Understanding Oscilloscope F unctions
the oscillosco
pe displays a waveform with a frequency lower than the actual input
waveform, or triggers and displays an unstable waveform.
Actual high-f
requency waveform
Apparent low-frequency
waveformduetoaliasing
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 f requency. The sample rate is called the Nyquist rate, and
is twice the Nyquist frequency.
Oscilloscope models with 100 MHz bandwidths sample at rates up to 1 GS/s.
Models with 200 MHz bandwidths sample at rates up to 2 GS/s. In both cases,
these m
aximum 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 Scale knob to change the horizontal scale. If the shape of the
wave
form changes drastically, you may have aliasing.
Select the Peak D etect acquisition mode. (See page 30, Peak Detect.) This
mode
samples the highest and lowest values so that the oscilloscope c an
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 o ccur, 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.
32 TPS2000B Series Digital Oscilloscope User Manual
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