage readings will be off by factors of 10.

More fun

These scopes have many other useful features, including built in FFTs, the ability to do math between different channels, and on-screen cursors for measuring differences between two locations. Playing around (or reading the Tektronix primer) is probably the best way to figure things out. Perhaps the most non-intuitive part of the digital scopes (at least for someone who grew up with analog scopes) is the fact that many of the options show up on the screen after pushing a button somewhere else. As long as you pay attention to the screen, you should be able to find just about anything.

Don't hesitate to ask for help either.