>> How to use an oscilloscope
IAN: Hi! This is Tony and Ian from Tektronix.
TONY: Today we are going to show you a video on an oscilloscope, oscilloscope basics and
the main features.
IAN: We're going to do that by starting with something that brings back a lot of fond memories
for electrical engineers - A simple RC circuit from the first day of undergrad engineering
class. That's going to serve as our spring board for moving on to how to use an oscilloscope.
So at its heart, an oscilloscope is a device for seeing how the voltage of the signal varies
over time. For example, imagine a simple RC circuit with battery, a capacitor and resistor.
TONY: And technically, it's not a circuit yet, right?
IAN: Exactly. We haven't taken the last step of connecting the battery yet. First we're
going to add one more piece to the puzzle. We're going to connect an oscilloscope probe
across the leads of the capacitor. Now when we connect the battery and complete the circuit,
you would expect the voltage to start at zero volts and then rise as the capacitor is charged
until it reaches one and a half volts. And if you have a correctly configured oscilloscope
connected, that's exactly what you would see. And it would look something like this... it
starts at zero and goes up to one and a half volts. Now, this signal is pretty stable.
Real world signals change a lot over time. So that's where knowing your way around the
front panel of an oscilloscope comes in handy.
TONY: So we'll take a look at an oscilloscope with a repeating signal?
IAN: Right, here's a much more realistic signal -- a sine wave. If you watch the sine wave
long enough maybe you'll see some glitches in it, or some inconsistencies, and that's
what as an embedded designer, you're trying to zero in on. So this particular oscilloscope
can display up to four analog channels from different inputs and each channel has its
own independent set of vertical controls. That means each waveform can be positioned
independently of the others on the screen. The two most important vertical controls are
the vertical position and the vertical scale. So the vertical position moves the waveform
up and down the screen, and the vertical scale expands or compresses the waveform vertically.
Vertical scale is expressed in volts per division. So this particular model of oscilloscope has
eight vertical divisions. Most of our oscilloscopes have ten, but if you have eight and you happen
to have it set to five volts per division, you could fit 40 volt peak to peak sine wave
on screen.
TONY: Can we take a look at that on the oscilloscope?
IAN: Good idea. Let's do that. So we've got a nice, stable square wave on the screen,
and I'm going to reach for the vertical controls that go with the channel we've got connected
- Channel one. And as I move the vertical position down, you can see the waveform moving
down on the screen. If I move it back up, you can see the waveform moving back up. So
this is a very continuous knob. You can position the waveform exactly where you want it. The
vertical scale is more granular, it moves in clicks. As I move the vertical scale one
click larger, the waveform gets smaller on screen. If I move the vertical scale one click
smaller, the waveform gets larger on screen.
TONY: That's great. Can we move on and see what the horizontal controls do?
IAN: Yes. So all of the input channels share one common set of horizontal scale parameters.
That means as you adjust the horizontal controls, all of the waveforms will move together on
the screen. The two most important horizontal controls are the horizontal position and the
horizontal scale. As you move the horizontal position, the trigger indicator moves left
or right and that causes all of the waveforms on the screen to move left or right together.
And as you adjust the horizontal scale, that causes all the waveforms on screen to expand
or contract horizontally. Horizontal scale is expressed in seconds per division. So if
you have ten horizontal divisions available on your screen and you have your oscilloscope
set to one second per division, then that means you can fit ten seconds worth of data
on your screen.
TONY: Wow, that's great. That's really similar to the vertical.
IAN: Exactly.
TONY: Can we see that on the screen also?
IAN: Yes. So now I've got my hand on the horizontal position knob. As I move it to the left, you'll
see the waveform move very slowly to the left. The trigger indicator, which I'll talk about
in a moment, also moves to the left. And I can move everything to the right as well.
Now like the vertical position knob, that's a very fine tune style of knob. The scale
moves in clicks like the vertical scale does. As I click it in one direction or another,
the waveform expands or contracts horizontally.
TONY: And now for the last of the controls, the trigger.
IAN: Yes. So these take a little more explanation. Modern oscilloscopes have really sophisticated
triggering systems. We're just going to talk about the bread and butter of triggering today:
The rising edge trigger. With rising edge trigger, you care mainly about the trigger
level. The trigger level is the voltage that your signal has to cross through for the oscilloscope
to consider it time to update the display. With the rising edge trigger you want your
trigger level to be somewhere between the top and bottom of the waveform. The reason
why it's important to set that correctly is because without triggering, your oscilloscope
just updates its data at arbitrary times, and your signal is repeating at arbitrary
times, and those probably won't line up. So for example, imagine your signal looks like
this [draws signal] and that's what it looked like the first time the oscilloscope happened
to update. And the next time it was time for the oscilloscope to update, it happened to
catch the signal a little earlier... so it looked like that [draws a second signal].
And then another time it updated, it happened to catch it at a completely opposite time
and it looked like that [draws a third signal]. So you get this chaotic, unfocussed display.
TONY: Which isn't too helpful, right?
IAN: It's not too helpful when you're trying to zoom in on what's going wrong in a circuitry
design. So instead, what you'd like is to be able to start the waveform at some particular
point [draws new signal]. And then the next time, that it's time to update the display,
you'd like it to update at exactly the same point on the signal so you get a much more
coherent display [draws new second signal].
TONY: Yeah, that's great. Can we see it on the oscilloscope here?
IAN: Definitely. So right now, we've got a triggered display. But I'm going to move the
trigger level way out of range, and after a few seconds the oscilloscope has gone to
an untriggered mode, and it says "Auto" here in the corner as a clue that there's something
that's been going on with your trigger set up. And as you can see, the waveform is just
flickering all over the screen. But as we bring the trigger level back down, we can
see immediately the waveform stabilizes. So whenever it is time for the oscilloscope to
update, it waits until the voltage crosses through this level [points to center of signal]
and then it marks that point with the trigger indicator. So this is the start of the signal
and it updates. Notice it's also able to display a little extra data before the start of the
signal. How it does that is a discussion for another day.
IAN: So we've really just scratched the surface of what an oscilloscope can do. We'd love
to do more videos like this one so it you have a specific topic you'd like us to cover,
take a look below this video or contact us on Twitter (http://twitter.com/tektronix).
TONY: And head right here to http://www.tektronix.com/learning/oscilloscope-tutorial/ -- there's a lot of resources here. Cheers!
IAN: Cheers!