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The sensitivity of an oscilloscope indicates the Y-axis voltage per division. The low-voltage sensitivity of most average-priced scopes is about 5 mV (millivolts, or thousandths of a volt) to 5 volts. You turn a dial to set the sensitivity that you want. When you set the dial to 5 mV, each mark on the face of the scope tube represents a difference of 5 mV. Voltage levels lower than 5 mV may appear, but you canít accurately measure them. Most scopes show very low voltage levels (microvolt range) as a slight ripple.
The ins and outs of using an oscilloscope
Although an oscilloscope can do some pretty cool things, you only have to perform a couple of steps to actually use one.
Hereís a quick rundown of the steps that you perform to measure the voltage of a DC signal with an oscilloscope:
1. Attach a test probe to the scope input.
Note: Some scopes have several inputs, called channels; we assume youíre dealing with just one input for now. term LinG - live, informative, Non-cost and Genuine !
220 Part IV: Getting Your Hands Dirty
2. Adjust the Volts Per Division control to set the amplitude or voltage range.
For example, if the voltage youíre testing is 0-5 volts, use the 1 volt per division range. With that setting, each volt corresponds to one tick mark on the screen of the scope.
3. Adjust the Sweep/Time Per Division control to set the time slice of the signal.
The time slice is the duration of the part of the signal thatís shown on the scope. A shorter time slice shows only a brief portion of the signal, whereas a longer time slice shows you more of it.
If youíre testing a DC signal, you donít need this control because the signal doesnít change (much) over time. You can choose a medium-range setting to ensure consistent readings, such as 1 millisecond per division (a millisecond is one one-thousandth of a second).
4. Select the signal type, either AC or DC, and the input channel.
Note that you donít get an input channel selector if you buy a singlechannel oscilloscope.
5. Most scopes have a trigger switch. If yours does, set it to Auto.
6. When youíve set up the oscilloscope properly, connect the test probe to the signal that you want to test.
7. Connect the ground of the probe to the ground of the circuit.
8. Connect the probe itself to the circuit point that you want to test (you can see this setup in Figure 10-8).
9. Read the waveform displayed on the screen.
Unless your scope has a direct read-out function that displays voltages on the screen, you need to correlate what youíre seeing with the settings of the s.
If youíre testing a low-voltage AC or pulsing digital signal, set the Sweep/Time Per Division control so that you can adequately see each cycle of the signal. Donít worry . . . you can experiment with the Sweep/Time Per Division control until the signal looks the way that you want it to.
Do not test AC voltage coming from a wall outlet using an oscilloscope, unless you first take special precautions. The manual that came with your scope should outline those precautions. We assume you are using your oscilloscope only to test low-voltage DC circuits, and low-voltage AC signals, such as those from a microphone. If you connect your scope directly to 117 VAC from a wall outlet you can injure both you and your scope!
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Chapter 10: Getting Down with Logic Probes and Oscilloscopes 221
When using an oscilloscope, touch the tip of the scope probe to the circuit point that you want to test.
What all the Wiggly lines mean
Oscilloscopes give you a visual representation of an electrical signal. The vertical axis indicates the amount of voltage (also called amplitude), and the horizontal axis represents time. Oscilloscopes always sweep left to right, so you read the timeline of the signal from left to right, just as you read a line in a book.
The signal that you observe on the oscilloscope is a waveform. Some waveforms are simple, some are complex. (We introduce the concept of waveforms and different signal types in Chapter 1.) Figure 10-9 shows the four most common waveforms that you encounter in electronics and what these waveforms look like on an oscilloscope screen:
DC (direct current) waveform: A flat, straight line, like the one that you see here. A DC waveformís amplitude, which is the voltage reading, is critical.
AC (alternating current) waveform: This waveform undulates over time. The most common waveform is a sine wave (see Chapter 1 for more about sine waves). AC waveforms vary in frequency. Some move quite slowly, such as 60 Hz (60 cycles per second), the frequency of house current in North America. Or AC waveforms can move very quickly, on the order of several million or billion Hertz.
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222 Part IV: Getting Your Hands Dirty
^ Digital waveform: A DC signal that varies between no volts (low) to some pre-determined voltage (high). The digital circuitry interprets the timing and spacing of the low and high marks. When you plug in a digital camera to your computer, the computer copies the pictures stored in the camera to its hard drive by using such a waveform. The waveform changes very quickly so that you can transmit the data in a short period of time.