EET 114 Unit 9. More on the Oscilloscope

Unit 9. More on the Oscilloscope

In the previous two units you've been using the oscilloscope and multimeter to make measurements in ac circuits. As you know from the first half of this course, the multimeter can also make measurements in dc circuits. The same is true for the oscilloscope. In this unit we'll look more closely at how to set the meter or the scope to measure ac or dc.

Work through the e-Lesson and self-test questions below.

After completing the e-Lesson, take Quiz #9, perform Lab #9, and do Homework #9.

Unit 8 Review

This unit will build on material that you studied in Unit 8. So let's begin by taking this self-test to review what you learned in that unit.

DC or AC?

The digital multimeter and the oscilloscope can each be used to measure either DC voltages or AC voltages.
On the multimeter, the AC/DC switch lets you choose between the two types of measurements.
On the oscilloscope, the input coupling switch lets you choose.
Let's look more closely at how these two switches work.

Multimeter's AC/DC Switch

The multimeter's AC/DC Switch has only two positions: DC and AC.
The photo below shows the location of the AC/DC switch (in the yellow box) on a Fluke 8050 digital multimeter.
In DC mode, the meter measures only DC voltages or currents, and it ignores AC voltages or currents.
In AC mode, the meter measures only AC voltages or currents, and it ignores DC voltages or currents.
At this point you might be saying to yourself, "That's obvious." Maybe it is, but when we turn to the oscilloscope, we'll find that things are not always as obvious as they might seem. So please don't skip over the self-test questions below.

Combined AC/DC Circuits

So far in this course, all the circuits we've built and studied have had either DC voltage sources or AC voltage sources, but not both.
Many real-life circuits contain both AC voltage sources and DC voltage sources. In such a circuit, a resistor will have both an AC current and a DC current running through it at the same time. Also, the resistor will have both an AC voltage drop and a DC voltage drop.
For a circuit that has both AC and DC, it's especially important to remember that the multimeter in DC mode shows you only DC voltages or currents, and that the multimeter in AC mode shows you only AC voltages or currents. The self-test will give you practice with some examples.

Oscilloscope for DC

We've seen that the multimeter can be used to measure both DC and AC currents and voltages. It's also true that the oscilloscope can be used to display either AC or DC voltages.
In Unit 6 we saw examples of sine waves, triangle waves, and square waves displayed on the oscilloscope screen. These are all AC voltages. But what would a DC voltage look like on the scope's screen?
Answer: When you display a fixed DC voltage on the oscilloscope screen, you'll just see a flat line, as in the picture below.
This makes sense if you remember that the oscilloscope displays a graph of voltage versus time. If voltage has a constant, unchanging value (such as 5 V), then the graph will be pretty boring—just a flat horizontal line.
Still, even though we just see a flat line, we can use the scope to measure the value of a DC voltage by seeing how far this flat line is above or below an imaginary line on the screen that represents 0 volts.
Take the self-test to see how to do it.

Negative DC Voltages on the Oscilloscope

On the oscilloscope, positive voltages will appear above the 0-volt reference line, and negative voltages will appear below the 0-volt reference line. (As mentioned in the previous self-test, you can choose the position of this 0-volt reference line. We'll see below how to do this.)
So if you display a negative DC voltage on the scope, it will still show up as a flat line, but this flat line will lie below the 0-volt reference line, not above it.
Example: Suppose you've adjusted the scope so that its centerline represents 0 volts. If you saw the image shown below, you'd be looking at a negative voltage, since the trace lies below the 0-volt line.

Oscilloscope's Input Coupling Switch

The input coupling switch on an oscilloscope lets you control the type of voltage (AC or DC) displayed on the screen. An oscilloscope has one input coupling switch for each of its input channels. For example, a dual-input scope has two input coupling switches, one for Channel 1 and one for Channel 2.
The photo below shows the location of the two input coupling switches (in the yellow boxes) on a Tektronix 2213.
Each input coupling switch has three positions: AC, DC, and GROUND.
Continue reading for information on each of these three positions.

Input Coupling Switch Set to AC

When the switch is set to AC, only AC voltage is displayed; DC voltage is blocked.
Use this position when you wish to measure AC voltage only, ignoring any DC voltage present in the circuit.

Input Coupling Switch Set to DC

When the input coupling switch is set to DC, both AC and DC voltages are displayed.
Here's why I said earlier that things are not always as obvious as they seem. Many students make the mistake of thinking that when this switch is set to DC, the oscilloscope will only display DC voltage.
Use this position when you wish to measure DC voltage, or when you want to measure a combination of AC and DC voltages.

Input Coupling Switch Set to GROUND

When the input coupling switch is set to GROUND, no input voltage is displayed; both AC and DC are blocked.
Use this GROUND setting to establish a 0-volt reference line. Here's how:
1. Set the input coupling switch to GROUND.
2. Use the vertical position control to move the trace to the position on the screen that you wish to use as your 0-volt reference line.
3. Switch the input coupling switch back to either AC or DC, depending on the type of measurement you're making.

Where Should You Set Your 0-Volt Reference Line?

Remember, you control where the 0-volt line lies on the oscilloscope screen. By using the input coupling switch and the vertical position control, you can adjust the scope so that 0 volts is at the bottom of the screen, in the center of the screen, or anywhere else on the screen.
If you expect to be dealing with both positive and negative voltages, it's usually best to use the centerline of the screen as your 0-volt reference. That way, positive voltages will be displayed in the screen's upper half, and negative voltages will be displayed in the screen's lower half.
If you expect to be dealing with positive voltages only, it's probably best to use the screen's bottom line as your 0-volt reference, since all positive voltages will be displayed above this line.
Although it's not a very common situation, if you expect to be dealing with negative voltages only, it's probably best to use the screen's top line as your 0-volt reference, since all negative voltages will be displayed below this line.

Oscilloscope for Combined AC/DC Circuits

Let's think about what you'll see on the oscilloscope screen if you measure a voltage that contains both AC and DC.
For example, suppose we're measuring a voltage that combines a 12 V_pp sine wave with a 2 V DC voltage. Let's assume that the scope's VOLTS/DIV switch is set to 2 V, and that the centerline has been set as the 0-volt reference line.
We know that the 12 V_pp sine wave by itself will look like this:
And we know that the 2 V DC voltage by itself will look like this:
So what will we see if we look at the combination of these two voltages? Answer: we'll see the sine wave pushed upward by 2 volts. In other words, it will look like this:
Notice that the sine wave still has a peak-to-peak voltage of 12 volts, but instead of going from a low value of -6 volts to a high value of +6 volts, the sine wave goes from a low value of -4 volts to a high value of +8 volts. If you think about it, this should make sense, because it's what you would expect to get if you took a normal sine wave and pushed it upward by 2 volts.
Of course, you'll only see that picture if the input coupling switch is set to DC. If the switch is set to AC, then only the AC voltage will be displayed, and if the switch is set to GROUND, then neither the AC nor the DC voltage will be displayed.

Unit 9 Review

This e-Lesson has covered some important points on using the oscilloscope to measure DC voltages or a combination of DC and AC voltages.
To finish the e-Lesson, take this self-test to check your understanding of these topics.

Congratulations! You've completed the e-Lesson for this unit. What's next?

Take Quiz #9.
Perform Lab #9.
Do Homework #9.
Keep practicing your skills by playing the games on the Games page.

Then you will have completed all of the required work for this course, except for the final exam. Congratulations on making it through to the end! For a good, thorough review, I suggest that you go back and re-take each of the Unit Review self-tests (located at the end of the e-Lessons).

Of course, there's plenty more to learn. In this course we've concentrated mainly on building circuits and making measurements on them. We've also looked at a few of the equations (such as Ohm's law) used in analyzing circuits. In several other courses that you'll take (including EET 150, EET 155, and EET 205), you'll learn many more equations and techniques for analyzing circuits. If you'd like get a head start on those classes, take a look at the material that you'll study in EET 150.

Nick Reeder | Electronics Engineering Technology | Sinclair Community College

Send comments to nick.reeder@sinclair.edu