An oscilloscope, or simply a 'scope', is a device that displays electrical signals. Fast scopes can be used to examine very short events, down to the microsecond or even the nanosecond level. In its most basic form, a scope is simply a cathode ray tube connected to an amplifier.

Surely it's not that simple?

No, it's not. If the signal is to be displayed against time, as it most commonly will be, then a 'sweep' signal that moves the beam of the CRT across the screen must be generated. The length of the sweep is easily set with a dial, but when do we make it start?

Concievably, we could just have it start when the scope is turned on and run continuously from that point. This is impractical, though, since the periodic signal being viewed won't have the same period as the sweep, causing the signal to shift around the display with each sweep. We could have the user carefully tweak the sweep frequency until it's the same as the signal frequency, but would you want to carefully tweak the display every time the signal changed?

No...

That's why scopes have a 'trigger' built into them. You turn on the trigger and set a signal level. When the signal passes that level, the sweep begins. If you set the trigger level right, then the trigger will activate at the same point on the input signal each time giving you a stable display. Proper use of the trigger will let you scope out just about anything you want.

What if I want to look at two signals?

No problem. Most scopes are built with the idea that, once you see the waveforms you'd want to compare them, so they've got two or more inputs, usually called 'channels'. These channels can be displayed separately, simultaneously, simultaneously with one inverted, added, subtracted, or even (most interestingly of all) with one controlling vertical beam motion and one controlling horizontal motion. This last is called 'X-Y mode' and is wonderful for seeing Lissajous figures, or it can be used as a vector display for an external device.

I want to make measurements, not look at pretty pictures!

No problem. A scope display has a grid marked on it, and the different scalings of the scope are calibrated to the grid. So when setting the horizontal and vertical scaling you select them as, say, 5 mV/division vertically and 10 ms/div horizontally. The two input signals can be set to different vertical scaling, and offset independently to line up better with the grid.

OK, now how do I get my signal into the scope?

Every scope I've ever seen takes inputs from coaxial cable with standard BNC connectors. So you can either put the signal directly into the scope with a coax cable or you can use an oscilloscope probe, which is a device designed to clip into a circuit and measure the voltage without affecting the operation of the circuit. The probe has a BNC connector to connect to the scope and is supplied with an alligator clip for grounding.

Now, what about those nifty new digital oscilloscopes?

A Digital Sampling Oscilloscope or DSO is a very different thing than a traditional analog scope. It is, essentially, a fast analog to digital converter connected to a computer that emulates a scope. This permits it to do some very nifty things that an ordinary scope cannot do, including making automatic quantitative measurements of waveforms and automatically setting itself up based on the input waveforms. A DSO can have an LCD screen making it much more compact, it can include a floppy drive to output waveforms, it can connect to other equipment and computers through IEEE 488 or even Ethernet.

So why haven't we all thrown out our analog scopes and got DSOs?

Well, there are some drawbacks to the DSO approach. The first and most important is the resolution of the scope. DSOs generally have a lower effective resolution than analog scopes, both in voltage levels and in time. Fast analog to digital converters can be very expensive, and beyond a certain speed they are presently impractical. Voltage resolution versus temporal resolution then is much more of a trade-off for a DSO. Also, due to much of the DSO's functionality being implemented in software, the front panel controls may feel laggy relative to an analog scope, though this is a comparatively minor issue.

Analog or digital, what applications do these have?

Well, any time there is an electrical signal to be observed, a scope can be useful. In electronic construction, testing, and repair, there are many signals to watch and so an oscilloscope is an essential tool for serious work in those areas. In a laboratory there may be any number of transducers whose output could be monitored with an oscilloscope. Finally, biological signals with medical importance, such as ECG signals, are often displayed using oscilloscopes.


(CC)
This writeup is copyright 2004 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at http://creativecommons.org/licenses/by-nd-nc/2.0/ .

Os*cil"lo*scope (?), n. [L. oscillare to swing + -scope.] (Elec.)

An instrument for showing visually the changes in a varying current; an oscillograph.

 

© Webster 1913

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