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Electronics for dummies - McComb G.

McComb G., Boes E. Electronics for dummies - Wiley publishing, 2005. - 433 p.
ISBN: 0-7645-7660-7
Download (direct link): electronicsfordummies2005.pdf
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^ Pulse waveform: This waveform shows a sudden change between a signalís low and high states. Most pulse waveforms are digital and usually serve as a timing mark, like the starterís gun in a 440 yard dash. When the gun goes off (the pulse) other parts of the circuit react and generate even more signals.
DC WAVEFORM
DIGITAL WAVEFORM
Figure 10-9:
Four
common
waveform
forms
PULSE WAVEFORM
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Chapter 10: Getting Down with Logic Probes and Oscilloscopes 223
So, When Do I Use an Oscilloscope?
When youíre testing voltage levels, you can often use multimeters and oscilloscopes interchangeably. The choice of which tool you use is yours, though for routine testing procedures, you may find the multimeter a little easier. In general, you may opt to use an oscilloscope for
Visually determining if an AC or digital signal has the proper timing.
For example, you often need this test when you troubleshoot radio and television equipment. The service manuals and schematics for these devices often show the expected oscilloscope waveform at various points in the circuit so that you can compare. Very handy!
Testing pulsating signals that change too rapidly for a logic probe to detect. Generally these are signals that change faster than about five million times a second (5 MHz).
Visually testing the relationship between two input signals, when using an oscilloscope with two input channels. You may need to do this test when you work with some digital circuits, for example. One signal may trigger the circuit to generate yet another signal. In fact, this is quite common. Being able to see both signals together helps you determine whether the circuit is working as it should.
Testing voltages, if the scope is handy; but you can use your multimeter for testing voltages, too.
Rather than whipping out your oscilloscope for every test, youíre better off using a multimeter for the following:
Testing the resistance of a circuit
Determining if a wire or other circuit is shorted (0 ohms resistance) or open (infinite ohms resistance)
Measuring current
Testing voltages and various components, such as capacitors and transistors
Putting the Oscilloscope to Work: Testing, 1-2-3!
So if youíve been reading along in this chapter, you now know a little bit about what an oscilloscope is for and what it does. In the following sections, we show you how to do a couple of quick tests. These tests demonstrate how you use a scope for a variety of simple chores. After you work through these tests, youíre well on your way to becoming a master scope user.
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224 Part IV: Getting Your Hands Dirty_
Basic setup and initial testing
Before you use your oscilloscope for any actual testing, set its controls to a normal or neutral setting. You then calibrate the scope, using its built-in test point, so that youíre sure itís working correctly.
Here are the steps for setting up your scope. Refer to Figure 10-6, earlier in this chapter, to reference the various knobs and buttons on your scope as you go through these steps. Remember that your oscilloscope may look a bit different, and its knobs and controls may have slightly different names.
1. Turn the scope on.
If itís the CRT bench top variety, allow time for the tube to warm up. You may or may not see a dot or line on the screen.
2. Set the Sweep/Time Per Division knob to 1 millisecond.
This setting is a good middle value for initial calibration.
3. Set the Volts Per Division knob to 0.5 volts.
This setting is also a good middle value to use for initial calibration when testing low-voltage DC circuits.
4. Set the Trigger Level control to Automatic (or midway, if it doesnít have an Automatic setting). Select AC Sync and Internal Sweep.
5. Select the Auto setting for both Horizontal Position and Vertical Position; or you can crank the knobs to their midpoint if your scope doesnít have an Auto setting.
6. Connect a test probe to the input.
If your scope has multiple channels (sometimes called inputs), use Channel A.
7. Select Gnd (Ground) for the Signal Clamp, if your scope has this control.
On some scopes, this control may be called Signal Coupling.
8. Connect the ground clip of the test probe to the designated ground connection on the scope (see Figure 10-10).
If your oscilloscope doesnít have a designated ground connection, clip the lead to any exposed metal point, such as the head of a screw.
9. If your scope has a Signal Clamp switch, attach the center of the test probe to the calibration test point. If your scope lacks a Signal Clamp switch, attach the center of the test probe to the ground point.
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Chapter 10: Getting Down with Logic Probes and Oscilloscopes 225
Figure 10-10:
Connect the ground of the probe to the ground connection on the scope.
10. Adjust the Vertical Position knob until the beam sits on the first division on the screen (Figure 10-11).
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