Understanding Oscilloscope Functions
Time Domain Aliasing.
Aliasing occurs when the oscilloscope does not sample the
signal fast enough to construct an accurate waveform record. When this happens,
the oscilloscope displays a waveform with a frequency lower than the actual input
waveform, or triggers and displays an unstable waveform.
Actual high-frequency waveform
Apparent low-frequency
waveform due to aliasing
Sample points
The oscilloscope accurately represents signals, but is limited by the probe
bandwidth, the oscilloscope bandwidth, and the sample rate. To avoid aliasing,
the oscilloscope must sample the signal more than twice as fast as the highest
frequency component of the signal.
The highest frequency that the oscilloscope sampling rate can theoretically
represent is the Nyquist frequency. The sample rate is called the Nyquist rate, and
is twice the Nyquist frequency.
The oscilloscope maximum sample rates are at least ten times the bandwidth.
These high sample rates help reduce the possibility of aliasing.
There are several ways to check for aliasing:
Turn the horizontal
Scale
knob to change the horizontal scale. If the shape of
the waveform changes drastically, you may have aliasing.
Select the Peak Detect acquisition mode. (See page 20,
.) This
mode samples the highest and lowest values so that the oscilloscope can
detect faster signals. If the shape of the waveform changes drastically, you
may have aliasing.
If the trigger frequency is faster than the display information, you may have
aliasing or a waveform that crosses the trigger level multiple times. Examining
the waveform allows you to identify whether the shape of the signal is going
to allow a single trigger crossing per cycle at the selected trigger level.
If multiple triggers are likely to occur, select a trigger level that will generate
only a single trigger per cycle. If the trigger frequency is still faster than the
display indicates, you may have aliasing.
If the trigger frequency is slower, this test is not useful.
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