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- Table des matières
- MARQUE LIVRES
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30
Sampling
Because the idea of sampling is so fundamental to the operation of a digital oscilloscope, let's look at it
in more detail.
An analogy for sampling is to think of the A/D converter as a camera that takes a picture of the
amplitude of the waveform. The "shutter time" is very short and the camera "assumes" that the
waveform is constant over the time that the "shutter" is open.
As an example, you might behave like this "sampling camera" if you were monitoring the outside air
temperature with a thermometer. You'd look at your watch, write down the time, then write down the
thermometer's reading.
If your thermometer was near the exhaust of a heat engine, the air temperature could vary more
rapidly than your thermometer is capable of responding. You would reconstruct the temperature
waveform as a function of time by using your samples -- and, depending on your knowledge of what
you were measuring, you may or may not be aware that your reconstruction of the waveform might
not accurately represent the "real" air temperature. The term "real" has quotes around it because we
can only know it through measurement -- and different measurement methods have different
capabilities, leading to different "versions" of reality.
Let's look at two cases of sampling, one where the sampling is adequate and one where it is not. These
qualitative examples will give you a feel for what good sampling can be like. Here's a waveform that
was constructed by a program that used 1000 points to display the waveform. The "Adequate
sampling" waveform shows the sampling points taken by the "camera" at 1 out of every 20 points in the
waveform. The "Inadequate sampling" waveform was 1 out of every 10 points.
Figure 19
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- CONTENTS 2
- Introduction 4
- Notation 6
- Signals 7
- Amplitude measurements 8
- Non-sinusoidal waveforms 10
- Analog oscilloscopes 11
- Theory of operation 13
- Vertical circuits 14
- Figure 9 15
- Figure 10 15
- Graticule 16
- Delayed sweep 17
- Operation of an analog scope 18
- Vertical coupling 19
- Variable adjustments 19
- Trigger controls 19
- Slow waveforms 20
- XY display mode 20
- Component test 21
- Digital oscilloscopes 23
- Figure 16 24
- Figure 17 27
- Sampling 30
- Figure 20 31
- Figure 21 32
- Dead time 33
- Operation and features 34
- The AUTO button 35
- Quick help 35
- Saving waveforms and data 37
- Network operation 38
- Math calculations 39
- Recording transient events 40
- Video triggering 40
- Averaging 40
- Peak detection 41
- Automatic Measurements 42
- Reference waveform 43
- Recording waveforms 44
- Digital filter 45
- Pass/fail testing 46
- WaveXpress® 47
- Figure 36 48
- Probe compensation 50
- Probe types 50
- Figure 37 51
- Figure 38 51
- Figure 39 52
- Figure 40 53
- Floating a scope 54
- Good measurement practices 55
- Oscilloscope safety 56
- Glossary 57
- References 64
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