Figure 11.5: Vertical resolution loss due to inadequate sampling of high-frequency
signal.
11.1.3. Other Effects
There are many other effects of the measurement equipment that can affect measurement
accuracy. Although a detailed account of each of these effects is beyond the scope of this
book, some of them are mentioned here briefly.
Internal Oscillator Accuracy and Stability.
The timing reference points inside the oscilloscope are typically based off a crystal oscillator.
The quality of the crystal chosen by the manufacturer will affect the accuracy and frequency
stability of the instrument. The oscillator will have a nominal accuracy and will also change
slightly with time. Oscilloscopes tend to be far more accurate with relative time
measurements (such as comparing the edge rates of similar signals) than with absolute time
measurements (such as measuring a long period very accurately). With relative time
measurements, the nominal accuracy may not matter. However, since the frequency
accuracy of the internal oscillator can vary with time, relative measurements separated over
long lengths of time may yield inaccurate results. The oscillator may also have temperature
sensitivities and other imperfections.
Analog-to-Digital Converter Resolution.
The voltage amplitude has a finite resolution due to the analog-to-digital converter used
internally. Most oscilloscopes use an 8-bit analog-to-digital converter which allows a
granularity of 256 levels of amplitude. Typically, the 256 levels are scaled to the display.
Thus, at any time the fraction of the display used is the fraction of the analog-to-digital
converter that is being used. Thus, for accurate amplitude measurements, the waveform
should be scaled to fill a good fraction of the display.
Trigger Jitter.
This is the variation in placement of the trigger by the instrument. This can lead to apparent
time variation of a measurement.
Interpolation.
How the instrument chooses to "connect the dots" can affect the shape of a reconstructed
waveform. Various interpolation algorithms exist and are usually selectable. The user may
also choose to shut off interpolation and see only the captured points.
Summary :
Oscilloscopes tend to be far more accurate with relative time measurements (such as comparing the edge rates of similar signals) than with absolute time measurements (such as measuring a long period very accurately). However, since the frequency accuracy of the internal oscillator can vary with time, relative measurements separated over long lengths of time may yield inaccurate results.
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