The Oscilloscope

This section is designed to help you to use the oscilloscope provided on your bench. Various types of oscilloscope are used in the lab. They don't all have identical facilities or call their controls the same names. This makes it impossible to give a precise description of them all! Fortunately, they have some things in common. If you already know how to use a 'scope, skip this section.

The object of a 'scope is to give you a picture of how one or two voltages are varying with time. This is particularly useful when you want to measure periodic signals like sinewaves. When it is working properly, it draws a pair of lines, or traces, on the screen which display the shape of the input signals. The vertical position of a point on a trace depends upon the voltage, the horizontal position upon time. The traces are drawn left-to-right at a speed set by the timebase controls (these are the ones calibrated in units like “milliseconds/cm”).

So, if the 'scope screen is blank — have you turned it on?... If it is on, has had a few minutes to warm up, and you still can't see anything, it may be due to one of the following:—

The trace may be amplified too much. This will mean it is trying to spend all its time above or below the screen! Try turning down the channel ‘gain’ knobs. These are usually marked in units like “Volts/cm”. A setting like “5V/cm” is usually low enough to see the traces.
The trace position may be wrong. You can think of the 'scope traces as being graphs of input voltage against time. Time is scaled along the horizontal (x) axis, and voltage against the vertical (y) axis. However, the 'scope doesn't show you the axes or the graph origin. The ‘position’ controls let you move the origin around the screen. When you start, it may be set in a place which causes the 'scope to try and draw traces somewhere off-screen. Try adjusting these controls to their middle settings. (They may be called ‘offset’ or ‘shift’ controls, or something like that, on your 'scope.)
The traces may be being drawn too swiftly across the screen. If the timebase setting is too ‘fast’ the spot which draws out the trace may zip across the screen so quickly that it doesn't light it up properly. Try slowing down the time base setting. i.e., if it was set at, say, 1 sec/cm, change it to 10 msec/cm.

Hopefully, by now you should see something on-screen — even if it is just one or two flat horizontal lines. To go further you need to put some sort of signal voltage into the 'scope. When actually using the 'scope, you should alter the brightness, gain, etc, to suit whatever you are doing. For example, if you turned up the brightness to help find the trace, it may now be rather high. If so, just turn it down to a comfortable level. The lab should be well-lit and you shouldn't need to use the 'scope as an extra light source!

Your 'scope is a dual-trace oscilloscope. This means it can show two waves on-screen at the same time. It may, however, have been set to only show one of them. There should be a row of buttons or a knob somewhere marked with things like, “Ch A, Ch B, Chop, Alt”. Check these are set to Chop or Alt to get both traces.

Set up your signal generator to give a sinewave of about 1 kHz of moderate size (e.g. a Volt or so) and connect it to the 'scope. (If you have an R.S. signal generator, don't use its ‘TTL’ output as this can only provide squarewaves!) One of the traces should now show sines of life (ouch! sorry about that!).

You can alter the vertical size of the wave using the ‘volts/div’ controls. Adjust these so the wave amplitude covers, say, half the screen. The number of cycles shown across the screen depends upon the timebase setting. Adjust this so that there are just a few cycles across the width of the screen. All being well, one trace will still be flat, and the other will show a clear sinewave pattern. If you see a jumble of sinewaves — or a wave that ‘drifts’ across the screen — the triggering controls need adjusting. These help the 'scope to display the waveform by controlling when the instant when each trace scan begins.

The 'scope contains special circuits which examine the input voltage and decide when a scan should begin. Their details vary from one 'scope to another and the details aren't important. The most important triggering controls are the one that selects the trigger signal input and the one marked “Auto”. Look for an “Auto” button, or a knob called something like “Level/Auto”. Set this to Auto. If this doesn't work, you may be putting a signal into one input, but making the trigger circuits look at the other one for their cue! As a result they can't see anything and don't know what to do. See if you can find any triggering controls marked “Ch. A, Ch. B, Alt”, etc. Use them to select the channel you are using for your sinewave signal.

The trigger controls also include some filters which are meant to help when you want to examine some types of complex waveform. Sometimes they are simply marked “High” or “Low” or “DC”. On other 'scopes they may be called something like “TV Frame” and are intended to help when the 'scope is used to repair TV's. It doesn't matter what they are called, play around with them and choose whichever gives you the best results.

Oscilloscopes tend to have a front stuffed with knobs and buttons & every 'scope design has its own combination of mysterious-sounding names for what they do. Don't be put off. Use a sinewave input and play around with the 'scope controls until you get a clear display. So far as we know, no combination of settings causes the 'scope to explode or fly around the room making whooping noises! (No, don't take that as a challenge!...)

Health Warning! Some 'scope controls can be uncalibrated. For example, the gain or timebase rate can be smoothly altered. This is useful if you want, for example, to adjust a displayed wave so it is exactly 3 cm divisions high. Note, however, that this means that the ‘volts/div’ setting can't now be correct. If you want to use the height to measure a voltage, or use a horizontal length to measure a time these controls must be set to calibrated. This is usually marked on the appropriate knob and is at a ‘click-stop’ position — or at one end of the knob range. Make sure your controls are calibrated, otherwise your measurements won't mean anything! You have been warned!

Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.